RE: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

On Thu, 12 May 2022, Peter Coghlan via cctalk wrote:

> > Wayne:  I just got an Dec AlphaStation 200. Look like its running NT 
> > though. 
> >
> 
> It's probably ok but check the power supply for leaky capacitors!

 Definitely, watch out for possible damage!

 Prompted I had a peek in mine (an Astec SA180-3505) and it does have lots 
of the dangerous Chemi-con LXF parts!  While mine looks clean and tidy yet 
(LXF series seem sturdier than older SXF ones), I need to get this sorted 
sooner rather than later.  Thankfully last time I booted it a couple weeks 
ago it worked just fine.

 (Strictly speaking I have either an AlphaServer 300 or an AlphaStation 
250 system, which are hard to tell apart and the only difference mentioned 
in DEC documentation between the two mainboards is the maximum amount of 
DRAM supported, 512MiB vs 256MiB.  However both are essentially the same, 
up to the dedicated sound connector (unused with the AlphaServer), so it's 
not clear to me if it was a manufacturing option such as a wire bridge put 
somewhere into the PCB or just a matter of software support.  Both kinds 
of systems use the same firmware, which does not report the system model!  
And in any case my mainboard has a 54-23787-01 designation, which I'm told 
is an AlphaStation 250 (an AlphaServer 300 would be 54-23787-02), and also 
an early version, dated week 51 of 1994, S/N 60, and with lots of hardware 
patching present, however without the sound option and with an AlphaServer 
300 badge and a 1997 date code on the case.  Which in any case is exactly 
the same as with the AlphaStation 200, so the PSU is likely the same as 
well, so again, do check yours!)

> Have a look around for batteries that might leak while it was/is in
> storage too!

 There's a 2032 Lithium coin cell in these systems only, so while it might 
have been depleted it won't do any harm.

  Maciej

Re: DEC H7822 power supply

[Paul Koning via cctalk]

> On May 13, 2022, at 3:36 AM, Christian Corti via cctalk 
>  wrote:
> 
> On Thu, 12 May 2022, Peter Coghlan wrote:
> ...
>> A variac does not provide isolation.  If it is not wired correctly, it is
> 
> Good "variacs" provide galvanic isolation from the mains, i.e. they are not 
> autotransformers.

There may be variable transformers that aren't autotransformers, but I've never 
seen one.  The commonly seen Variac devices (either that brand, or analogous 
devices from other makers) are autotransformers.  

It should be obvious by inspection which kind you have, and take precautions as 
needed.

paul

Re: DEC H7822 power supply

[Christian Corti via cctalk]

On Thu, 12 May 2022, Peter Coghlan wrote:

Can we agree that there is such a thing as an "AC current" and there
is such a thing as a "DC current" and the two of them can be added
together?


Ouch... an alternating current current and direct current current ;-)
And you usually add the voltages, not the currents.


A variac does not provide isolation.  If it is not wired correctly, it is


Good "variacs" provide galvanic isolation from the mains, i.e. they are 
not autotransformers.



I just wanted to describe the obscure (to me) circuit I found in a DEC H7821
power supply in case anyone else needs to know how it seems to work.


That indeed is really interesting. The only reason that I can imagine why 
they used the ripple voltage to generate the transceiver voltage is that 
they "forgot" to implement an extra secondary winding on the transformer 
(or they reused the transformer from another design).


Christian

Re: DEC H7822 power supply

[Christian Corti via cctalk]

On Thu, 12 May 2022, Toby Thain wrote:

It does not, due to unidirectionality.


But in general, diodes can be (and are) used as controlled AC switches. 
One good example are the head select diodes in disk drives. You apply a 
direct voltage and thus "open" the diode. You then have a overlayed 
alternating voltage coming from the head coils passing through the diodes.

This is then AC coupled to the preamplifier.

Christian

Re: DEC H7822 power supply

[Christian Corti via cctalk]

On Thu, 12 May 2022, Toby Thain wrote:
I also hate the English/American expression "inverter" for voltage 
converters, because really nothing is being inverted in any way.
Generating AC from DC _does_ always involve generation of "inverted" voltages 
relative to the input DC (AC has positive and negative cycles), so the term 
isn't really strange?


It is very strange and wrong, because alternating != inverting.
Making -9V out of +9V would be inverting, making "AC" from "DC" is
chopping (along with the chopper circuit, chopper transformer...).
Inverting is making something, well, inside out, upside down and the like. 
That's not appropriate for AC to DC.


But this is a cultural thing. Like other terms, e.g. "shipping" although 
most of the time there is no ship involved.


Christian

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

>> 
>> If I am trying to calculate the height a sea harbour wall needs to be, can I
>> not consider the height of the tide and the height of the waves separately
>> and add them together, just to make it easier to work it out even though I
>> know that it would be physically impossible to distinguish the water that
>> makes up the tide from the water that makes up the waves?
>> 
> Using this analogy if you measured 10 waves at 10 feet high you would need
> a wall 100 feet high, correct? Or 20 waves at 7 feet it would need to be
> 140 feet. 
> Not valid at all.
>

The idea is to measure the maximum height of the waves on a windy day by
examining how far up they make splash marks on a vertical pole stuck in the
seabed for the purpose.  Then consult a tide table to see how high the tide
was at the time the wave measurement was made.  Subtracting the latter figure
from the former figure gives the height of the waves above the tide.  (This
is the AC component.  The tide is the DC component.)  Adding the height of
the highest expected tide to the figure calculated for the maximum height of
the waves would give a figure for the minimum height needed for the harbour
wall.

>
>> (It's not even that bad in the case of electric current because instruments
>> exist which can measure the DC component of a current and the AC component
>> of a current separately or together.)
> I have no idea what you are talking about. What instruments are you referring
> to? I’d like to read about them.
>

A suitable moving coil ammeter will register the average value of a current
flowing through it and will indicate which direction it is flowing in.  This
is the DC component of the current.  A suitable moving iron ammeter will
(crudely) register the RMS value of the AC current flowing through it.  I'm
not certain but as far as I recall, it will also register the actual value
of DC current.  There are probably lots of fancy computerised oscilloscope
like instruments that can be used to measure all sorts of oddball current
waveforms given suitable current probes.


>> 
>> How about we take a 3V doorbell transformer and make a circuit consisting
>> of the secondary of the transformer, a 6V battery and a 9V light bulb all
>> connected in series?  How would the current in this circuit be described?
>> A varying current? An AC current? A DC current?
>>
>
> The current would be described by ohm’s law. Incidentally, if the primary
> of a xformer was actually connected it would probably blow up the battery.
>

Maybe I should have stated that I was assuming that power is applied to the
primary for the test?  Perhaps I should have also stated that I am assuming
that the light bulb is selected such that the current it draws at it's rated
voltage will not exceed the capability of either the transformer or the
battery?  I don't expect anything to blow up assuming we are understanding
each other correctly which may not be a valid assumption.

>
> You would describe it as Open. If not, its just a 6 volt DC circuit.
>

I don't understand.

>
>> How about we use a 6V doorbell transformer and a 3V battery instead?  Would
>> that change how the current is described?
> No it wouldn’t.  
>
>> I would say that in both cases, the current flowing has an AC component and
>> it also has a DC component.
> As stated and not rectified it’s not a valid circuit.
> In ps circuits there is a fuse on the secondary side of the circuit usually
> after the rectifier-filter circuit to prevent any rectifier shorts from
> damaging other components.

The transformer secondary, battery and light bulb are all connected in series
ie in a circle, end to end.  The battery is not going to be bothered by the
AC current flowing through it and the presence of the light bulb will limit
the current to a safe value.

If the battery and the transformer secondary and the light bulb were connected
in parallel ie the two terminals of the battery were connected directly to the
two terminals of the transformer secondary and the two terminals of the light
bulb, that would be a different matter.  Either the transformer or the battery
or both could be damaged in this case.  This is not what I was suggesting.

>
> Well, the previous example seemed to imply that the output from a rectifier
> was pure dc.
>

Pretty much everything I have said is to try to illustrate that what flows
through the rectifier has a DC component and an AC component.

>
> Its not, it’s pulsed DC
>

Pulsed DC sounds like something that has a DC component and an AC component.

