Re: VAX 4000-500 PSU Overload?

[Holm Tiffe]

Robert Jarratt wrote:

> 
> 
> > -Original Message-
> > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Holm
> Tiffe
> > Sent: 25 December 2015 13:15
> > To: General Discussion: On-Topic and Off-Topic Posts
> > Subject: Re: VAX 4000-500 PSU Overload?
> > 
> > This Capacitor was bad in my PSU too and I've changed it.
> 
> In the light of what you say below, do you mean you only changed this one,
> or did you change all of the ones on the logic board? Does that include the
> really big ones too?

No, I've changed all smaller electrolytics, the big brown 6800µF and the
big filter caps after the rectifiers remained untouched.

The bigger caps  aren't loaded that much in relation to ther volumina
with pulsed currents as the smaller ones and the se works better on big ones.
The ability to dissipiate heat is also better for big capacitors and that's
the cause that they are still wet internally and therefore still working.

Regards,
Holm

-- 
  Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe, 
 Freiberger Straße 42, 09600 Oberschöna, USt-Id: DE253710583
  www.tsht.de, i...@tsht.de, Fax +49 3731 74200, Mobil: 0172 8790 741

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Holm
Tiffe
> Sent: 25 December 2015 13:15
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> This Capacitor was bad in my PSU too and I've changed it.

In the light of what you say below, do you mean you only changed this one,
or did you change all of the ones on the logic board? Does that include the
really big ones too?

> 
> So finally, what was wrong with my idea that you sould simply change all
> reachable electrolytic capacitors in that PSU to get it working again?
> 

Nothing at all was wrong with your idea. I had indeed followed your advice
on the 12V board, on the basis that I could reach them and that it seemed to
be the 12V side that was having the problem, so implying that other parts of
the PSU were OK. I did not change any on the 5V side as that side seems to
be OK and I don't really like the approach of pulling and soldering them
from above all that much.

> In my Eyes it really isn't worth the effort of testing each one, change
them all
> to get that thing working halfways reliable again, it is an really bad
desing
> anyways...
> 

Yes, and I may yet have to change those on the 5V side, because after
posting the good news, I had one brief episode where the PSU switched itself
off again after running for about a minute. On powering it up again it
worked for a good hour without issue. So perhaps I should bite the bullet
and replace those too, and also the other small electrolytics on the logic
board too.

However, I don't think I will replace the really big ones. Generally, I get
the impression that these big ones suffer fewer problems, certainly their
ESR is fine. I am also reluctant to change them because they are generally
connected to large heat-sucking planes, making them hard for me to desolder
without making a mess of the board and possibly damaging it too.

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Holm Tiffe]

Robert Jarratt wrote:

> Good news! It looks like I have managed to get this working again. I
> realised I had not checked the electrolytic capacitors on the main logic
> board, ones situated far from the main power output boards. I found one
> which had a higher ESR than two other identical ones, although the ESR was
> still theoretically OK according to the table on my meter it was at the
> limit, so I replaced it and the PSU now appears to work. It really goes to
> show, *all* electrolytic capacitors should be checked. Picture showing the
> one I replaced here: http://1drv.ms/1PlfKIV.
> 
> Interestingly, I had had a similar failure in another of these PSUs, which I
> handed to a friend along with a VAX 4000-300  I gave him, in the hope that
> he would be able to fix it. As that PSU was already partially dismantled
> from my previous investigations, I asked to borrow it back for the purposes
> of doing the reverse engineering. I noticed that the same capacitor on that
> PSU also has a higher ESR, but again within range of what should be OK. So,
> looks like I might try this as a fix for the second bad PSU too, if he is OK
> with me trying that.
> 
> Regards
> 
> Rob

This Capacitor was bad in my PSU too and I've changed it.

So finally, what was wrong with my idea that you sould simply change all
reachable electrolytic capacitors in that PSU to get it working again?

In my Eyes it really isn't worth the effort of testing each one, change them
all to get that thing working halfways reliable again, it is an really bad
desing anyways...


Regards,

Holm

-- 
  Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe, 
 Freiberger Straße 42, 09600 Oberschöna, USt-Id: DE253710583
  www.tsht.de, i...@tsht.de, Fax +49 3731 74200, Mobil: 0172 8790 741

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

Good news! It looks like I have managed to get this working again. I
realised I had not checked the electrolytic capacitors on the main logic
board, ones situated far from the main power output boards. I found one
which had a higher ESR than two other identical ones, although the ESR was
still theoretically OK according to the table on my meter it was at the
limit, so I replaced it and the PSU now appears to work. It really goes to
show, *all* electrolytic capacitors should be checked. Picture showing the
one I replaced here: http://1drv.ms/1PlfKIV.

Interestingly, I had had a similar failure in another of these PSUs, which I
handed to a friend along with a VAX 4000-300  I gave him, in the hope that
he would be able to fix it. As that PSU was already partially dismantled
from my previous investigations, I asked to borrow it back for the purposes
of doing the reverse engineering. I noticed that the same capacitor on that
PSU also has a higher ESR, but again within range of what should be OK. So,
looks like I might try this as a fix for the second bad PSU too, if he is OK
with me trying that.

Regards

Rob

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Brent
> Hilpert
> Sent: 21 December 2015 22:33
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> SMD markings and packagings are ambiguous, unclear, and all over the
> spectrum.
> Here's a good site for chasing SMD markings:
>   http://www.s-manuals.com/smd
> 
> REing boards like this is an iterative process with consideration of the
device
> markings, pin use, measurements, what makes sense electronically, and
> design period.
> 
> In the two cases you mention it looks like the H's are a slightly
different font
> or size and spaced slightly differently, I suspect the id codes there are
A4 and
> 2F.
> 
> An initial guess for the "A4H" device that's right above the LM339 is an
A4-
> code dual diode with only one diode used, as it looks (from the photo)
like
> one pin is unconnected.
>   http://www.s-manuals.com/smd/a4
> 
> A good guess for the "2FH" devices are 2F-code PNP transistors:
>   http://www.s-manuals.com/smd/2f
> 
> Similarly, the "1PG" or  "1PC" device may be a 1P-code NPN transistor.
> 
> Diodes and transistors are best double-checked by first characterising
your
> multimeter with PN junctions in known transistors or diodes so you know
> what ohms-range and what reading to expect on your MM when you have a
> PN junction in forward-conduction. Look for the lowest ohms range that
will
> give you a reading for forward conduction. Then use that knowledge to
check
> the SMD transistors and diodes for sensibility.
> 
> Some strategy:
> 
>   - Sort out all the network connections for the board, by visual
> observation and continuity measurement.
> 
>   - Identify the power supply pins to the board by chasing the power
> supply pins of the IC packages.
> This will include or begin with identifying the ground
connection(s)
> to the board in part by chasing the
> ground from the main unit to the pins which this board plugs into.
> Is an internal layer on the board a ground plane?
> 
>   -  From what I can discern it may be that the two ICs are supplied
by
> +/- supplies, that is, the V- pin for the
> ICs may go to a negative supply rather than ground, so there may
be
> ground(s), V- and V+ connections
> to the board, rather than just ground and V+.
> 
>   - I'd suggest drawing the two op amps one above the other with a
> positive bus horizontally at the top of the page.
> If there is a negative supply draw a negative bus horizontally at
the
> bottom of the page.
> Draw in the networks around the op amps, it looks like they are
> similar and feed into the 3 terminal device located to the upper-right of
the
> LM339,
> and thence into one of the comparators. That should take care of a
> lot of the board, then do the remaining 3 comparators.
> 
>   - within sensibility, try to draw everything so electron flow is
'up' the
> page, this means for example that PNP transistors
>will be drawn upside down, with the emitter at top-right, while
NPNs
> will be conventional with emitter at lower-right.
>Everything should sit vertically in between the negative bus below
> and positive bus above or between grounds towards the bottom and the
> positive bus above.
> 
>   - within sensibility, try to keep signal flow left-to-right with
only
> feedback paths going 'backwards' right-to-left.

I reckon I have got about as far as I can with reverse engineering the riser
board that connects to the current sensing resistors. I am less certain now
that the fault could be in this area, but I thought that I might as well
finish the schematic since I have got so far with it. The result is here:
http://1drv.ms/1Yxy8nG.
 
The part under the "Riser Board" label is as complete as I can make it, the
part under the "Output Stage" label is just the very last part of the board
that the riser is connected to. The X3-L and X3-R connectors are the
connectors on the riser board. There is much more on the board the riser is
attached to,  mainly transformers, large resistors and capacitors, plus
another little riser board with a UC3825 PWM controller on it; it also has
tracks in a middle layer, I can see them a bit, but they are very hard to
trace.

There are likely some mistakes. For example, not having the datasheets for
what are probably transistors and dual diodes, I may have got them the wrong
way round, although I am pretty sure the pin that is on its own on one side
of the package is the common cathode for the dual diodes, and the base for
the transistors. I 

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: Robert Jarratt [mailto:robert.jarr...@ntlworld.com]
> Sent: 20 December 2015 07:04
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Subject: RE: VAX 4000-500 PSU Overload?
> 
> 
> 
> The problem is just how to physically do it. But in any case I don't think
it is
> necessary to do this now, as I have been able to reproduce the problem on
> the bench now. It looks like the 12V side starts up and then shuts down,
> sometimes after a few seconds, sometimes after a couple of minutes,
> sometimes it does not even start to output the right voltage. I am pretty
sure
> that it is falsely detecting overcurrent. The problem is going to be
finding and
> fixing the part that is not working, there are a lot of tiny surface mount
> components that seem to process the signal from the current sense
> resistor
> 


I was just reading the BA440 enclosure manual, and realised that the PSU
will shut down the system if it thinks the fan has failed. When I have the
PSU on the bench I don't have the fan connected of course. This would
explain the shut downs on the bench even when under low loads, it could be
just that it is detecting fan failure. At this point I realise I could be
barking up completely the wrong tree in suspecting faulty overcurrent
detection.

This PSU is really hard to diagnose, because it is so complex, especially
because it has a pretty complex interface to the backplane, not just power,
but also, seemingly, a fair number of control signals. I can't really tell
why it is shutting down, which makes it difficult to know where to begin
once the basics have been checked.

Regards

Rob 

Re: VAX 4000-500 PSU Overload?

[Brent Hilpert]

On 2015-Dec-21, at 3:10 AM, Robert Jarratt wrote:
>> -Original Message-
>> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Brent
>> Hilpert
>> Sent: 21 December 2015 10:12
>> To: General Discussion: On-Topic and Off-Topic Posts
>> Subject: Re: VAX 4000-500 PSU Overload?
>> 
>> The lighter-beige ones, some of which are wider, are likely caps, for
> example
>> the one beside the 34072 dual op amp looks to be a simple power decoupling
>> cap (across  the IC power pins).
>> The reddish-brown units with 4-digit numbers are resistors: 3 sig figs &
>> zeroes multiplier, e.g. 1002 = 10,000 ohms, 1330 = 133 ohms, 1181 = 1180
>> ohms.
>> The unmarked(?) 'redder' ones I expect are resistors, for example the one
>> above the "di" in "digital" looks like the feedback gain control resistor
> for the
>> op amp, although in other places that would make 4 resistors in parallel
> which
>> is unusual.
>> Resistors can generally be identified with a multimeter, if you can come
> up
>> with the same value when swapping the multimeter leads (polarity) and
>> stepping through the multimeter ohm ranges you've probably got the
>> resistance - making the multiple measurements to identify non-linear
>> responses to eliminate conductivity readings from semiconductor junctions.
>> 
>> There looks to be two very similar networks around the two op amps in the
>> 34072. It wouldn't really be that difficult a board to RE, although it
> does look
>> like some poking around with a multimeter for continuity will be necessary
> to
>> track internal traces, but they look to be relatively limited in number.
> 
> Many thanks. My first measurements with a multimeter had suggested to me
> that the darker ones were the caps. Thanks for putting me right before
> getting too far with the reverse engineering. The multi-layers mean I won't
> necessarily know where there is a connection, I can deduce that there is one
> where a through hole appears to go nowhere, but who knows what other
> connections I might miss.
> 
> Any idea what an A4H and a 2FH are? They must be some kind of transistor,
> but I am not sure what kind.

SMD markings and packagings are ambiguous, unclear, and all over the spectrum.
Here's a good site for chasing SMD markings:
http://www.s-manuals.com/smd

REing boards like this is an iterative process with consideration of the device 
markings, pin use, measurements, what makes sense electronically, and design 
period.

In the two cases you mention it looks like the H's are a slightly different 
font or size and spaced slightly differently, I suspect the id codes there are 
A4 and 2F.

An initial guess for the "A4H" device that's right above the LM339 is an 
A4-code dual diode with only one diode used, as it looks (from the photo) like 
one pin is unconnected.
http://www.s-manuals.com/smd/a4

A good guess for the "2FH" devices are 2F-code PNP transistors:
http://www.s-manuals.com/smd/2f

Similarly, the "1PG" or  "1PC" device may be a 1P-code NPN transistor.

Diodes and transistors are best double-checked by first characterising your 
multimeter with PN junctions in known transistors or diodes so you know what 
ohms-range and what reading to expect on your MM when you have a PN junction in 
forward-conduction. Look for the lowest ohms range that will give you a reading 
for forward conduction. Then use that knowledge to check the SMD transistors 
and diodes for sensibility.

Some strategy:

- Sort out all the network connections for the board, by visual 
observation and continuity measurement.

- Identify the power supply pins to the board by chasing the power 
supply pins of the IC packages.
  This will include or begin with identifying the ground connection(s) 
to the board in part by chasing the
  ground from the main unit to the pins which this board plugs into.
  Is an internal layer on the board a ground plane?

-  From what I can discern it may be that the two ICs are supplied by 
+/- supplies, that is, the V- pin for the
  ICs may go to a negative supply rather than ground, so there may be 
ground(s), V- and V+ connections
  to the board, rather than just ground and V+.

- I'd suggest drawing the two op amps one above the other with a 
positive bus horizontally at the top of the page.
  If there is a negative supply draw a negative bus horizontally at the 
bottom of the page.
  Draw in the networks around the op amps, it looks like they are 
similar and feed into the 3 terminal device located to the upper-right of the 
LM339,
  and thence into one of the comparators. Th

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Brent
> Hilpert
> Sent: 21 December 2015 10:12
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> The lighter-beige ones, some of which are wider, are likely caps, for
example
> the one beside the 34072 dual op amp looks to be a simple power decoupling
> cap (across  the IC power pins).
> The reddish-brown units with 4-digit numbers are resistors: 3 sig figs &
> zeroes multiplier, e.g. 1002 = 10,000 ohms, 1330 = 133 ohms, 1181 = 1180
> ohms.
> The unmarked(?) 'redder' ones I expect are resistors, for example the one
> above the "di" in "digital" looks like the feedback gain control resistor
for the
> op amp, although in other places that would make 4 resistors in parallel
which
> is unusual.
> Resistors can generally be identified with a multimeter, if you can come
up
> with the same value when swapping the multimeter leads (polarity) and
> stepping through the multimeter ohm ranges you've probably got the
> resistance - making the multiple measurements to identify non-linear
> responses to eliminate conductivity readings from semiconductor junctions.
> 
> There looks to be two very similar networks around the two op amps in the
> 34072. It wouldn't really be that difficult a board to RE, although it
does look
> like some poking around with a multimeter for continuity will be necessary
to
> track internal traces, but they look to be relatively limited in number.

Many thanks. My first measurements with a multimeter had suggested to me
that the darker ones were the caps. Thanks for putting me right before
getting too far with the reverse engineering. The multi-layers mean I won't
necessarily know where there is a connection, I can deduce that there is one
where a through hole appears to go nowhere, but who knows what other
connections I might miss.

Any idea what an A4H and a 2FH are? They must be some kind of transistor,
but I am not sure what kind.

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Brent Hilpert]

On 2015-Dec-21, at 1:14 AM, Robert Jarratt wrote:
>> -Original Message-
>> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Holm
> Tiffe
>> Sent: 21 December 2015 08:51
>> To: General Discussion: On-Topic and Off-Topic Posts
>> Subject: Re: VAX 4000-500 PSU Overload?
>> 
>> Robert Jarratt wrote:
>> 
>> Just resolder the SMD Caps, they sometimes get a loose pad.
>> You see if that happened while resoldering.
>> 
> 
> Shouldn't I be able to detect a loose component by pushing on it with a
> small screwdriver or something? I suspect the caps are the darker coloured
> (orange) ones, but I am not sure, is that right?

The lighter-beige ones, some of which are wider, are likely caps, for example 
the one beside the 34072 dual op amp looks to be a simple power decoupling cap 
(across  the IC power pins).
The reddish-brown units with 4-digit numbers are resistors: 3 sig figs & zeroes 
multiplier, e.g. 1002 = 10,000 ohms, 1330 = 133 ohms, 1181 = 1180 ohms.
The unmarked(?) 'redder' ones I expect are resistors, for example the one above 
the "di" in "digital" looks like the feedback gain control resistor for the op 
amp, although in other places that would make 4 resistors in parallel which is 
unusual. 
Resistors can generally be identified with a multimeter, if you can come up 
with the same value when swapping the multimeter leads (polarity) and stepping 
through the multimeter ohm ranges you've probably got the resistance - making 
the multiple measurements to identify non-linear responses to eliminate 
conductivity readings from semiconductor junctions.

There looks to be two very similar networks around the two op amps in the 
34072. It wouldn't really be that difficult a board to RE, although it does 
look like some poking around with a multimeter for continuity will be necessary 
to track internal traces, but they look to be relatively limited in number.

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Holm
Tiffe
> Sent: 21 December 2015 08:51
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> Robert Jarratt wrote:
> 
> >
> >
> 
> Just resolder the SMD Caps, they sometimes get a loose pad.
> You see if that happened while resoldering.
> 

Shouldn't I be able to detect a loose component by pushing on it with a
small screwdriver or something? I suspect the caps are the darker coloured
(orange) ones, but I am not sure, is that right?

Thanks

Rob

Re: VAX 4000-500 PSU Overload?

[Holm Tiffe]

Robert Jarratt wrote:

> 
> 
> > -Original Message-
> > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Paul
> > Koning
> > Sent: 20 December 2015 22:32
> > To: General Discussion: On-Topic and Off-Topic Posts
> > Subject: Re: VAX 4000-500 PSU Overload?
> > 
> > 
> > > On Dec 19, 2015, at 4:05 PM, Robert Jarratt
> 
> > wrote:
> > >
> > > ...
> > > I can't tell if this is a Kelvin connected one or not, although I
> > > suspect it probably is. You can see a picture with two here:
> > http://1drv.ms/1micVN7.
> > > These are from a second board in the PSU, but they look identical to
> > > the one on the problem board. It would appear to have 4 connections,
> > > but doesn't look anything like the only Kelvin connected one I could
> find.
> > 
> > It does look like a Kelvin hookup.  Yes, usually they are two fat and two
> > skinny connections, but so long as two connections carry the current while
> > the other two do not, it's valid.
> > 
> 
> Thanks Paul, I have been assuming that it is Kelvin connected for the time
> being.
> 
> I have no idea if the resistor is faulty or if the fault is in the detection
> logic. I am not entirely sure how to test it, and if it is faulty then
> finding a replacement may be hard. If it is the detection logic, then I am
> really in trouble because it is all SMD stuff, the board it is on appears to
> have multiple layers and I can barely make out the markings on the
> components, so I can't identify their type (resistor or capacitor) or value,
> and I have never desoldered/resoldered SMD before. There is an op amp and a
> comparator, which are easily identifiable, but there also appear to be some
> transistors, and although I can see the markings "A4H" I have been unable to
> identify what type they are, what the pinout is or if they are even
> transistors at all (but I think they must be). Pictures of the logic board
> here: http://1drv.ms/1QT7641.
> 
> I also noticed another very low value resistor, 0.01R, which looks far more
> conventional (not kelvin connected for sure, it looks just like any ordinary
> resistor, only a bit chunkier), but sadly, it seems to measure fine using my
> ESR/Low Ohms meter, when measured in circuit.
> 
> Regards
> 
> Rob

Just resolder the SMD Caps, they sometimes get a loose pad.
You see if that happened while resoldering.