>
> so i thought you might be referring
> to the rectifier and associated filter and possibly regulator components as
> the whole rectifier”
> Sometimes in talking about electronics the specific function of a portion
> of connected components is cited. IE filter circuit,  Detector circuit,
> rectifier circuit. It’s just shorthand speak.
>

Sorry, I didn't follow.

I'm going to bow out at this point because I don't there is anything
further I can 

Re: DEC H7822 power supply

[Will Cooke via cctalk]

> On 05/12/2022 2:14 PM Peter Coghlan via cctalk  wrote:
> 
> 
> Wayne S wrote:
> >> On May 12, 2022, at 09:56, Peter Coghlan via cctalk 
> >>  wrote:
> >>
> >> Toby Thain via cctalk wrote:
>  On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:
>  Given the normal usage that has evolved for the terms DC and AC rather 
>  than
>  their dictionary definitions, I would suggest that the current that gets
>  passed by a rectifier has both a DC component and an AC component. When
> 
> >>>
> >>> It does not, due to unidirectionality.
> >>>
> >>
> >> Consider the current through the rectifier as the sum of a "DC" current 
> >> plus
> >> an "AC" current. The "DC" current has a steady positive value and the "AC"
> >> current varies above and below zero with a magnitude less than or equal to
> >> that of the "DC" current.
> >
> > I’m really trying to understand what you’re getting at here.
> > In practical terms, The assumption that there is a sum of a DC and AC
> > component is incorrect.
> Why? What evidence do you have that the assumption is incorrect?
> 
> If I am trying to calculate the height a sea harbour wall needs to be, can I
> not consider the height of the tide and the height of the waves separately
> and add them together, just to make it easier to work it out even though I
> know that it would be physically impossible to distinguish the water that
> makes up the tide from the water that makes up the waves?
> 
> (It's not even that bad in the case of electric current because instruments
> exist which can measure the DC component of a current and the AC component
> of a current separately or together.)
> 
>
Peter,
You are quite correct.  It is called superposition.
https://en.wikipedia.org/wiki/Superposition_principle
It applies to all linear systems and is one of the principles that make many 
difficult problems practicable.
My background is primarily physics.  Superposition is extremely important there 
in many realms, including signal processing, electric circuits, and 
differential equations, among many others.

I'll say no more.  I didn't want to get into this quagmire, either.

Will

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Wayne S wrote:
>> On May 12, 2022, at 09:56, Peter Coghlan via cctalk  
>> wrote:
>> 
>> Toby Thain via cctalk wrote:
 On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:
 Given the normal usage that has evolved for the terms DC and AC rather than
 their dictionary definitions, I would suggest that the current that gets
 passed by a rectifier has both a DC component and an AC component.  When
 
>>> 
>>> It does not, due to unidirectionality.
>>> 
>> 
>> Consider the current through the rectifier as the sum of a "DC" current plus
>> an "AC" current.  The "DC" current has a steady positive value and the "AC"
>> current varies above and below zero with a magnitude less than or equal to
>> that of the "DC" current.
>
> I’m really trying to understand what you’re getting at here.
> In practical terms, The assumption that there is a sum of a DC and AC
> component is incorrect.

Why? What evidence do you have that the assumption is incorrect?

If I am trying to calculate the height a sea harbour wall needs to be, can I
not consider the height of the tide and the height of the waves separately
and add them together, just to make it easier to work it out even though I
know that it would be physically impossible to distinguish the water that
makes up the tide from the water that makes up the waves?

(It's not even that bad in the case of electric current because instruments
exist which can measure the DC component of a current and the AC component
of a current separately or together.)

>
> There is no DC component in an AC wave. A wave is either AC meaning
> reversing polarity or it is DC meaning there is no reversal of polarity.
>

Can we agree that there is such a thing as an "AC current" and there
is such a thing as a "DC current" and the two of them can be added
together?

How about we take a 3V doorbell transformer and make a circuit consisting
of the secondary of the transformer, a 6V battery and a 9V light bulb all
connected in series?  How would the current in this circuit be described?
A varying current? An AC current? A DC current?

How about we use a 6V doorbell transformer and a 3V battery instead?  Would
that change how the current is described?

I would say that in both cases, the current flowing has an AC component and
it also has a DC component.

>
> And a rectifier does not sum anything.
>

I'm not saying the rectifier sums anything.  What I am saying is that the
current that goes through the rectifier can be considered as the sum of
two component currents.  It's just a way of looking at it, like considering
the tide and the waves separately in the harbour wall calculation.

>
> Are you maybe using “Rectifier” as the whole circuit?  A Rectifier
> can be a single component, like a diode used to pass one half of an
> alternating current or it can be a few components like 4 diodes connected
> as a bridge to pass both halfs.
>

Strictly speaking, a circuit involves a closed path for a current to flow in.
However, "circuit" also gets used to describe a substantial group of components
that needs to be connected to something else to form a fully closed path.  I
don't think a rectifier would be enough to be considered a whole circuit. 

>
> This may help to visualize.
> Do an experiment. If you have an Oscope take a small battery maybe 9 volt and
> wire the negative lead to the neg scope input. Touch the positive lead to the
> scope positive and watch the deflection. You should see a quick trace  rise
> and the trace should remain above the zero point.
> That is what pure DC looks like. No ripple at all. Power Supplies try to make
> that from your wall power input. 
> Next, untouch the positive and watch the trace fall. At no point does it fall
> below the zero line.
> NoW touch and untouch the positive as fast as you can and watch the trace.
> That is pulsating DC although with irregular frequency. That would be the
> output from the rectifier component.
> Reverse the leads and do it again.
> This will show the equivalent of a negative polarity.
>  
> If you have a variac, set it to 9 volts or so, plug in a light or something to
> provide a load and using your oscilloscope probes touch across the light and
> you should see a sine wave with regular frequency.  Notice the negative going
> half. That is AC.
> 
>
> It’s important to use a variac for safety ( or an isolation transformer very
> carefully) as wall voltages can hurt you.
>

A variac does not provide isolation.  If it is not wired correctly, it is
possible to end up scooping 9V from the live end of the mains supply instead
of from the neutral end.  If there is a bad connection or a break in the
winding near or at the neutral end, it is possible to end up with the whole
mains supply at the output.  I don't want to connect the earth lead of my
scope probe to the neutral output of a variac either.  If I were doing this,
I would prefer to use a transformer designed to provide an isolated fixed low
voltage output.

>
> 

Re: DEC H7822 power supply

[Toby Thain via cctalk]

On 2022-05-12 11:37 a.m., Peter Coghlan via cctalk wrote:

Toby Thain via cctalk wrote:

On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:


Given the normal usage that has evolved for the terms DC and AC 
rather than

their dictionary definitions, I would suggest that the current that gets
passed by a rectifier has both a DC component and an AC component.  When



It does not, due to unidirectionality.



Consider the current through the rectifier as the sum of a "DC" current 
plus

an "AC" current.  The "DC" current has a steady positive value and the "AC"
current varies above and below zero with a magnitude less than or equal to
that of the "DC" current.

When the two are summed, the result is a varying current which does not go
below zero.  This is what I mean by it having a DC component and an AC
component.


Sure, if you change the reference point, a DC signal under one reference 
("zero") potential becomes AC under a different potential. The 
definitions are relative to a reference, in order to define the current 
direction.





This sort of analysis often used in electronic engineering to break down
more complex entities into simpler ones which can be analysed separately
with greater ease.  Didn't somebody have a theorem or an axiom or something
that says this is a valid way to do it? I forget who.  It was a long time
ago.  Kirchoff maybe? No, it wasn't him, maybe he was the one that said
the sum of currents into and out of nodes is zero and stuff like that...
It is a particularly suitable strategy for the case in hand because a
transformer will not pass "DC" from primary to secondary so the effect of
this component can be ignored (except that it could cause the transformer
core to saturate which must be allowed for.  This is only relevant to the
designer of the power supply but if I don't mention it, someone will
surely tell me that I should have.)

(I am starting to regret making the effort to accurately describe this
unusual and confusing (to me anyway) circuit for the benefit of others who
might find themselves struggling with this power supply as I was and might
find some hints on how it operates to be helpful.)