Regards,

Holm

-- 
  Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe, 
 Freiberger Straße 42, 09600 Oberschöna, USt-Id: DE253710583
  www.tsht.de, i...@tsht.de, Fax +49 3731 74200, Mobil: 0172 8790 741

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Paul
> Koning
> Sent: 20 December 2015 22:32
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> 
> > On Dec 19, 2015, at 4:05 PM, Robert Jarratt

> wrote:
> >
> > ...
> > I can't tell if this is a Kelvin connected one or not, although I
> > suspect it probably is. You can see a picture with two here:
> http://1drv.ms/1micVN7.
> > These are from a second board in the PSU, but they look identical to
> > the one on the problem board. It would appear to have 4 connections,
> > but doesn't look anything like the only Kelvin connected one I could
find.
> 
> It does look like a Kelvin hookup.  Yes, usually they are two fat and two
> skinny connections, but so long as two connections carry the current while
> the other two do not, it's valid.
> 

Thanks Paul, I have been assuming that it is Kelvin connected for the time
being.

I have no idea if the resistor is faulty or if the fault is in the detection
logic. I am not entirely sure how to test it, and if it is faulty then
finding a replacement may be hard. If it is the detection logic, then I am
really in trouble because it is all SMD stuff, the board it is on appears to
have multiple layers and I can barely make out the markings on the
components, so I can't identify their type (resistor or capacitor) or value,
and I have never desoldered/resoldered SMD before. There is an op amp and a
comparator, which are easily identifiable, but there also appear to be some
transistors, and although I can see the markings "A4H" I have been unable to
identify what type they are, what the pinout is or if they are even
transistors at all (but I think they must be). Pictures of the logic board
here: http://1drv.ms/1QT7641.

I also noticed another very low value resistor, 0.01R, which looks far more
conventional (not kelvin connected for sure, it looks just like any ordinary
resistor, only a bit chunkier), but sadly, it seems to measure fine using my
ESR/Low Ohms meter, when measured in circuit.

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Paul Koning]

> On Dec 19, 2015, at 4:05 PM, Robert Jarratt  
> wrote:
> 
> ...
> I can't tell if this is a Kelvin connected one or not, although I suspect it
> probably is. You can see a picture with two here: http://1drv.ms/1micVN7.
> These are from a second board in the PSU, but they look identical to the one
> on the problem board. It would appear to have 4 connections, but doesn't
> look anything like the only Kelvin connected one I could find.

It does look like a Kelvin hookup.  Yes, usually they are two fat and two 
skinny connections, but so long as two connections carry the current while the 
other two do not, it's valid.  

paul

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Mouse
> Sent: 20 December 2015 01:46
> To: cctalk@classiccmp.org
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> >>> But getting something in series to measure load is going to be
> >>> really awkward, [...]
> >> As in, you don't have any such thing to put in series, or you don't
> >> see a good way to get it in series?  [...]
> > I think the suggestion was to put an ammeter between the PSU and the
> > backplane.
> 
> Yes...but ammeters (for nontrivial currents) are generally just voltmeters
> across current-sense resistors, and it's the current-sense resistor that
needs
> to carry the current and thus requires attention.
> 

The problem is just how to physically do it. But in any case I don't think
it is necessary to do this now, as I have been able to reproduce the problem
on the bench now. It looks like the 12V side starts up and then shuts down,
sometimes after a few seconds, sometimes after a couple of minutes,
sometimes it does not even start to output the right voltage. I am pretty
sure that it is falsely detecting overcurrent. The problem is going to be
finding and fixing the part that is not working, there are a lot of tiny
surface mount components that seem to process the signal from the current
sense resistor

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Mouse]

>>> But getting something in series to measure load is going to be
>>> really awkward, [...]
>> As in, you don't have any such thing to put in series, or you don't
>> see a good way to get it in series?  [...]
> I think the suggestion was to put an ammeter between the PSU and the
> backplane.

Yes...but ammeters (for nontrivial currents) are generally just
voltmeters across current-sense resistors, and it's the current-sense
resistor that needs to carry the current and thus requires attention.

/~\ The ASCII Mouse
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RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Dave
> Wade
> Sent: 19 December 2015 20:44
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Subject: RE: VAX 4000-500 PSU Overload?
> 
> 
> 
> 
> I wonder if the voltage level detection thinks its out of spec, rather
than the
> over current parts. These PSUs afre exceedingly complex...
> 

Yes, that is my suspicion, but, as I said in another reply, it looks like
this involves tiny surface mount stuff. No idea how to test it, and reverse
engineering the schematic is going to be tough, especially as I can't make
out the markings or identify the device types, so even being able to measure
the components is not going to be easy.

Regards

Rob

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Paul
> Koning
> Sent: 19 December 2015 19:26
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> 
> Something to watch out for with high current sense resistors: they often
> have four connections rather than two.  This is a "Kelvin connection": two
> connections are for the current to be sensed, and the other two pick up
the
> resulting voltage difference.  The idea is that you get the voltage drop
across
> the resistor itself, as opposed to the voltage drop across the resistor
PLUS
> the solder joints.  That makes a difference if you have a sense resistor
of just
> a few milliohms.
> 
> If you have a high current supply but it doesn't use Kelvin connected
current
> sense, you're vulnerable to false shutdown if the solder joints are less
than
> perfect.  If so, switching to the Kelvin hookup, if you can see how, is a
good
> answer; failing that, make sure the joints are in excellent condition.
> 

I can't tell if this is a Kelvin connected one or not, although I suspect it
probably is. You can see a picture with two here: http://1drv.ms/1micVN7.
These are from a second board in the PSU, but they look identical to the one
on the problem board. It would appear to have 4 connections, but doesn't
look anything like the only Kelvin connected one I could find.

I tried testing it with my ESR/Low Ohms Meter, but the nominal value appears
to be 0.003R, which is below the range of my meter, the meter shows a value
between 0.00 and 0.02, but mostly 0.01. This is with the resistor in
circuit. It would be quite difficult to remove it. But I suspect that it may
be more the sense circuitry around the resistor than the resistor itself,
since the value seems to be OK. If that is the case then I may be in
trouble, the sense circuitry seems to be on a little riser board with loads
of surface mount components, including an LM339 comparator.

Regards

Rob

RE: VAX 4000-500 PSU Overload?

[Dave Wade]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Robert
> Jarratt
> Sent: 19 December 2015 19:19
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> 
> Subject: RE: VAX 4000-500 PSU Overload?
> 
> 
> 
> > -Original Message-
> > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Dave
> > Wade
> > Sent: 19 December 2015 18:32
> > To: 'General Discussion: On-Topic and Off-Topic Posts'
> > Subject: RE: VAX 4000-500 PSU Overload?
> >
> >
> >
> >
> > Seeing as it already seems to have over current sensing, I guess it
> > has
> some
> > small resistors in there so perhaps it is possible to measure the
current.
> I
> > notices one looks like this:-
> >
> > http://uk.farnell.com/welwyn/oar1-r020fi/resistor-1-0r020/dp/1200359
> >
> 
> 
> Ah! So that is what those things are! I definitely think this one is
detecting
> something wrong and shutting down the 12V side. I have just been doing
> some tests with a dummy load and the 12V side will come up and then go
> down a few moments later, sometimes it will stay up, it seems a bit
random.

I wonder if the voltage level detection thinks its out of spec, rather than
the over current parts. These PSUs afre exceedingly complex...

> Presumably then, there is something wrong with the overcurrent detection.
> 
> I will have to look into how you test these, or perhaps just replace them
and
> see what happens.
> 

I can't see the resistors themselves being the problem. Perhaps re-soldering
them 


> Regards
> 
> Rob

Dave
G4UGM

Re: VAX 4000-500 PSU Overload?

[Paul Koning]

> On Dec 19, 2015, at 2:18 PM, Robert Jarratt  
> wrote:
> 
>> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Dave
>> Wade
>> 
>> Seeing as it already seems to have over current sensing, I guess it has some
>> small resistors in there so perhaps it is possible to measure the current. I
>> notices one looks like this:-
>> 
>> http://uk.farnell.com/welwyn/oar1-r020fi/resistor-1-0r020/dp/1200359
>> 
> 
> 
> Ah! So that is what those things are! I definitely think this one is
> detecting something wrong and shutting down the 12V side. 

Something to watch out for with high current sense resistors: they often have 
four connections rather than two.  This is a "Kelvin connection": two 
connections are for the current to be sensed, and the other two pick up the 
resulting voltage difference.  The idea is that you get the voltage drop across 
the resistor itself, as opposed to the voltage drop across the resistor PLUS 
the solder joints.  That makes a difference if you have a sense resistor of 
just a few milliohms.

If you have a high current supply but it doesn't use Kelvin connected current 
sense, you're vulnerable to false shutdown if the solder joints are less than 
perfect.  If so, switching to the Kelvin hookup, if you can see how, is a good 
answer; failing that, make sure the joints are in excellent condition.

paul

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Dave
> Wade
> Sent: 19 December 2015 18:32
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Subject: RE: VAX 4000-500 PSU Overload?
> 
> 
> 
> 
> Seeing as it already seems to have over current sensing, I guess it has
some
> small resistors in there so perhaps it is possible to measure the current.
I
> notices one looks like this:-
> 
> http://uk.farnell.com/welwyn/oar1-r020fi/resistor-1-0r020/dp/1200359
> 


Ah! So that is what those things are! I definitely think this one is
detecting something wrong and shutting down the 12V side. I have just been
doing some tests with a dummy load and the 12V side will come up and then go
down a few moments later, sometimes it will stay up, it seems a bit random.
Presumably then, there is something wrong with the overcurrent detection.

I will have to look into how you test these, or perhaps just replace them
and see what happens.

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Jon Elson]

On 12/19/2015 04:56 AM, Robert Jarratt wrote:


Actually, I may have just narrowed it down a bit. I connected a 12V DC bulb
to the -12 output, in parallel to a 15R resistor, and the PSU shuts down
immediately. The 12V bulb has a 21W rating. If my calculations are correct,
that means the bulb draws 1.75A. The 15R resistor would draw 0.8A. So a
total of 2.5A, which is well under the spec of 4A, but causes the PSU to
shutdown. Using the 5W filament of the bulb does not cause the PSU to shut
down.

Is there a flaw in my calculations, or am I really not overloading the -12V
supply? Just to recap I had a 15R resistor and a 12V DC 21W bulb in
parallel, on the -12V output.


Cold light bulbs draw WAY more current than when the 
filament is hot.  Not sure if a 21 W 12V bulb could draw 20 
A cold, but it might.


Jon

RE: VAX 4000-500 PSU Overload?

[Dave Wade]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Robert
> Jarratt
> Sent: 19 December 2015 17:20
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> 
> Subject: RE: VAX 4000-500 PSU Overload?
> 
> 
> 
> > -Original Message-
> > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Mouse
> > Sent: 19 December 2015 16:09
> > To: cctalk@classiccmp.org
> > Subject: Re: VAX 4000-500 PSU Overload?
> >
> > > But getting something in series to measure load is going to be
> > > really awkward, [...]
> >
> > As in, you don't have any such thing to put in series, or you don't
> > see a
> good
> > way to get it in series?  I've got a couple of 0R1 resistors which
> > work
> nicely for
> > that in conjunction with a voltmeter that can handle millivolt
> measurements;
> > you might be able to get something similar.  Mine are 10W, and, while
> > they are designed for bolting to a heatsink, they can probably handle
> > at least
> 2 or 3
> > watts, which means some 5-6 amps, in free air.  (Though that then
> > drops about half a volt, which can be a problem when you can't put it
> > on the
> input
> > side of the
> > regulation.)
> >
> 
> I think the suggestion was to put an ammeter between the PSU and the
> backplane. That would be awkward to make all the connections with the PSU
> out of the machine. Also difficult is to make a dummy load for a bench
test
> that draws enough current to test that it can sustain its rated output at
full
> load (or some decent proportion of full load at any rate).
> 
> Regards
> 
> Rob

Rob,

Seeing as it already seems to have over current sensing, I guess it has some
small resistors in there so perhaps it is possible to measure the current. I
notices one looks like this:-

http://uk.farnell.com/welwyn/oar1-r020fi/resistor-1-0r020/dp/1200359

Dave

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Mouse
> Sent: 19 December 2015 16:09
> To: cctalk@classiccmp.org
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> > But getting something in series to measure load is going to be really
> > awkward, [...]
> 
> As in, you don't have any such thing to put in series, or you don't see a
good
> way to get it in series?  I've got a couple of 0R1 resistors which work
nicely for
> that in conjunction with a voltmeter that can handle millivolt
measurements;
> you might be able to get something similar.  Mine are 10W, and, while they
> are designed for bolting to a heatsink, they can probably handle at least
2 or 3
> watts, which means some 5-6 amps, in free air.  (Though that then drops
> about half a volt, which can be a problem when you can't put it on the
input
> side of the
> regulation.)
> 

I think the suggestion was to put an ammeter between the PSU and the
backplane. That would be awkward to make all the connections with the PSU
out of the machine. Also difficult is to make a dummy load for a bench test
that draws enough current to test that it can sustain its rated output at
full load (or some decent proportion of full load at any rate).

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Mouse]

>> Bulbs have a lower than normal resistance when cold [...]
>> Three or four 15 ohm resistors in parallel would probably make a
>> better load for the -12V line.
> Unfortunately I don't have enough of these to hand, and the local
> shop (Maplin) does not have enough in stock.

:-(

> But getting something in series to measure load is going to be really
> awkward, [...]

As in, you don't have any such thing to put in series, or you don't see
a good way to get it in series?  I've got a couple of 0R1 resistors
which work nicely for that in conjunction with a voltmeter that can
handle millivolt measurements; you might be able to get something
similar.  Mine are 10W, and, while they are designed for bolting to a
heatsink, they can probably handle at least 2 or 3 watts, which means
some 5-6 amps, in free air.  (Though that then drops about half a volt,
which can be a problem when you can't put it on the input side of the
regulation.)

/~\ The ASCII Mouse
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RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

 
> Bulbs have a lower than normal resistance when cold so it is possible that
you
> may be drawing more than 4A from the -12V line at the instant that you
apply
> power.
> 
> Bulbs are great for supplying a minimum load for a power supply that won't
> regulate without it but they are not so great for supplying a load near
the
> maximum.  In the latter case, the maximum load will be exceeded for a
short
> time at startup.  This is not a problem if the power supply is designed to
cope
> with this (a power supply for motors for example) but computer power
> supplies will typically be designed to react quickly to overload
conditions.
> 
> Three or four 15 ohm resistors in parallel would probably make a better
load
> for the -12V line.

Unfortunately I don't have enough of these to hand, and the local shop
(Maplin) does not have enough in stock. 

> 
> It seems hard to imagine what could normally draw something approaching
> 4A on the -12V line in a VAX 4000 though.  If it was possible to get an
> ammeter in series with that line maybe you could get an idea of what the
> normal draw is on that line, if the power supply stays running long
enough.
> This would give a better idea of what sort of dummy load is needed to
> simulate it.
> 

I checked the resistance of the backplane across the -12V inputs and it is
very high, so it seems unlikely that it could be that. But getting something
in series to measure load is going to be really awkward, I will see if I can
do it somehow. What would really help would be a way to bench test all the
outputs at high-ish loads to see if it is the PSU or not, but I don't know
of a practical way to do this with the high currents required.

Regards

Rob

RE: VAX 4000-500 PSU Overload?

[Peter Coghlan]

>
> Actually, I may have just narrowed it down a bit. I connected a 12V DC bulb
> to the -12 output, in parallel to a 15R resistor, and the PSU shuts down
> immediately. The 12V bulb has a 21W rating. If my calculations are correct,
> that means the bulb draws 1.75A. The 15R resistor would draw 0.8A. So a
> total of 2.5A, which is well under the spec of 4A, but causes the PSU to
> shutdown. Using the 5W filament of the bulb does not cause the PSU to shut
> down.
>
> Is there a flaw in my calculations, or am I really not overloading the -12V
> supply? Just to recap I had a 15R resistor and a 12V DC 21W bulb in
> parallel, on the -12V output.
>

Bulbs have a lower than normal resistance when cold so it is possible that
you may be drawing more than 4A from the -12V line at the instant that you
apply power.

Bulbs are great for supplying a minimum load for a power supply that won't
regulate without it but they are not so great for supplying a load near the
maximum.  In the latter case, the maximum load will be exceeded for a short
time at startup.  This is not a problem if the power supply is designed to
cope with this (a power supply for motors for example) but computer power
supplies will typically be designed to react quickly to overload conditions.

Three or four 15 ohm resistors in parallel would probably make a better
load for the -12V line.

It seems hard to imagine what could normally draw something approaching 4A
on the -12V line in a VAX 4000 though.  If it was possible to get an ammeter
in series with that line maybe you could get an idea of what the normal draw
is on that line, if the power supply stays running long enough.  This would
give a better idea of what sort of dummy load is needed to simulate it.

Regards,
Peter Coghlan.

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: Robert Jarratt [mailto:robert.jarr...@ntlworld.com]
> Sent: 19 December 2015 09:56
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Cc: 'cct...@classiccmp.org'
> Subject: RE: VAX 4000-500 PSU Overload?
> 
> A bench test of the PSU with a small dummy load saw the PSU appear to
> work fine. It might help to load it more to see if it still operates
correctly
> under load. The specs for the PSU are:
> 
> 
> 22 amperes at +12.1 Vdc
> 60 amperes at +5 Vdc
> 7 amperes at +3.4 Vdc
> 4 amperes at -12.1 Vdc
> 
> Has anyone got any suggestions for how to create a higher load in a fairly
> simple manner? It would take a lot of resistors or vehicle light bulbs to
create
> some of those loads, and something heftier than a breadboard to connect
> them together.
> 
> Thanks
> 

Actually, I may have just narrowed it down a bit. I connected a 12V DC bulb
to the -12 output, in parallel to a 15R resistor, and the PSU shuts down
immediately. The 12V bulb has a 21W rating. If my calculations are correct,
that means the bulb draws 1.75A. The 15R resistor would draw 0.8A. So a
total of 2.5A, which is well under the spec of 4A, but causes the PSU to
shutdown. Using the 5W filament of the bulb does not cause the PSU to shut
down.

Is there a flaw in my calculations, or am I really not overloading the -12V
supply? Just to recap I had a 15R resistor and a 12V DC 21W bulb in
parallel, on the -12V output.