Regards,
Peter Coghlan.

Re: DEC H7822 power supply

[Paul Koning via cctalk]

> On May 12, 2022, at 11:37 AM, Peter Coghlan via cctalk 
>  wrote:
> 
> Toby Thain via cctalk wrote:
>> On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:
>>> Given the normal usage that has evolved for the terms DC and AC rather than
>>> their dictionary definitions, I would suggest that the current that gets
>>> passed by a rectifier has both a DC component and an AC component.  When
>>> 
>> 
>> It does not, due to unidirectionality.
>> 
> 
> Consider the current through the rectifier as the sum of a "DC" current plus
> an "AC" current.  The "DC" current has a steady positive value and the "AC"
> current varies above and below zero with a magnitude less than or equal to
> that of the "DC" current.
> 
> When the two are summed, the result is a varying current which does not go
> below zero.  This is what I mean by it having a DC component and an AC
> component.

Sure, that's the standard way to look at a signal.  It becomes very obvious 
when you do a Fourier transform; you see a term at f=0 (the DC offset) and 
terms at f equal to multiples of the mains frequency.

paul

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Toby Thain via cctalk wrote:

On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:


Given the normal usage that has evolved for the terms DC and AC rather than
their dictionary definitions, I would suggest that the current that gets
passed by a rectifier has both a DC component and an AC component.  When



It does not, due to unidirectionality.



Consider the current through the rectifier as the sum of a "DC" current plus
an "AC" current.  The "DC" current has a steady positive value and the "AC"
current varies above and below zero with a magnitude less than or equal to
that of the "DC" current.

When the two are summed, the result is a varying current which does not go
below zero.  This is what I mean by it having a DC component and an AC
component.

This sort of analysis often used in electronic engineering to break down
more complex entities into simpler ones which can be analysed separately
with greater ease.  Didn't somebody have a theorem or an axiom or something
that says this is a valid way to do it? I forget who.  It was a long time
ago.  Kirchoff maybe? No, it wasn't him, maybe he was the one that said
the sum of currents into and out of nodes is zero and stuff like that...
  
It is a particularly suitable strategy for the case in hand because a

transformer will not pass "DC" from primary to secondary so the effect of
this component can be ignored (except that it could cause the transformer
core to saturate which must be allowed for.  This is only relevant to the
designer of the power supply but if I don't mention it, someone will
surely tell me that I should have.)

(I am starting to regret making the effort to accurately describe this
unusual and confusing (to me anyway) circuit for the benefit of others who
might find themselves struggling with this power supply as I was and might
find some hints on how it operates to be helpful.)

Regards,
Peter Coghlan.

Re: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

On Thu, 12 May 2022, Peter Coghlan via cctalk wrote:

> I replaced the capacitor in question with a blue Nichicon SE of the same
> value scavanged from another H7821 until I order some spares.  I now have
> 8.7V available on the 9V supply, a little less than I hoped for but it is
> enough to make the internal thinwire tranceiver happy.  Now there is just
> the seized fan to deal with.

 Tolerance is 5% on this line, so you're well within.

> Thanks again to you and Tony for leading me in the direction of the likely
> source of this fault.  I would never have thought of looking in the power
> supply to find it.

 You are welcome!  I wish I had the skills to actually diagnose a faulty 
PSU myself like you do.

 Experience has taught me to look into the PSU first for all kinds of odd 
phenomena.  For example I had a WiFi+5-port 100BASE-TX Ethernet bridge 
which stopped talking to one particular device, a 10BASE-T to 10BASE2 
media converter/repeater regardless of the port it was wired to; IIRC 
frame reception worked, but transmission did not make it through.

 It cost me a lot of hair pulling to sort it, including actually buying a 
brand new identical media converter, the first suspect, as it was the only 
device that caused the bridge to exhibit its odd behaviour.  Which did not 
help of course and the new device triggered exactly the same symptoms.

 Eventually it has turned out to be the bridge's PSU being on its way out. 
It was one of those nasty plug top PSUs that are not worth even looking 
inside, and just replacing it with a new one (which of course cost more 
than the bridge combined with its PSU originally did!) cured the problem.  
Out of 4 such bridges I bought ~15 years ago 3 are still in service, but 
all their PSUs had to be replaced.  At least they had a standard connector 
for the power plug (the newer replacement devices don't anymore, sigh).

 The 4th bridge, the same that caused problems, started having more issues 
with its wired ports later on if more than 2 devices were plugged IIRC, 
with no clear reason except for suspision of a power deficit, so I put it 
aside with a plan to recap it and see whether it helps (this discussion 
just reminded me about it, so maybe I'll try doing it over the coming 
weekend as I have since got the parts required).

 And it was just one of many incidents I had with PSUs playing tricks, so 
the PSU is now my first and primary suspect if a device misbehaves in a 
way that cannot be clearly attributed to a software bug (which is by far 
the most common case nowadays, a clear sign of a generation change to me).

  Maciej

Re: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

On Wed, 11 May 2022, Paul Koning wrote:

> > I guess especially for standard capacitors factories ordered whatever was 
> > readily available whereas with the high-ripple low-impedance/ESR parts the 
> > choice was much more limited.
> 
> I wonder if nowadays a good replacement for the case where low ESR is 
> needed would be a big ceramic capacitor.  Unlike the old days those now 
> can be had in many-µF capacitances.

 Possibly, in the lower capacitance range, if not for the horrendous 
price, e.g. $50/£40 each for a 470µF/25V part: 



(at least you get free shipping for just one piece).  There's still little 
choice for the higher range parts found in DEC PSUs, such as 1800µF/25V, 
4700µF/10V or 6800µF/20V even.  With aluminium organic polymer capacitors 
rated voltage also quickly drops for higher capacitances, so you won't get 
any of these as replacements either.  I don't know offhand to what extent 
these limitations come from current technology vs the laws of physics.

  Maciej

Re: DEC H7822 power supply

[Toby Thain via cctalk]

On 2022-05-12 4:40 a.m., Christian Corti via cctalk wrote:

On Thu, 12 May 2022, Peter Coghlan wrote:
about it?  Maybe it would be more accurate to use terms like steady 
voltage

and alternating voltage? Alternating doesn't seem like that good a term


In German it's exactly like you suggest it. We say "Gleichspannung" (= 
constant/steady voltage) and "Wechselspannung" (=alternating voltage)


I also hate the English/American expression "inverter" for voltage 
converters, because really nothing is being inverted in any way.




Generating AC from DC _does_ always involve generation of "inverted" 
voltages relative to the input DC (AC has positive and negative cycles), 
so the term isn't really strange?



But hey, they still have inches and feet and ounzes (be they fluid or 
solid) and gallons, so what can I expect? ;-


Christian

Re: DEC H7822 power supply

[Toby Thain via cctalk]

On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:

It's hard to come up with suitable terms for this sort of stuff that
convey the meaning of what is going on.  I struggled with it for a while
and that was what I ended up with.

With the strict meaning of DC and AC being direct current and alternating
current and given that we are often talking about voltages rather than
currents, meanings are already getting stretched.  Besides, what's direct
about it?  Maybe it would be more accurate to use terms like steady voltage


Many textbooks (and wikipedia) define DC as "in one direction", which is 
accurate. There is no need to refer to voltage; DC is indeed descriptive 
of the _current_ not voltage. "Constant voltage" is not a synonym for 
"DC", although it is a subset of DC we frequently encounter.



and alternating voltage? Alternating doesn't seem like that good a term
either.  To me it suggests some sort of square wave switching very rapidly
between one extreme and another, not a nice lazy sine wave which is the
normally accepted meaning.


Likewise, alternating means "alternating direction".



Given the normal usage that has evolved for the terms DC and AC rather than
their dictionary definitions, I would suggest that the current that gets
passed by a rectifier has both a DC component and an AC component.  When


It does not, due to unidirectionality.


this mixture is fed into a transformer primary as in this case, the DC
component does not pass through from the primary to the secondary but the
AC component does.  Transformers only work on AC, right?

Or maybe it could equally be said that a transformer can be used to convert
pulsed DC to AC?
...
Regards,
Peter Coghlan.