Looks like I have an avenue to explore.

Thanks

Rob

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

A bench test of the PSU with a small dummy load saw the PSU appear to work
fine. It might help to load it more to see if it still operates correctly
under load. The specs for the PSU are:


22 amperes at +12.1 Vdc
60 amperes at +5 Vdc
7 amperes at +3.4 Vdc
4 amperes at -12.1 Vdc

Has anyone got any suggestions for how to create a higher load in a fairly
simple manner? It would take a lot of resistors or vehicle light bulbs to
create some of those loads, and something heftier than a breadboard to
connect them together.

Thanks

Rob

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Maciej W.
> Rozycki
> Sent: 17 December 2015 16:18
> To: cctalk@classiccmp.org
> Cc: cct...@classiccmp.org
> Subject: RE: VAX 4000-500 PSU Overload?
> 
>  NB the two brown capacitors seen in the middle top of your photo are
> among ones that leaked in my PSU; an SXF marking is barely recognisable on
> the edge of the lower one in your photo.  After considerable effort
earlier
> this year I succeeded with removing the intermediate heatsink blocking
> access to these caps, by desoldering the four rectifier dual-diodes
holding
> the heatsink in place -- at the cost of losing a leg from one of the
smaller ones
> (a Motorola MBR3045PT) and some damage to the PCB.

These caps are on the 5V/3.3 board, and after close inspection, do not
appear to have leaked. I have been unable to reach them with the probes from
my ESR meter either. Given that the leaked ones were on the 12V board I had
just replaced those, and cleaned up the leaked electrolyte and then tested
the PSU again. It did work briefly, with no ripple on 5V, but I did not have
access to measure 3.3V ripple. I have had suggestions to just pull these out
from above and solder them in from above, rather than go through the
horrible procedure you describe. I am going to use a dummy load today to
test the PSU on the bench to see if it shuts down outside the machine. The
problem will be to know which of the outputs is causing the shutdown. I
wonder if it is possible to test the PSU with one output board removed, so
that I can identify which board is the bad one? I know I already asked this,
but if anyone happens to know that would be great!

> 
>  Fortunately MBR3045PT parts are still available and I was able to get a
> replacement, and the damage to the PCB is I believe not critical.  What's
> important the larger rectifier dual-diodes (84CNQ045; no clear indication
of
> the manufacturer) have survived intact, as these seem to have become
> unobtainium now.  Apparently the last die foundry capable of making these
> parts has discontinued them earlier this year due to lack of customer
> interest, so the only source remaining might be part recovery from
otherwise
> broken equipment.
> 
>  Overall I think I'll need better tools to be able to desolder such stuff
in a
> more repeatable and less destructive way.  The thick legs of the
> 84CNQ045 parts combined with the large volume and consequently thermal
> capacity of the intermediate heatsink seem to be able to take heat away
> virtually instantaneously.  I'll appreciate your advice on choosing a good
> soldering/desoldering station, capable of handling such high-current (and
> consequently highly heat-conducting) parts.

Well, when I tried to do this on a previous H7874 I had, a friend of mine
(on this list) helped me out with this on the 12V board. We used one of
these:
http://cpc.farnell.com/duratool/d00672/digital-desoldering-station-80w/dp/SD
01384?MER=e-bb45-1001 without causing damage. I now have one and it
works OK, though be careful if, like me, you use some pressure to bend pins
upright while trying to desolder, the barrel is not actually that strong!

> 
>  For the curious and possibly to provide some information on the parts
> involved I've documented progress with the disassembly of this module at:
> <ftp://ftp.linux-mips.org/pub/linux/mips/people/macro/private/h7874/>
> (large photos!).  Removing the offending caps revealed C322 and C323
> designation underneath; these are 330µF/25V parts.  I decided not to move
> forward with installing replacements and reassembling the module, or
> proceeding with disassembling the other module without upgrading my
> soldering/desoldering tools first though.
> 


Those pics show the full horror of this PSU and getting access to the parts!
Not for the faint of heart :-)


>  No idea.  I've only recently started finding my way with this system; I'm
> mostly a software person.  Admittedly quite a low-level one, but still on
the
> software side.  And it's been only earlier this year I discovered the
issue with
> the dreaded SXF caps, even though I've been using various DEC
> hardware/PSUs for over 15 years now.
> 


Yes, I am similar, mainly a software person really, did a bit of electronics
at University, but not much at all. Rapidly having to learn more about PSUs!

Regards

Rob

RE: VAX 4000-500 PSU Overload?

[Maciej W. Rozycki]

On Sun, 13 Dec 2015, Robert Jarratt wrote:

> >  No leaking ChemiCon SXF-series capacitors inside?  They seem to suffer
> > from some kind of design or manufacturing defect and consequently the
> > electrolyte gets through the seal eventually even in parts never used.  A
> > number of DEC PSUs have these capacitors, including the H7874 PSU which
> > BA430/BA440 VAX 4000 cabinets use.
> 
> 
> Just opened it up and there are indeed leaking caps, the brown ones. There
> is also a strange deposit on a couple of screws (picture here:
> http://1drv.ms/1micVN7), but the screws are nowhere near the leaking
> capacitors. Any thoughts on that?

 Obviously zinc plating of the screw has come in reaction with something.

 It doesn't seem from the photo electrolyte could have reached there, 
however I'd double check that, in particular inspect the surface of the 
PCB throughout looking for any difference in appearance.  This stuff can 
reach far, e.g. in one of my failed H7821 PSUs which was unfortunately 
stored at an angle to its proper operational position (or otherwise the 
upside down position of problematic capacitors have prevented the seal 
from failing), electrolyte travelled at least 15cm/6" away from the failed 
capacitor.  And this screw is located in a bottom corner of the PSU when 
in its operational position so gravity will drive leaked electrolyte 
towards it even if capillary action does not.  Other than that I have no 
idea.

 Lead or tin from the soldering alloy aren't as reactive as zinc is and 
therefore they don't show signs of corrosion so quickly as zinc does, 
although as I observed in the failed PSUs I have they eventually do too.  
As does copper from any exposed traces and can the solder mask or the 
epoxy substrate of the PCB itself too.

> >  Unfortunately replacing the parts is not easy as heatsinks block access
> to the
> > soldering pads and you need to desolder main rectifiers first to gain
> access.
> > You need to clean any electrolyte spills too as they will cause corrosion
> and
> > shorts.
> 
> Yes, I have had one of these PSUs apart before and know how horrible it is
> to get at these parts. I will however go ahead and replace these caps.

 Indeed, and good luck!

 NB the two brown capacitors seen in the middle top of your photo are 
among ones that leaked in my PSU; an SXF marking is barely recognisable on 
the edge of the lower one in your photo.  After considerable effort 
earlier this year I succeeded with removing the intermediate heatsink 
blocking access to these caps, by desoldering the four rectifier 
dual-diodes holding the heatsink in place -- at the cost of losing a leg 
from one of the smaller ones (a Motorola MBR3045PT) and some damage to the 
PCB.

 Fortunately MBR3045PT parts are still available and I was able to get a 
replacement, and the damage to the PCB is I believe not critical.  What's 
important the larger rectifier dual-diodes (84CNQ045; no clear indication 
of the manufacturer) have survived intact, as these seem to have become 
unobtainium now.  Apparently the last die foundry capable of making these 
parts has discontinued them earlier this year due to lack of customer 
interest, so the only source remaining might be part recovery from 
otherwise broken equipment.

 Overall I think I'll need better tools to be able to desolder such stuff 
in a more repeatable and less destructive way.  The thick legs of the 
84CNQ045 parts combined with the large volume and consequently thermal 
capacity of the intermediate heatsink seem to be able to take heat away 
virtually instantaneously.  I'll appreciate your advice on choosing a good 
soldering/desoldering station, capable of handling such high-current (and
consequently highly heat-conducting) parts.

 For the curious and possibly to provide some information on the parts 
involved I've documented progress with the disassembly of this module at: 
 
(large photos!).  Removing the offending caps revealed C322 and C323 
designation underneath; these are 330µF/25V parts.  I decided not to move 
forward with installing replacements and reassembling the module, or 
proceeding with disassembling the other module without upgrading my 
soldering/desoldering tools first though.

> >  I observed similar symptoms with the failed PSU as you do: it started up
> > briefly, enough for diagnostic output to start coming from the system
> > through the console port, and then within a couple of seconds the PSU shut
> > down.
> 
> I don't have an easy way to apply test loads to all the outputs at the same
> time. Is it possible to remove the final output boards for each "half" of
> the output and test one at a time without causing the PSU to shut down?

 No idea.  I've only recently started finding my way with this system; I'm 
mostly a software person.  Admittedly quite a low-level one, but still on 
the software side.  And it's been only earlier this year I discove

Re: VAX 4000-500 PSU Overload?

[Holm Tiffe]

Robert Jarratt wrote:

> 
> 
> > -Original Message-
> > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Holm
> Tiffe
> > Sent: 16 December 2015 17:27
> > To: General Discussion: On-Topic and Off-Topic Posts
> > Subject: Re: VAX 4000-500 PSU Overload?
> > 
> > Robert Jarratt wrote:
> > 
> > ...sounds very similar to my experience with that PSU. The cure seems to
> be
> > simple run that machine then mine has stopped powering off now it seems.
> > 
> > I don't even know if the power off is because of an detected overload at
> all, I
> > had problems to even power on the machine the first time before changing
> > the caps. It would start only the approx 5 time if I switched it on and I
> think
> > that those problems are related.
> > 
> 
> I intend to do some more analysis and testing before putting the PSU back in
> the machine. I just tried using a current-limited bench PSU on the outputs
> of the PSU to see if one of them behaves oddly. I suspect the -12V output is
> not right, it sucked up more than 1A with just 0.3V across it. The other
> outputs sucked up much less current and increased slowly.

..sounds at least interresting. Maybe you have a bad Tantal Capacitor
somewhere on the boards..
Try to pull boards until the current decreases...

> 
> Of course it could have been a totally invalid test, but I know in some
> circumstances it can help to see if the output stage and the crowbar is
> working. 

Whatever you want..  I'm curious what you will find out.
> 
> > I've installed VMS 7.3 and some additional packages and played for some
> > days with that machine and it is running stable now if I power it on.
> > The only thing that I still have to complain about is that the NiCad Pack
> is
> > empty again a little to fast for my taste, the Battery was a new one..
> > 
> 
> Some people suggest removing those batteries altogether, to avoid damaging
> leaks.
> 
> Regards
> 
> Rob
> 

The machine is build to contain those NC Packs, and yes I know that gases
from the Cells will corrode the electronics around it.
But the machine had some years running time on it and the corrosion effects
are pretty small, so I think with the fitted new battery it will last some
additional and I could handle this.


Regards,

Holm

-- 
  Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe, 
 Freiberger Straße 42, 09600 Oberschöna, USt-Id: DE253710583
  www.tsht.de, i...@tsht.de, Fax +49 3731 74200, Mobil: 0172 8790 741

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Holm
Tiffe
> Sent: 16 December 2015 17:27
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> Robert Jarratt wrote:
> 
> ...sounds very similar to my experience with that PSU. The cure seems to
be
> simple run that machine then mine has stopped powering off now it seems.
> 
> I don't even know if the power off is because of an detected overload at
all, I
> had problems to even power on the machine the first time before changing
> the caps. It would start only the approx 5 time if I switched it on and I
think
> that those problems are related.
> 

I intend to do some more analysis and testing before putting the PSU back in
the machine. I just tried using a current-limited bench PSU on the outputs
of the PSU to see if one of them behaves oddly. I suspect the -12V output is
not right, it sucked up more than 1A with just 0.3V across it. The other
outputs sucked up much less current and increased slowly.

Of course it could have been a totally invalid test, but I know in some
circumstances it can help to see if the output stage and the crowbar is
working. 

> I've installed VMS 7.3 and some additional packages and played for some
> days with that machine and it is running stable now if I power it on.
> The only thing that I still have to complain about is that the NiCad Pack
is
> empty again a little to fast for my taste, the Battery was a new one..
> 

Some people suggest removing those batteries altogether, to avoid damaging
leaks.

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Holm Tiffe]

Robert Jarratt wrote:

> > Tonight I replaced the leaking capacitors on the +12V/-12V board, and also
> > replaced the two similar ones that looked and measured fine. I put the PSU
> > back together, put in a couple of less important boards and drives, and
> the
> > machine now seems to power on OK. I measured the ripple using the power
> > connector for the front panel and that looks OK too. Unfortunately though,
> > that connector only sends out +12V, +5V and -12V. It does not have output
> > for +3.3V, so I have not, so far, been able to check the ripple for this.
> It is a bit
> > awkward to test on the bench with a dummy load.
> > 
> > As I did not replace the capacitors on the +5V/3.3V board, because the
> > underside is very hard to access, and I am reluctant to pull them off and
> > solder from above, I would like to be sure there is no ripple on the 3.3V
> > supply. So, I was wondering if anyone has any neat tricks for probing the
> > +3.3V supply with the PSU installed in the machine?
> > 
> > Next job is to replace the capacitors on the little DSSI terminator, they
> are
> > easy to change.
> > 
> 
> 
> I spoke too soon :-(
> 
> The machine stayed on for a couple of minutes and then powered off. I
> suspect there is a problem with it detecting an overload that may not
> actually be there. Looks like I will need to get a dummy load and put it on
> the bench to see if it still happens there.
> 
> I do recall that when it first failed it would stay on for shorter and
> shorter periods each time I tried it. So perhaps there is some component
> warming up and then causing a failure?
> 
> Regards
> 
> Rob


...sounds very similar to my experience with that PSU. The cure seems to be
simple run that machine then mine has stopped powering off now it seems.

I don't even know if the power off is because of an detected overload at
all, I had problems to even power on the machine the first time before
changing the caps. It would start only the approx 5 time if I switched it
on and I think that those problems are related.

I've installed VMS 7.3 and some additional packages and played for some
days with that machine and it is running stable now if I power it on.
The only thing that I still have to complain about is that the NiCad Pack 
is empty again a little to fast for my taste, the Battery was a new one..


Regards,

Holm

-- 
  Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe, 
 Freiberger Straße 42, 09600 Oberschöna, USt-Id: DE253710583
  www.tsht.de, i...@tsht.de, Fax +49 3731 74200, Mobil: 0172 8790 741

Re: VAX 4000-500 PSU Overload?

[Glen Slick]

On Tue, Dec 15, 2015 at 12:38 PM, Robert Jarratt
 wrote:
>
> Tonight I replaced the leaking capacitors on the +12V/-12V board, and also
> replaced the two similar ones that looked and measured fine. I put the PSU
> back together, put in a couple of less important boards and drives, and the
> machine now seems to power on OK. I measured the ripple using the power
> connector for the front panel and that looks OK too. Unfortunately though,
> that connector only sends out +12V, +5V and -12V. It does not have output
> for +3.3V, so I have not, so far, been able to check the ripple for this. It
> is a bit awkward to test on the bench with a dummy load.
>

Nice work and good outcome. Gives me some hope I might be able to get
the H7874 supply from my BA430 VAX 4000-200 repaired and running
again.


> As I did not replace the capacitors on the +5V/3.3V board, because the
> underside is very hard to access, and I am reluctant to pull them off and
> solder from above, I would like to be sure there is no ripple on the 3.3V
> supply. So, I was wondering if anyone has any neat tricks for probing the
> +3.3V supply with the PSU installed in the machine?

On the BA440 pedestal chassis lift off the two front door panels and
then remove the 5 screws on the left side and the 6 screws on the
right side that hold the front door mounting brackets in place and
then you can lift off the outer skin panels in one piece. After you
remove the outer skin there are 18 screws (if I counted correctly)
that hold the rear metal skin over the backplane area. After you
remove that you'll have full access to probe the square pads on the
backplane next to the power supply connectors. The pads are not
labeled on the back so make a note of the voltages when looking at
them in from the front when the power supply is removed.

It might sound like a lot of work but it's really not bad, assuming
the BA440 chassis isn't buried under stuff where you can only get at
it from the front.

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> Tonight I replaced the leaking capacitors on the +12V/-12V board, and also
> replaced the two similar ones that looked and measured fine. I put the PSU
> back together, put in a couple of less important boards and drives, and
the
> machine now seems to power on OK. I measured the ripple using the power
> connector for the front panel and that looks OK too. Unfortunately though,
> that connector only sends out +12V, +5V and -12V. It does not have output
> for +3.3V, so I have not, so far, been able to check the ripple for this.
It is a bit
> awkward to test on the bench with a dummy load.
> 
> As I did not replace the capacitors on the +5V/3.3V board, because the
> underside is very hard to access, and I am reluctant to pull them off and
> solder from above, I would like to be sure there is no ripple on the 3.3V
> supply. So, I was wondering if anyone has any neat tricks for probing the
> +3.3V supply with the PSU installed in the machine?
> 
> Next job is to replace the capacitors on the little DSSI terminator, they
are
> easy to change.
> 


I spoke too soon :-(

The machine stayed on for a couple of minutes and then powered off. I
suspect there is a problem with it detecting an overload that may not
actually be there. Looks like I will need to get a dummy load and put it on
the bench to see if it still happens there.

I do recall that when it first failed it would stay on for shorter and
shorter periods each time I tried it. So perhaps there is some component
warming up and then causing a failure?

Regards

Rob

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Robert
> Jarratt
> Sent: 13 December 2015 15:47
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Subject: RE: VAX 4000-500 PSU Overload?
> 
> 
> 
> 
> I have been able to remove the leaking ones from the +12V and -12V board,
I
> just have to wait for the new parts to arrive. The two on the 5V and 3.3V
> board have not leaked, are not bulging, and are harder to remove without
> removing the bottom heatsink or pulling from above as you suggest. I have
> not been able to reach them with my ESR meter either, unfortunately. I am
> tempted to leave them, although pulling from above is an option. Does
> anyone else agree with the pull from above solution? In case you have not
> seen it, the design makes it very hard to get at the underside, you have
to
> desolder a load of transistors to do so, and they are not in easy
locations
> either.
> 


Tonight I replaced the leaking capacitors on the +12V/-12V board, and also
replaced the two similar ones that looked and measured fine. I put the PSU
back together, put in a couple of less important boards and drives, and the
machine now seems to power on OK. I measured the ripple using the power
connector for the front panel and that looks OK too. Unfortunately though,
that connector only sends out +12V, +5V and -12V. It does not have output
for +3.3V, so I have not, so far, been able to check the ripple for this. It
is a bit awkward to test on the bench with a dummy load.

As I did not replace the capacitors on the +5V/3.3V board, because the
underside is very hard to access, and I am reluctant to pull them off and
solder from above, I would like to be sure there is no ripple on the 3.3V
supply. So, I was wondering if anyone has any neat tricks for probing the
+3.3V supply with the PSU installed in the machine? 

Next job is to replace the capacitors on the little DSSI terminator, they
are easy to change.

Thanks

Rob

Re: VAX 4000-500 PSU Overload?