Sent from my DEC Alphaserver 800

Wayne S wrote:


“ The ripple on the rectified 5V and 12V supplies gets transformed into an
isolated AC source for the 9V supply.  ”
Shouldn’t that be “pulsed DC” instead of “AC” as rectification
changes AC to DC ?


Sent from my iPhone


On May 11, 2022, at 01:36, Peter Coghlan via cctalk  
wrote:

The ripple on the rectified 5V and 12V supplies gets transformed into an
isolated AC source for the 9V supply.

RE: DEC H7822 power supply

[Peter Coghlan via cctalk]

>
> Wayne: AC DC  terminology has been well documented since the 1800's. 
> Don't try to reinvent the terms or no one will know what you are talking 
> about.
> I answered a few things below...
> 

They didn't have switch mode power supplies in the 1800s.  Terms like AC and
DC as understood back then can be used to describe what goes in to and comes
out of this power supply but they are not ideal for describing what's going
on in the middle of it.

It seems like an unusual sort of design to me.  I haven't come across the
idea of extracting ripple from one output and using it to create another
output before.  It took me a long time to figure out what was going on.
Maybe it is a common thing to do and I haven't seen it because of my
lack of experience with this sort of thing?  Could anyone who is more
familiar with switch mode power supplies comment?  Anyway, I was just
trying to describe what I've seen. 

>
> Wayne: Alternating Current  is a continuously varying sine wave. The polarity
> does reverse over time.  Perhaps Alternating Current isn't a good term and
> should be Alternating voltage instead but AC is the terminology and it
> describes the form of the wave. Is basically says that there is a zero to
> positive component and a zero to negative component of the voltage as 
> measured over time in 2 half waves
>

Ok. I doubt we will find many true sine waves inside this power supply though.

>
> Wayne:  There is no AC component.  The output from a rectifier is pulsed DC,
> either half wave or full wave. An additional circuit after the rectifier
> provides the smoothing to provide nearly pure DC. Nearly pure meaning the
> voltage remains constant and does not drop much when measured over time.
> The most pure DC source is a battery.  Transformers work on AC or Pulsed DC.
>

It depends on your definition of AC component.  How about we say the output
of a rectifier consists of a steady component plus a varying component?
I guess you could call it pulsed DC if you want.  Whatever it is, when you
put it through a transformer, what you get out the other side will have a
varying component and no steady component.  The output of the transformer
is probably vaguely sinusoidal because the performance of the core is likely
to fall off as frequency rises.  It could be described as AC.  It could not
be described as pulsed DC.  

>
> Wayne: There is no negative component of Pulsed DC so no AC.
>

As before, how about we call it a steady component plus a varying component?

>
> Wayne.  Lotta good text on this out there. Basically a variable current
> induced through a wire generates a magnetic field. Any wire placed next
> to it, has that field induced in it as well.  The catch is that the field
> has to pulse or alternate to keep generating the field and being induced
> into the other wire. It's the movement IE the up and down motion of the 
> voltage that causes the field to be induced in the other wire.
>

In a transformer, it is the variation of the current in one wire which
creates a varying magnetic field which induces a current in the other wire.

>
> Wayne:  I just got an Dec AlphaStation 200. Look like its running NT though. 
>

It's probably ok but check the power supply for leaky capacitors!
Have a look around for batteries that might leak while it was/is in
storage too!

>
> >
> > “ The ripple on the rectified 5V and 12V supplies gets transformed 
> > into an isolated AC source for the 9V supply.  ”
> > Shouldn’t that be “pulsed DC” instead of “AC” as rectification
> > changes AC to DC ?
>

No, it shouldn't.  What goes into the transformer could be described as
pulsed DC.  What comes out of the transformer cannot be described as pulsed DC.
It has a varying component and no steady component, therefore I described it
as AC.

Regards,
Peter Coghlan.

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Maciej W. Rozycki wrote:
> On Thu, 12 May 2022, Peter Coghlan via cctalk wrote:
> 
>>  Anyway, the good news is that I think I have found the source of the
>> problem.  One of the capacitors used to filter the (DC?, pulsed DC?,
>> rippled DC?, biased AC?) supply to the 9V regulator is marked 330uF/25V.
>> It reads 6uF on the capacitance range on my multimeter.  This can't be
>> helping the cause.  It's not showing any signs of leakage but it's got
>> a brown sleeve and the same logo as the nasty, leaky SXF capacitors but
>> it is marked KME.  (I said there was only one capacitor in the filter
>> in a previous posting.  Originally I managed to spot a little 10uF/100V
>> capacitor but somehow failed to notice the chubbier 330uF/25V capacitor
>> completing a PI filter with a small choke...)
>
>  Likely just a general purpose capacitor.  I only have 2 330uF/25V 85°C 
> Nichicon SE parts listed for the H7821 (and I have a note about one PSU of 
> this kind having a 220uF/35V part in place of one of those too).  Also no 
> 100V parts at all, but 4 10uF/35V 105°C parts, either Teapo SE or Daewoo 
> RS.  Teapo SE and Chemi-con KME series are standard general purpose parts 
> and given the low temperature rating Nichicon SE are likely such as well.
> 
>  I guess especially for standard capacitors factories ordered whatever was 
> readily available whereas with the high-ripple low-impedance/ESR parts the 
> choice was much more limited.
> 

Hi Maciej,

I replaced the capacitor in question with a blue Nichicon SE of the same
value scavanged from another H7821 until I order some spares.  I now have
8.7V available on the 9V supply, a little less than I hoped for but it is
enough to make the internal thinwire tranceiver happy.  Now there is just
the seized fan to deal with.

Thanks again to you and Tony for leading me in the direction of the likely
source of this fault.  I would never have thought of looking in the power
supply to find it.

Regards,
Peter.

>
>   Maciej
>

Re: DEC H7822 power supply

[Christian Corti via cctalk]

On Thu, 12 May 2022, Peter Coghlan wrote:

about it?  Maybe it would be more accurate to use terms like steady voltage
and alternating voltage? Alternating doesn't seem like that good a term


In German it's exactly like you suggest it. We say "Gleichspannung" (= 
constant/steady voltage) and "Wechselspannung" (=alternating voltage)


I also hate the English/American expression "inverter" for voltage 
converters, because really nothing is being inverted in any way.


But hey, they still have inches and feet and ounzes (be they fluid or 
solid) and gallons, so what can I expect? ;-


Christian

Re: DEC H7822 power supply

[Paul Koning via cctalk]

> On May 11, 2022, at 9:00 PM, Maciej W. Rozycki via cctalk 
>  wrote:
> 
> ...
> I guess especially for standard capacitors factories ordered whatever was 
> readily available whereas with the high-ripple low-impedance/ESR parts the 
> choice was much more limited.

I wonder if nowadays a good replacement for the case where low ESR is needed 
would be a big ceramic capacitor.  Unlike the old days those now can be had in 
many-µF capacitances.

paul

Re: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

On Thu, 12 May 2022, Peter Coghlan via cctalk wrote:

> Anyway, the good news is that I think I have found the source of the
> problem.  One of the capacitors used to filter the (DC?, pulsed DC?,
> rippled DC?, biased AC?) supply to the 9V regulator is marked 330uF/25V.
> It reads 6uF on the capacitance range on my multimeter.  This can't be
> helping the cause.  It's not showing any signs of leakage but it's got
> a brown sleeve and the same logo as the nasty, leaky SXF capacitors but
> it is marked KME.  (I said there was only one capacitor in the filter
> in a previous posting.  Originally I managed to spot a little 10uF/100V
> capacitor but somehow failed to notice the chubbier 330uF/25V capacitor
> completing a PI filter with a small choke...)

 Likely just a general purpose capacitor.  I only have 2 330uF/25V 85°C 
Nichicon SE parts listed for the H7821 (and I have a note about one PSU of 
this kind having a 220uF/35V part in place of one of those too).  Also no 
100V parts at all, but 4 10uF/35V 105°C parts, either Teapo SE or Daewoo 
RS.  Teapo SE and Chemi-con KME series are standard general purpose parts 
and given the low temperature rating Nichicon SE are likely such as well.

 I guess especially for standard capacitors factories ordered whatever was 
readily available whereas with the high-ripple low-impedance/ESR parts the 
choice was much more limited.

  Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

It's hard to come up with suitable terms for this sort of stuff that
convey the meaning of what is going on.  I struggled with it for a while
and that was what I ended up with.

With the strict meaning of DC and AC being direct current and alternating
current and given that we are often talking about voltages rather than
currents, meanings are already getting stretched.  Besides, what's direct
about it?  Maybe it would be more accurate to use terms like steady voltage
and alternating voltage? Alternating doesn't seem like that good a term
either.  To me it suggests some sort of square wave switching very rapidly
between one extreme and another, not a nice lazy sine wave which is the
normally accepted meaning.

Given the normal usage that has evolved for the terms DC and AC rather than
their dictionary definitions, I would suggest that the current that gets
passed by a rectifier has both a DC component and an AC component.  When
this mixture is fed into a transformer primary as in this case, the DC
component does not pass through from the primary to the secondary but the
AC component does.  Transformers only work on AC, right?

Or maybe it could equally be said that a transformer can be used to convert
pulsed DC to AC?

(The transformer has to be kinda special to avoid the core getting
saturated by the large DC current (What's a "direct current current"?)
flowing through the primaries...)

Anyway, the good news is that I think I have found the source of the
problem.  One of the capacitors used to filter the (DC?, pulsed DC?,
rippled DC?, biased AC?) supply to the 9V regulator is marked 330uF/25V.
It reads 6uF on the capacitance range on my multimeter.  This can't be
helping the cause.  It's not showing any signs of leakage but it's got
a brown sleeve and the same logo as the nasty, leaky SXF capacitors but
it is marked KME.  (I said there was only one capacitor in the filter
in a previous posting.  Originally I managed to spot a little 10uF/100V
capacitor but somehow failed to notice the chubbier 330uF/25V capacitor
completing a PI filter with a small choke...)

Regards,
Peter Coghlan.


Sent from my DEC Alphaserver 800

Wayne S wrote:
>
> “ The ripple on the rectified 5V and 12V supplies gets transformed into an
> isolated AC source for the 9V supply.  ”
> Shouldn’t that be “pulsed DC” instead of “AC” as rectification
> changes AC to DC ?
> 
> 
> Sent from my iPhone
>
>> On May 11, 2022, at 01:36, Peter Coghlan via cctalk  
>> wrote:
>> 
>> The ripple on the rectified 5V and 12V supplies gets transformed into an
>> isolated AC source for the 9V supply.  
>

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Christian Corti via cctalk wrote:

On Tue, 10 May 2022, Peter Coghlan wrote:
> Ahead of the 78M09 is another TO220 package marked D45H8 which seems to
> be a transistor.  Then I am completely lost.  I can't find the rectifier

That TO220 part would be the rectifier. It either has two legs (simple 
diode) or three (two-wave rectifier with presumably common cathode).




I thought that too at first.  However, it is definately a transistor.

I've made a bit more progress figuring out the circuit.  There is a
large torroid with three windings on it on the output side.  I thought
it was some sort of triple wound smoothing choke for the 5V, 12V and 9V
supplies but it is not.  It turns out that this is actually the source of
the 9V supply.  It is effectively a current transformer with two primaries.
The ripple on the rectified 5V and 12V supplies gets transformed into an
isolated AC source for the 9V supply.  This AC goes to a simple rectifier
which looks like a 1N4001 but isn't, a small choke and a smoothing capacitor.
The resulting DC goes to the emitter of the D45H8 whose collector feeds the
input of the 78(M)09.  I haven't figured out how the base of the D45H8 is
controlled yet.

Regards,
Peter.



Christian
   

Re: DEC H7822 power supply

[Christian Corti via cctalk]

On Tue, 10 May 2022, Peter Coghlan wrote:

Ahead of the 78M09 is another TO220 package marked D45H8 which seems to
be a transistor.  Then I am completely lost.  I can't find the rectifier


That TO220 part would be the rectifier. It either has two legs (simple 
diode) or three (two-wave rectifier with presumably common cathode).


Christian

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Peter Coghlan wrote:
> Tony Duell wrote:
> > On Sun, May 8, 2022 at 1:02 PM Maciej W. Rozycki via cctalk
> >  wrote:
> >
> > >  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
> > > wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
> > > across the suspicious connections as any reference to ground may not be
> > > indicative.
> > >
> > >  This voltage is also present with the H7819 PSU.  It's not clear to me
> > > what it is used for.
> > 
> > It's only a guess (this is rather more modern than the DEC hardware I
> > have), but it may be a supply for a thinwire ethernet transceiver
> > circuit. This would be insolated from system ground of course, and
> > I've seen something similar in an X-terminal.
> >
> 
> Hi Tony,
> 
> After years of rediscovering the same failures over and over again,
> I finally managed to get in the habit of making notes while I am
> trying to diagnose faults.  Now I just need to get in the habit of
> reading my old notes when I start to work on a new fault.  I did
> that just now and I found that I have a MicroVAX 3100 model 76 whose
> internal thinwire tranceiver failed last summer.  Not finding any
> obvious reason for the failure, I replaced it with an external
> tranceiver plugged into the AUI port and then forgot all about it.
> 
> I should check that this floating 9V output from it's H7821 PSU is
> working correctly.
>

I checked it.  It showed 4.4V and also seemed to be running rather
hotter than the other H7821 PSUs I've been working with.  I confirmed
that the internal thinwire tranceiver is still not working.  I swapped
out the H7821 in favour of a known working H7821 which gave me 8.9V
and the internal tranceiver then worked again.

I put the faulty H7821 into the system that the good H7821 came from
and confirmed that the internal tranceiver in that system now didn't
work and the 9V supply was around 5V.  With the different layout in
this system, I was able to see that the rear fan in the power supply
was not rotating which could account for it's higher temperature.
On the offchance that this might also account for the low 9V supply,
I tried unplugging the seized fan but this didn't make any difference.

The 9V supply comes from a 78M09 which I hoped would be faulty because
I can't make much sense of the circuit that feeds it.  I pulled the same
component from another older H7821 and it turned out to be a 7809 which
I tried fitting in place of the 78M09 but the symptoms remained the same.
Ahead of the 78M09 is another TO220 package marked D45H8 which seems to
be a transistor.  Then I am completely lost.  I can't find the rectifier
or the winding on the chopper transformer that is supposed to produce power
for this part of the circuit.  Making comparison resistance tests between
the faulty H7821 and another one showed up some discrepancies around the
78M09 / 7809 and the D45H8 however the former were probably because of
the differences between a 78M09 and a 7809 and in the latter case, the
D45H8 looked reasonable when tested out of circuit.

The faulty H7821 has seen orders of magnitude more usage than any of
my others but it still has it's original SXF electrolytics which are
not showing any signs of leakage yet that I can see without unsoldering
them.  It's been the right way up pretty much forever.

Thanks Maciej and Tony for leading me to the cause of this issue.
Another mystery (almost) solved.

Regards,
Peter.

> 
> (I wonder why it is described as -9V on the label of the 30-35042-01 PSU
> in my VAX 4000/100A?  If it had been shown as +/-9V for example or even
> just plain old 9V, I would have been less inclined to assume it was
> referenced to ground.)
> 
> Regards,
> Peter.
> 
> >
> > -tony
> > 
> > 
> > 
> > >
> > >   Maciej

Re: DEC H7822 power supply

[Tony Duell via cctalk]

On Mon, May 9, 2022 at 2:27 PM Paul Koning via cctalk
 wrote:
>
> Why would you expect a transceiver power supply to be isolated from system 
> ground?  The transceiver logic ground (via the AUI connector) is tied to the 
> system ground.  It is the coax that is isolated from the system ground, not 
> the bulk of the transceiver circuitry.


An external transceiver has an internal _isolated_ DC-DC converter and
the signal lines are transformer-isolated. So the electronics is
isolated from system ground.

If you have an internal transceiver you either have to keep that DC-DC
converter or run the electronics off an isolated output on the power
supply.

Note that the outer of the BNC connector is isolated from the system
casing (which is grounded). In some cases those insulating bushings
are actually ceramic capactors (!).