[Holm Tiffe]

Robert Jarratt wrote:

> 
> 
> > -Original Message-
> > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Glen Slick
> > Sent: 14 December 2015 01:36
> > To: General Discussion: On-Topic and Off-Topic Posts
> > Subject: Re: VAX 4000-500 PSU Overload?
> > 
> > I took a look at the DSSI & SCSI termination power supply board in my VAX
> > 4000 BA440 chassis. It's the small board in the storage area of the 
> > backplane
> > mounted in place by a single screw. There are two identical circuits based 
> > on
> > the LT-1086 with the adjustable output voltage set at 5.30V  [1.25V * (1 +
> > 392/121)]. Each circuit has a 100uF 35V 105C nichicon cap between the input
> > and ground and the output and ground. Both circuits on my board still output
> > 5.3V from a 12V input, but I think the caps are starting to leak 
> > electrolyte on
> > the PCB and I should replace them all (4 total) before things get ugly.
> >
> 
> 
> I just got the board out. The caps on mine look OK, but I see the ESR is at 
> or just above the value suggested on the table printed on my meter. So I will 
> replace them all.

All of the Nichicons in my PSU and that Terminator Power Source looked ok,
but that was all...totally dead from the ESR side of view.

Don't bother with what you measure, replace them all if you can.

The leaking Caps in my PSU where from another Manufacturer (don't remember
wich one) and had a black color.


Regards,

Holm
-- 
  Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe, 
 Freiberger Straße 42, 09600 Oberschöna, USt-Id: DE253710583
  www.tsht.de, i...@tsht.de, Fax +49 3731 74200, Mobil: 0172 8790 741

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Glen Slick
> Sent: 14 December 2015 01:36
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> I took a look at the DSSI & SCSI termination power supply board in my VAX
> 4000 BA440 chassis. It's the small board in the storage area of the backplane
> mounted in place by a single screw. There are two identical circuits based on
> the LT-1086 with the adjustable output voltage set at 5.30V  [1.25V * (1 +
> 392/121)]. Each circuit has a 100uF 35V 105C nichicon cap between the input
> and ground and the output and ground. Both circuits on my board still output
> 5.3V from a 12V input, but I think the caps are starting to leak electrolyte 
> on
> the PCB and I should replace them all (4 total) before things get ugly.
>


I just got the board out. The caps on mine look OK, but I see the ESR is at or 
just above the value suggested on the table printed on my meter. So I will 
replace them all.

 
> Each circuit has three diodes, an SB520 and a 1N4735A for which I can find
> datasheets, and a third one which looks like a General Instrument
> MP654 for which I can't find a datasheet and I'm not sure if that is the 
> correct
> part number. I can't quite read the marking all the way around the body of
> the diode. If someone else takes a look at their
> BA440 DSSI & SCSI termination power supply board let me know if you can
> make out what that part number is, or if MP654 is a GI part for which you can
> find a datasheet.
> 

I had a look and all I can make out on one is"P654" with "143" underneath. On 
the second one I can see "MP" with "GI" underneath. So, presumably, the full 
marking is MP654/ GI 143. Presumably, one of these parts is the date code.

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Glen Slick]

On Thu, Dec 10, 2015 at 11:42 PM, Holm Tiffe  wrote:
>
> Another thing is that small PCB on the DSSI storage Backplane, the Caps
> there where dead too. I thing it is the termination Power regulator and in
> my case the bad caps "simulated" a bad disk, I could'nt initialize this
> disk properly before I've changed the Caps there too..
>

I took a look at the DSSI & SCSI termination power supply board in my
VAX 4000 BA440 chassis. It's the small board in the storage area of
the backplane mounted in place by a single screw. There are two
identical circuits based on the LT-1086 with the adjustable output
voltage set at 5.30V  [1.25V * (1 + 392/121)]. Each circuit has a
100uF 35V 105C nichicon cap between the input and ground and the
output and ground. Both circuits on my board still output 5.3V from a
12V input, but I think the caps are starting to leak electrolyte on
the PCB and I should replace them all (4 total) before things get
ugly.

Each circuit has three diodes, an SB520 and a 1N4735A for which I can
find datasheets, and a third one which looks like a General Instrument
MP654 for which I can't find a datasheet and I'm not sure if that is
the correct part number. I can't quite read the marking all the way
around the body of the diode. If someone else takes a look at their
BA440 DSSI & SCSI termination power supply board let me know if you
can make out what that part number is, or if MP654 is a GI part for
which you can find a datasheet.

-Glen

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Holm
Tiffe
> Sent: 11 December 2015 07:42
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> There is a big rectangular Cap in that PSU that I've left there since
> I don't have a usable Replacement and the primary caps where still good so
> far. The PSU itself is a really bad design from sight of repair, you can't
> properly replace the caps on the switcher PCB since you could'nt dismount
> the heat spreader so I suggest that you pull the Caps from above and
solder
> new one from above as I finally did.
> 

I have been able to remove the leaking ones from the +12V and -12V board, I
just have to wait for the new parts to arrive. The two on the 5V and 3.3V
board have not leaked, are not bulging, and are harder to remove without
removing the bottom heatsink or pulling from above as you suggest. I have
not been able to reach them with my ESR meter either, unfortunately. I am
tempted to leave them, although pulling from above is an option. Does anyone
else agree with the pull from above solution? In case you have not seen it,
the design makes it very hard to get at the underside, you have to desolder
a load of transistors to do so, and they are not in easy locations either.

Regards

Rob

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Maciej W.
> Rozycki
> Sent: 11 December 2015 01:48
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 
>  No leaking ChemiCon SXF-series capacitors inside?  They seem to suffer
> from some kind of design or manufacturing defect and consequently the
> electrolyte gets through the seal eventually even in parts never used.  A
> number of DEC PSUs have these capacitors, including the H7874 PSU which
> BA430/BA440 VAX 4000 cabinets use.


Just opened it up and there are indeed leaking caps, the brown ones. There
is also a strange deposit on a couple of screws (picture here:
http://1drv.ms/1micVN7), but the screws are nowhere near the leaking
capacitors. Any thoughts on that?



>  Unfortunately replacing the parts is not easy as heatsinks block access
to the
> soldering pads and you need to desolder main rectifiers first to gain
access.
> You need to clean any electrolyte spills too as they will cause corrosion
and
> shorts.

Yes, I have had one of these PSUs apart before and know how horrible it is
to get at these parts. I will however go ahead and replace these caps.

> 
>  I observed similar symptoms with the failed PSU as you do: it started up
> briefly, enough for diagnostic output to start coming from the system
> through the console port, and then within a couple of seconds the PSU shut
> down.

I don't have an easy way to apply test loads to all the outputs at the same
time. Is it possible to remove the final output boards for each "half" of
the output and test one at a time without causing the PSU to shut down?

Thanks

Rob

RE: VAX 4000-500 PSU Overload?

[Richard Loken]

On Fri, 11 Dec 2015, Robert Jarratt wrote:

> Well, I have the exception that proves the rule. My H7864 PSU (from an
> rtVAX 1000, same PSU as MicroVAX II) had a switching transistor and
> resistor fail, no cap failures

Well, I did admit I was exagerrating. :)

-- 
   Richard Loken VE6BSV, Unix System Administrator : "Anybody can be a father
   Athabasca University:  but you have to earn
   Athabasca, Alberta Canada   :  the title of 'daddy'"
   ** richar...@admin.athabascau.ca ** :  - Lynn Johnston

RE: VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Richard
> Loken
> Sent: 11 December 2015 16:45
> To: General Discussion: On-Topic and Off-Topic Posts
> Cc: gene...@classiccmp.org
> Subject: Re: VAX 4000-500 PSU Overload?
> 
> ...
> 
> My doctor once said to me that all problems in the head involve teeth and
I
> can safely make a similar exageration:
> 
>   All electronic failures involve capacitors
> 
> And this becomes (most repulsively) even more true when switch mode
> power supplies are involved.  Keeping that in mind will usually help with
> diagnosis.
> 


Well, I have the exception that proves the rule. My H7864 PSU (from an rtVAX
1000, same PSU as MicroVAX II) had a switching transistor and resistor fail,
no cap failures

:-)

Regards

Rob

Re: VAX 4000-500 PSU Overload?

[Richard Loken]

On Thu, 10 Dec 2015, Jon Elson wrote:

> On 12/10/2015 04:30 PM, Robert Jarratt wrote:
>> My VAX4000-500 will no longer power up, with the PSU starting up and then
...

> Most likely there is a failed capacitor somewhere.  Look
...

My doctor once said to me that all problems in the head involve teeth and I
can safely make a similar exageration:

All electronic failures involve capacitors

And this becomes (most repulsively) even more true when switch mode power
supplies are involved.  Keeping that in mind will usually help with
diagnosis.

-- 
   Richard Loken VE6BSV, Unix System Administrator : "Anybody can be a father
   Athabasca University:  but you have to earn
   Athabasca, Alberta Canada   :  the title of 'daddy'"
   ** richar...@admin.athabascau.ca ** :  - Lynn Johnston

Re: VAX 4000-500 PSU Overload?

[Noel Chiappa]

> From: Jon Elson

> You should be able to safely power the machine with only a couple
> boards at a time to find which one is bad.

For debugging power supplies, a 'load module' (a card with only a bunch of
resistors on it) is invaluable. No trying to figure out where/how to connect
a large load resistor - just plug the board in.

DEC made several:

  M7556 dual - +5V 1A -15V 375mA
  M9049  
  M9060-YA  quad - +5V 5A
  M9713-AA  dual - +5V 2.75A -15V .75A

The M7556 (at least) can be used on both QBUS and UNIBUS backplanes (since
they share the same pins for +5 and -12V (QBUS) and -15V (UNIBUS). Probably
all the others can be used on both buses, too, but check the pins they draw
from first.

The M7556 can be easily modified (with a few etch cuts) into a +5/+12V load
module, but the resultant board can _only_be used in a QBUS.

If you can't find any of these (there are none left on eBay, but other
suppliers still seem to have them), another alternative is to get a blank
prototyping board from Douglas Electronics (http://www.douglas.com/),
and add your own resistors.