-tony



>
> paul
>

Re: DEC H7822 power supply

[Paul Koning via cctalk]

> On May 8, 2022, at 7:04 PM, Maciej W. Rozycki via cctalk 
>  wrote:
> 
> On Sun, 8 May 2022, Tony Duell wrote:
> 
>>> In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
>>> wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
>>> across the suspicious connections as any reference to ground may not be
>>> indicative.
>>> 
>>> This voltage is also present with the H7819 PSU.  It's not clear to me
>>> what it is used for.
>> 
>> It's only a guess (this is rather more modern than the DEC hardware I
>> have), but it may be a supply for a thinwire ethernet transceiver
>> circuit. This would be insolated from system ground of course, and
>> I've seen something similar in an X-terminal.

Why would you expect a transceiver power supply to be isolated from system 
ground?  The transceiver logic ground (via the AUI connector) is tied to the 
system ground.  It is the coax that is isolated from the system ground, not the 
bulk of the transceiver circuitry.

paul

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Tony Duell wrote:
> On Sun, May 8, 2022 at 1:02 PM Maciej W. Rozycki via cctalk
>  wrote:
>
> >  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
> > wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
> > across the suspicious connections as any reference to ground may not be
> > indicative.
> >
> >  This voltage is also present with the H7819 PSU.  It's not clear to me
> > what it is used for.
> 
> It's only a guess (this is rather more modern than the DEC hardware I
> have), but it may be a supply for a thinwire ethernet transceiver
> circuit. This would be insolated from system ground of course, and
> I've seen something similar in an X-terminal.
>

Hi Tony,

After years of rediscovering the same failures over and over again,
I finally managed to get in the habit of making notes while I am
trying to diagnose faults.  Now I just need to get in the habit of
reading my old notes when I start to work on a new fault.  I did
that just now and I found that I have a MicroVAX 3100 model 76 whose
internal thinwire tranceiver failed last summer.  Not finding any
obvious reason for the failure, I replaced it with an external
tranceiver plugged into the AUI port and then forgot all about it.

I should check that this floating 9V output from it's H7821 PSU is
working correctly.

(I wonder why it is described as -9V on the label of the 30-35042-01 PSU
in my VAX 4000/100A?  If it had been shown as +/-9V for example or even
just plain old 9V, I would have been less inclined to assume it was
referenced to ground.)

Regards,
Peter.

>
> -tony
> 
> 
> 
> >
> >   Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Hi Maciej,

>
> > 
> > What is H7826 used in?  I don't think I have any of those.
>
> DECstation 5000/1xx systems and TURBOchannel Extender boxes.  Not sure if 
> anything else.
>

Ok.  I have VAXStation/MicroVAX and Alpha systems but no DECStations so that
explains why I haven't seen them.

>
> > Do you know what the white wire is for?  I originally thought it might
> > be power-good but only because the white wire on a H7816 seems to be
> > power-good.  On the 30-35042-01, it is slightly above 0V.  On my H7822
> > it is about 5.6V and on my H7821 it is 8.8V.
>
>  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow 
> wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage 
> across the suspicious connections as any reference to ground may not be 
> indicative.
>

Aha - that explains the odd and slowly varying different voltages I've
found on these leads in different machines.  It didn't dawn on me that
they could be isolated from ground.  I should have noticed that there
were two adjacent leads which behaved like this.

This should help clarify some of the puzzles I came across when I was
trying to trace the circuit of the H7821 and H7822.

Regards,
Peter.

>
>  This voltage is also present with the H7819 PSU.  It's not clear to me 
> what it is used for.
>
>  Maciej

Re: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

On Sun, 8 May 2022, Tony Duell wrote:

> >  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
> > wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
> > across the suspicious connections as any reference to ground may not be
> > indicative.
> >
> >  This voltage is also present with the H7819 PSU.  It's not clear to me
> > what it is used for.
> 
> It's only a guess (this is rather more modern than the DEC hardware I
> have), but it may be a supply for a thinwire ethernet transceiver
> circuit. This would be insolated from system ground of course, and
> I've seen something similar in an X-terminal.

 That makes sense to me.  Both the VAXstation 3100 and the DECstation 3100 
systems (which are among the users of the H7821 PSU) and the VAXstation 
4000 system (likewise the H7819 PSU) have a built-in 10BASE2 interface.  
Where these PSUs are used for storage expansion boxes some voltages are of 
course unused.

 NB I have literally just recapped an H7816 PSU where I spotted a bunch of 
Chemi-con LXF parts starting to leak (both the DEC-usual 1800uF/25V parts 
and less common 2200uF/10V and 1000uF/25V ratings) as well as a pair of 
notorious Nichicon PL parts at 4700uF/10V (though these were clean and 
from the look of the closing cap a more modern variant; I've replaced them 
for extra safety anyway).

 Thankfully the PSU still was in a working condition and it was easy to 
get it disassembled enough to get at the parts, there was virtually no 
corrosion yet, and the PSU now continues working after the recap, so it 
was quite an easy task on this occasion.  The old parts, including the 
leaking ones, still showed in-range values of capacitance and ESR, not 
unusually, which is obviously why no damage to other circuitry has been 
caused.

 Lucky me, also to think to have a look inside!

  Maciej

Re: DEC H7822 power supply

[Tony Duell via cctalk]

On Sun, May 8, 2022 at 1:02 PM Maciej W. Rozycki via cctalk
 wrote:

>  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
> wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
> across the suspicious connections as any reference to ground may not be
> indicative.
>
>  This voltage is also present with the H7819 PSU.  It's not clear to me
> what it is used for.

It's only a guess (this is rather more modern than the DEC hardware I
have), but it may be a supply for a thinwire ethernet transceiver
circuit. This would be insolated from system ground of course, and
I've seen something similar in an X-terminal.

-tony



>
>   Maciej

Re: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

Hi Peter,

> > Right, my notes indicate Nichicon PL parts might be problematic too, e.g. 
> > one at 4700uF/10V on 5V output of the H7826 PSU.
> >
> 
> What is H7826 used in?  I don't think I have any of those.

 DECstation 5000/1xx systems and TURBOchannel Extender boxes.  Not sure if 
anything else.

> >  The symptom is exactly like with my broken H7821.  Check the power-good 
> > signal (brown wire with the H7821, possibly likewise with the H7822).  It 
> > should be driven high at the TTL level.
> >
> 
> The wire in the same position as the brown one on a H7821 is purple on a H7822
> and they both seem to have the same function.  I ended up temporarily fitting
> a two 1000uF/25V capacitors on the lower board of my H7822 and reconnecting it
> to the upper board.  This gave me +5.1V on the purple wire and the CPU then
> ran ok for test purposes.
> 
> I have a VAX 4000/100A which has a Zytec Model EP 071181 power supply.  The
> only Digital reference on it is 30-35042-01.  It is the same physical form
> factor as the H7822.  The output connectors look the same, including the
> wire colours except +12V is brown instead of orange and this one has an
> extra connector for 3.3V (which is not used for anything in the 4000/100A).
> The reason I mention it is there is a list of output voltages on the label:
> +5.1V, +12.1V, +3.3V, -12V and -9V.  By a process of elimination, the -9V
> supply must be the grey wire (which gives me -8.2V when I put the meter
> on it.  All the other voltages are much closer to specification).
> 
> I wonder if the grey wire in the same position on the H7822 is also supposed
> to be -9V?  When I try to measure it on my H7822, I get a negative voltage
> but it is wandering around -4V.  In contrast, the -12V supply on the blue
> wire is a steady -11.99V, probably due to the 7912 regulator.  The same wire
> as the grey on the H7822 is yellow on a H7821.  I measured the voltage on a
> working H7821 and found a steady -0.2V which seems odd.
> 
> Do you know what the white wire is for?  I originally thought it might
> be power-good but only because the white wire on a H7816 seems to be
> power-good.  On the 30-35042-01, it is slightly above 0V.  On my H7822
> it is about 5.6V and on my H7821 it is 8.8V.

 In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow 
wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage 
across the suspicious connections as any reference to ground may not be 
indicative.

 This voltage is also present with the H7819 PSU.  It's not clear to me 
what it is used for.

  Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Hi Maciej,

>
> Right, my notes indicate Nichicon PL parts might be problematic too, e.g. 
> one at 4700uF/10V on 5V output of the H7826 PSU.
>

What is H7826 used in?  I don't think I have any of those.

>
> I can confirm now Nichicon PF 47uF/35V parts to be the source of an issue 
> with my Bel Power.  All four leaked.  Thankfully I was able to fully 
> revive that PSU (now in 24/7 operation).
>

I got replacement capacitors for my POWER-ONE unit but I haven't managed
to obtain a replacement IRFPE50 chopper transistor yet.  I found a sort-of
local supplier that lists them but they have a EUR 30 minimum order and
they don't stock much other stuff I need or want :-(

>
> These are likely standard type parts.  I've yet to come across a low 
> impedance axial capacitor type.  I may have missed something of course.
>

I couldn't find any suitable axial replacements so I ended up using radials
which were roughly the same height as the width of the axials.

>
> >
> > > Heat dissipated by the cap itself under high ripple current never helps 
> > > and will surely speed up cap deterioration.  After all its service life 
> > > halvens with each 10°C temperature rise even with non-faulty parts.
> > >
> > 
> > When I was shopping for replacements, I was a bit alarmed to find that
> > the maximum specified "endurance" (whatever that is) I could find was
> > 5000 hours.  This isn't much more than a long life incandescent light bulb.
> 
>  Mind that it's at 105°C.  If you keep such caps operating at 65°C (which 
> is still rather hot), then endurance raises to 8h (~9 years continuous 
> use).
> 
>  Anyway try to chase replacements specified for at least 1h at 105°C.
> Nichicon HE/UHE and Panasonic FR seem suitable replacements for Chemi-con 
> SXF, Nichicon PL, surpassing old parts in terms of ESR/impedance/ripple 
> and dimension-wise.
>

When I was trying to order them, I found it hard to get anything suitable.
I ended up ordering 15 (out of 16 in stock at Mouser) Panasonic FC series
to do just three PSUs.

>
> I don't have any H7822 PSU.  Your experience with the H7821 is the same 
> as mine though (and I still need to figure out what's wrong with one which 
> still doesn't drive its power-good line active after recapping).
>

There is a 464 or 560 Ohm resistor near the capacitor in middle of the H7821
which has been damaged by leakage in several of my PSUs.  It is in series
with a thermistor mounted on a heatsink and as far as I recall it is part
of the fan control circuit.  It might also contribute to power-good though.
Other components could be damaged too or there could be goo lying under
some components.

>
> I used 
> Nichicon HE P/N UHE1E182MHD as the replacement for those.  There does 
> appear to be COVID-related shortage of this part (600 expected at Mouser 
> 15/03/2023, ugh!), which used to be readily available in large quantities 
> several years ago.  However Panasonic FR P/N EEUFR1E182 is available in a 
> small quantity (and is better).

I should grab some EEUFR1E182 before they disappear.

>
> I came across LXF parts in one H7826 only and they were clean, but I 
> chose to replace them as a precaution anyway as I've got stuck with trying 
> to repair a couple of broken H7826 PSUs already still not working after 
> cleaning the mess and replacing broken caps (mind that I'm a software 
> engineer with enough hassle to sort out on the software side already).
>

Sounds like it was better to not take a chance on them.

>
> I've seen leaks from SXF parts on the primary side too with the H7826, so 
> it is not that they only fail on the secondary side.  Also it is lower 
> frequency ripple that's more problematic, see frequency correction factors 
> for ripple current in datasheets, because impedance is higher at lower 
> frequencies.  That's an inherent property of capacitance.
>

That makes sense.

>
> > I replaced the five leaking capacitors on the upper board in my H7822,
> > disconnected the input to the lower board and moved the LED connection to
> > the upper board.  The machine powered up nicely afterwards, the fans spun
> > and the green LED came on after a short delay.  However, the diagnostic
> > LEDs all come on and stay on so it appears the CPU is being made aware that
> > the lower board is not functioning, even though I don't need it.  I should
> > to do some comparisons with a H7821 and see if I can work around this.  Or
> > maybe I could fit some not quite to specification electrolytics from the
> > junk box on the lower board, just to keep it happy?
> 
>  The symptom is exactly like with my broken H7821.  Check the power-good 
> signal (brown wire with the H7821, possibly likewise with the H7822).  It 
> should be driven high at the TTL level.
>

The wire in the same position as the brown one on a H7821 is purple on a H7822
and they both seem to have the same function.  I ended up temporarily fitting
a two 1000uF/25V capacitors on the lower board of my H7822 and reconnecting it

Re: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

Hi Peter,

> I still have the leaky electrolytics I removed from the POWER-ONE PSU in my
> Cisco IGS a while back.  I stored them with their leads up and goo seems to
> be still oozing out of some of them despite their inactivity and orientation.
> These ones are marked Nichicon PL(M) 4700uF/63V, 2200uF/16V and 330uF/35V and
> also have markings like H8950, H9018 and H8946 - maybe these are date codes?

 Right, my notes indicate Nichicon PL parts might be problematic too, e.g. 
one at 4700uF/10V on 5V output of the H7826 PSU.

> I also removed the smaller ones like 47uF/35V PF(M) H8952 for example but it
> is less clear to me whether these were leaking too or just got leaked on by
> the others.  There were only a few of them so I decided they were better out
> than in.  They all have similar coloured brown sleeves like the faulty ones
> in the DEC power supplies too.

 I can confirm now Nichicon PF 47uF/35V parts to be the source of an issue 
with my Bel Power.  All four leaked.  Thankfully I was able to fully 
revive that PSU (now in 24/7 operation).

> There are also the leaky 10uF/35V axial electrolytics in my LK201 keyboard.
> Those are in orange sleeves and marked "ESZ", whatever that is.  They have
> date codes like 8612.  I thought this might be a widespread problem but
> so far I have only found it in one keyboard.

 These are likely standard type parts.  I've yet to come across a low 
impedance axial capacitor type.  I may have missed something of course.

> Looks like I need to go back and recheck everything I thought wasn't leaking
> last time I checked :-(
> 
> Thing is, to check them, they have to come out of the case and that involves
> at least some change in orientation, except for contortionists...

 I doubt a temporary reorientation of parts you want to replace anyway is 
going to cause any trouble in that short amount of time.

> > Heat dissipated by the cap itself under high ripple current never helps 
> > and will surely speed up cap deterioration.  After all its service life 
> > halvens with each 10°C temperature rise even with non-faulty parts.
> >
> 
> When I was shopping for replacements, I was a bit alarmed to find that
> the maximum specified "endurance" (whatever that is) I could find was
> 5000 hours.  This isn't much more than a long life incandescent light bulb.

 Mind that it's at 105°C.  If you keep such caps operating at 65°C (which 
is still rather hot), then endurance raises to 8h (~9 years continuous 
use).

 Anyway try to chase replacements specified for at least 1h at 105°C.
Nichicon HE/UHE and Panasonic FR seem suitable replacements for Chemi-con 
SXF, Nichicon PL, surpassing old parts in terms of ESR/impedance/ripple 
and dimension-wise.

> All the leaking ones in DEC H7821 and H7822 PSUs I have come across so far
> are 1800uF/25V Chemicon with brown sleeves.  I think mine are all SXF
> but I am not 100% sure of that.  There are lots of other electrolytic
> capacitors in these power supplies but I've only looked closely at the
> larger ones.  All of the reservoir capacitors attached to the mains bridge
> rectifiers that I have seen look fine.  Maybe I need to go back and check
> the smaller ones though :-(

 I don't have any H7822 PSU.  Your experience with the H7821 is the same 
as mine though (and I still need to figure out what's wrong with one which 
still doesn't drive its power-good line active after recapping).  I used 
Nichicon HE P/N UHE1E182MHD as the replacement for those.  There does 
appear to be COVID-related shortage of this part (600 expected at Mouser 
15/03/2023, ugh!), which used to be readily available in large quantities 
several years ago.  However Panasonic FR P/N EEUFR1E182 is available in a 
small quantity (and is better).