Noel

Re: VAX 4000-500 PSU Overload?

[Holm Tiffe]

Robert Jarratt wrote:

> My VAX4000-500 will no longer power up, with the PSU starting up and then
> immediately shutting down. I suspect a possible short somewhere. I have
> measured the resistance of the load presented to the PSU by connecting
> probes to the backplane sockets used to power the machine. The odd one is
> the 5V load. With all the boards in and drives inserted I measure a
> resistance of about 4R. As I pulled out boards, drives and fans, it
> gradually crept up to 6R. So with nothing connected to the backplane I get a
> 6R load across the 5V supply.
> 
>  
> 
> To my inexperienced mind, that seems a bit low. Should I expect such a
> value, or should I be dismantling the box to investigate possible shorts or
> failed components on the backplane?
> 
>  
> 
> Additionally, the 12V side seems to be charging a capacitor as the
> resistance slowly climbs to about 130K. Is that reasonable? Again, nothing
> but the backplane.
> 
>  
> 
> The 3.3 and -12V show very high resistance at all times.
> 
>  
> 
> Thanks
> 
>  
> 
> Rob 


I have an 4000/300 which had PSU Problems too (don't know for now if this
uses the same PSU).
When I got the machine the PSU powered up only sometimes after switching on
and when it was running it shut off itself after some time.

I'm strongly suggest to change all the Electrolytics in
the PSU you can reach, all the brown Nichicons where dry an dead.
One bigger cap (100i0µ)on a PCB lost electrolyte and pissed it all over the
PCB.
There is a big rectangular Cap in that PSU that I've left there since
I don't have a usable Replacement and the primary caps where still good so
far. The PSU itself is a really bad design from sight of repair, you can't
properly replace the caps on the switcher PCB since you could'nt dismount
the heat spreader so I suggest that you pull the Caps from above and solder
new one from above as I finally did.

Now after some use of the machine the PSU has stabilized somehow, the
machine runs flawlessly.

Another thing is that small PCB on the DSSI storage Backplane, the Caps
there where dead too. I thing it is the termination Power regulator and in
my case the bad caps "simulated" a bad disk, I could'nt initialize this
disk properly before I've changed the Caps there too..

Regards,

Holm
-- 
  Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe, 
 Freiberger Straße 42, 09600 Oberschöna, USt-Id: DE253710583
  www.tsht.de, i...@tsht.de, Fax +49 3731 74200, Mobil: 0172 8790 741

Re: VAX 4000-500 PSU Overload?

[Jon Elson]

On 12/10/2015 04:30 PM, Robert Jarratt wrote:

My VAX4000-500 will no longer power up, with the PSU starting up and then
immediately shutting down. I suspect a possible short somewhere. I have
measured the resistance of the load presented to the PSU by connecting
probes to the backplane sockets used to power the machine. The odd one is
the 5V load. With all the boards in and drives inserted I measure a
resistance of about 4R. As I pulled out boards, drives and fans, it
gradually crept up to 6R. So with nothing connected to the backplane I get a
6R load across the 5V supply.

  



Most likely there is a failed capacitor somewhere.  Look 
over the boards for small Tantalum caps.  They may not show 
any outward signs of failure.  Finding the bad cap on a 
board with dozens of them in parallel is a nightmare!  You 
should be able to safely power the machine with only a 
couple boards at a time to find which one is bad.


And, of course, it could be the power supply itself!

Jon

Re: VAX 4000-500 PSU Overload?

[Maciej W. Rozycki]

On Thu, 10 Dec 2015, Robert Jarratt wrote:

> My VAX4000-500 will no longer power up, with the PSU starting up and then
> immediately shutting down. I suspect a possible short somewhere. I have
> measured the resistance of the load presented to the PSU by connecting
> probes to the backplane sockets used to power the machine. The odd one is
> the 5V load. With all the boards in and drives inserted I measure a
> resistance of about 4R. As I pulled out boards, drives and fans, it
> gradually crept up to 6R. So with nothing connected to the backplane I get a
> 6R load across the 5V supply.

 No leaking ChemiCon SXF-series capacitors inside?  They seem to suffer 
from some kind of design or manufacturing defect and consequently the 
electrolyte gets through the seal eventually even in parts never used.  A 
number of DEC PSUs have these capacitors, including the H7874 PSU which 
BA430/BA440 VAX 4000 cabinets use.

 Gravity helps this failure to happen apparently as I haven't seen this 
kind of failure in a number of PSUs, like the H7821 or H7819, which were 
kept in their proper position throughout their life and where the affected 
capacitors are mounted leads up.  The design of the H7874 PSU puts the 
capacitors in a horizontal position though when the unit is in its proper 
position.

 You may want to inspect your PSU internally to see if there are any signs 
of failure around SXF capacitors; these have a brown sleeve.  Initial 
disassembly of the PSU is easy, you just need to unfasten a bunch of 
screws and undo three internal connectors.  There are 5 330µF/25V parts 
and 1 1000µF/25V part of this kind total in the H7874, scattered across 
its PCBs.

 All the 330µF parts were failed in a PSU I have recently obtained, 
however replacing all SXF capacitors is I'm told advisable even if there 
is no sign of failure as one is inevitable sooner or later.  While at it 
replacing a pair of ChemiCon SXE capacitors these PSUs have is also a good 
idea I'm told as they might be prone to the same failure, although I 
haven't seen one to happen myself.  These are 100µF/35V, in the same shade 
of brown the SXF parts are.

 Unfortunately replacing the parts is not easy as heatsinks block access 
to the soldering pads and you need to desolder main rectifiers first to 
gain access.  You need to clean any electrolyte spills too as they will 
cause corrosion and shorts.

 I observed similar symptoms with the failed PSU as you do: it started up 
briefly, enough for diagnostic output to start coming from the system 
through the console port, and then within a couple of seconds the PSU shut 
down.

  Maciej

Re: VAX 4000-500 PSU Overload?

[Glen Slick]

On Thu, Dec 10, 2015 at 2:30 PM, Robert Jarratt
 wrote:
> My VAX4000-500 will no longer power up, with the PSU starting up and then
> immediately shutting down. I suspect a possible short somewhere. I have
> measured the resistance of the load presented to the PSU by connecting
> probes to the backplane sockets used to power the machine. The odd one is
> the 5V load. With all the boards in and drives inserted I measure a
> resistance of about 4R. As I pulled out boards, drives and fans, it
> gradually crept up to 6R. So with nothing connected to the backplane I get a
> 6R load across the 5V supply.
>
> Additionally, the 12V side seems to be charging a capacitor as the
> resistance slowly climbs to about 130K. Is that reasonable? Again, nothing
> but the backplane.

In the VAX 4000-200 in a BA430 chassis the M9715 board contains two
+5.3V supplies based on LT-1086 regulators and powered from the +12V
supply to provide two separate DSSI and SCSI bus termination voltages.
Each supply has a 100uF caps across the regulator inputs and outputs.

In addition the M9715 board contains a DS8641 bus transceiver powered
from the +5V supply. I'm not sure exactly what that does. I believe it
is monitoring the SCSI bus termination voltage and driving a bus
signal back to the H7874 supply, possibly to shut down the supply if
too much current on the SCSI bus termination supply causes it to drop
out.

I haven't pulled a BA440 chassis apart enough to take a close look at
the equivalent backplane circuitry there. In one photo I took a while
ago I can see two TO-220 packages and at least a couple of caps in the
middle of the storage backplane. Possibly the BA440 backplane contains
the same DSSI and SCSI bus termination supplies and monitoring
circuitry that is contained on the M9715 module in the BA430 chassis.

So with all boards, drives, and fans removed from a BA440 chassis
there will still be some load on the +5V and +12V lines.

Re: VAX 4000-500 PSU Overload?

[Peter Coghlan]

>
> My VAX4000-500 will no longer power up, with the PSU starting up and then
> immediately shutting down. I suspect a possible short somewhere. I have
> measured the resistance of the load presented to the PSU by connecting
> probes to the backplane sockets used to power the machine. The odd one is
> the 5V load. With all the boards in and drives inserted I measure a
> resistance of about 4R. As I pulled out boards, drives and fans, it
> gradually crept up to 6R. So with nothing connected to the backplane I get a
> 6R load across the 5V supply.
>
> 
>
> To my inexperienced mind, that seems a bit low. Should I expect such a
> value, or should I be dismantling the box to investigate possible shorts or
> failed components on the backplane?
>
>

Your meter used on a resistance range will not give you a good idea of the
load presented to the power supply.  Most of the load is semiconductors which
will not behave normally as they will not get enough voltage from the meter.
The one case where it may be helpful is if you measure a dead short or very
close to it, indicating something like a shorted decoupling capacitor or maybe
a sliver of solder or other conductive object bridging some contacts somewhere.
6R is not low enough to indicate this.  It will cause a bit under 1A to flow
from the power supply. 1A is likely much less than the machine normally draws
on the +5V supply.

I would suggest disconnecting the power supply completely from the machine and
putting a dummy load such as a 6V headlamp bulb or bulbs on it instead.  If the
PSU works normally on the dummy load, reconnect the machine and look for some
component getting hot.

If the PSU continues to trip when on a dummy load, try looking for shorted
rectifiers and/or smoothing capacitors on the output side of the PSU.  If the
output voltage is trying to climb too high, maybe a crowbar trip is operating?
(As Tony always says, don't disable the crowbar to try to find the problem!)
 
>
> Additionally, the 12V side seems to be charging a capacitor as the
> resistance slowly climbs to about 130K. Is that reasonable? Again, nothing
> but the backplane.
>

It seems reasonable but again, your meter on a resistance range is not going
to tell you much here due to the semiconductor load.

>
> The 3.3 and -12V show very high resistance at all times.
>

Ditto.

Regards,
Peter Coghlan.