> I think I came across some LXF ones that seemed to be ok, I can't remember
> where though.  I probably need to go find these and check them again :-(

 I came across LXF parts in one H7826 only and they were clean, but I 
chose to replace them as a precaution anyway as I've got stuck with trying 
to repair a couple of broken H7826 PSUs already still not working after 
cleaning the mess and replacing broken caps (mind that I'm a software 
engineer with enough hassle to sort out on the software side already).

> Here's a thought.  Apart from the keyboard, all the ones I have seen that
> are leaking are filtering the outputs of switch mode power supplies.  I
> wonder does the higher frequency of the ripple they are dealing with have
> a bearing on this?

 I've seen leaks from SXF parts on the primary side too with the H7826, so 
it is not that they only fail on the secondary side.  Also it is lower 
frequency ripple that's more problematic, see frequency correction factors 
for ripple current in datasheets, because impedance is higher at lower 
frequencies.  That's an inherent property of capacitance.

> I replaced the five leaking capacitors on the upper board in my H7822,
> disconnected the input to the lower 

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Hi Maciej,

Thanks for your input.

"Maciej W. Rozycki" wrote:
>
> Lying on a side would also permit leaking, I've seen an H7821 damaged in 
> storage that way.  Gravity only helps with the leads up.
>

I have lots of H7821s that were on their side for a few years and leaked a
lot but I also have H7822s that were stored the same way and did not show
any evidence of leaking until very recently.  I think I also have some
H7821s that were the right way up most of their lives and have leaked a
little.  It's getting hard to remember what happened to which :-(

>
> From experience Chemi-con SXF caps used with many DEC PSUs need to be 
> urgently replaced.  Other Chemi-con lines reported affected are LXF, SXE 
> and KME.  Products of the time from other manufacturers may be affected as 
> well.  I'd have to check what line were those that leaked in a Bel Power 
> PSU that I had to fix (I reckon you had a similar experience, right?).
>

I still have the leaky electrolytics I removed from the POWER-ONE PSU in my
Cisco IGS a while back.  I stored them with their leads up and goo seems to
be still oozing out of some of them despite their inactivity and orientation.
These ones are marked Nichicon PL(M) 4700uF/63V, 2200uF/16V and 330uF/35V and
also have markings like H8950, H9018 and H8946 - maybe these are date codes?
I also removed the smaller ones like 47uF/35V PF(M) H8952 for example but it
is less clear to me whether these were leaking too or just got leaked on by
the others.  There were only a few of them so I decided they were better out
than in.  They all have similar coloured brown sleeves like the faulty ones
in the DEC power supplies too.

There are also the leaky 10uF/35V axial electrolytics in my LK201 keyboard.
Those are in orange sleeves and marked "ESZ", whatever that is.  They have
date codes like 8612.  I thought this might be a widespread problem but
so far I have only found it in one keyboard.

>
> It was the composition of the electrolyte that was outright wrong, so I 
> doubt it's batch-related.
>

>
> Once the seal has broken I guess all odds are off.  I could imagine 
> capillary action to take effect.
>

Looks like I need to go back and recheck everything I thought wasn't leaking
last time I checked :-(

Thing is, to check them, they have to come out of the case and that involves
at least some change in orientation, except for contortionists...

>
> Heat dissipated by the cap itself under high ripple current never helps 
> and will surely speed up cap deterioration.  After all its service life 
> halvens with each 10°C temperature rise even with non-faulty parts.
>

When I was shopping for replacements, I was a bit alarmed to find that
the maximum specified "endurance" (whatever that is) I could find was
5000 hours.  This isn't much more than a long life incandescent light bulb.

>
> As a matter of interest what capacitance/voltage are those?  Are they of 
> the Chemi-con/SXF type too?
>

All the leaking ones in DEC H7821 and H7822 PSUs I have come across so far
are 1800uF/25V Chemicon with brown sleeves.  I think mine are all SXF
but I am not 100% sure of that.  There are lots of other electrolytic
capacitors in these power supplies but I've only looked closely at the
larger ones.  All of the reservoir capacitors attached to the mains bridge
rectifiers that I have seen look fine.  Maybe I need to go back and check
the smaller ones though :-(

I think I came across some LXF ones that seemed to be ok, I can't remember
where though.  I probably need to go find these and check them again :-(

Here's a thought.  Apart from the keyboard, all the ones I have seen that
are leaking are filtering the outputs of switch mode power supplies.  I
wonder does the higher frequency of the ripple they are dealing with have
a bearing on this?


I replaced the five leaking capacitors on the upper board in my H7822,
disconnected the input to the lower board and moved the LED connection to
the upper board.  The machine powered up nicely afterwards, the fans spun
and the green LED came on after a short delay.  However, the diagnostic
LEDs all come on and stay on so it appears the CPU is being made aware that
the lower board is not functioning, even though I don't need it.  I should
to do some comparisons with a H7821 and see if I can work around this.  Or
maybe I could fit some not quite to specification electrolytics from the
junk box on the lower board, just to keep it happy?

Regards,
Peter Coghlan.

>
>  Maciej
>

Re: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

On Tue, 3 May 2022, Peter Coghlan via cctalk wrote:

> > Would the system have been possibly stored upside down sometime?
> >
> 
> I don't think so.  It may have spent some time lying on it's side due to
> deteriorating rubber feet and for ease of access and but I can't see any
> reason for it ever being upside down.  Once I discovered this issue a few
> years ago, I checked all my power supplies, removed any leaking capacitors
> and changed to storing the machines the right way up in a vertical stack,
> with newspapers between them in place of the rubber feet.  I hoped this would
> prevent any capacitors which hadn't shown any signs of leaking from starting
> to leak.  (Now whichever machine I want to work on always seems to be at the
> bottom of the stack...)

 Lying on a side would also permit leaking, I've seen an H7821 damaged in 
storage that way.  Gravity only helps with the leads up.

> >  These nasty caps do leak even while in storage (and even if never used, 
> > not even soldered ever, according to one source, a repair professional).
> >
> 
> They are nasty and devious.  In my case, the ones that have been stored in
> any orientation but not used much seem to have fared better.  I only started
> using this machine with the H7822 for extended periods for the first time a
> few months ago.  One of the reasons I started using it more is because I
> thought it was immune to the leaky capacitor problem!  It never saw any
> serious use before that, even when it was new.

 From experience Chemi-con SXF caps used with many DEC PSUs need to be 
urgently replaced.  Other Chemi-con lines reported affected are LXF, SXE 
and KME.  Products of the time from other manufacturers may be affected as 
well.  I'd have to check what line were those that leaked in a Bel Power 
PSU that I had to fix (I reckon you had a similar experience, right?).

> I have at least two machines with H7822 power supplies.  Even though they
> have capacitors that look the same as the ones in the H7821, the ones in
> the H7822 power supplies didn't seem to be showing any signs of leaking
> when I examined them some time ago so I thought they might be from a batch
> that was unaffected by the problem.  It seems that this was not true :-(

 It was the composition of the electrolyte that was outright wrong, so I 
doubt it's batch-related.

> I unsoldered the other eight similar capacitors (four one each board) from
> the H7822 yesterday evening.  I found a small amount of leakage under most
> of them but it was was only evident after they were removed from the board.

 Yes, it's been a common case.

> In general, there was less damage visible under the ones on the lower board
> with leads facing down oddly enough.  I thought one of the capacitors from
> the upper board had not leaked at all.  I left the removed capacitors
> standing on the bench overnight with their leads upwards and they all have
> some signs of leakage visible on them today.  It's hard to draw any
> conclusions.

 Once the seal has broken I guess all odds are off.  I could imagine 
capillary action to take effect.

> The capacitor from the upper board that leaked enough for me to notice it
> might be under greater stress when operating than the others.  I think the
> same capacitor in the H7821 power supplies seems to leak more in those too.

 Heat dissipated by the cap itself under high ripple current never helps 
and will surely speed up cap deterioration.  After all its service life 
halvens with each 10°C temperature rise even with non-faulty parts.

> I don't have enough spare capacitors to replace the four on the lower
> board.  I am going to leave those out, leave the input lead to that board
> unplugged and plug the green LED into the upper board.  I hope it will then
> behave just like a H7821.  There won't be any power to the front disk drive
> connector but I am not using that so it doesn't matter unless the "power good"
> output is affected, in which case I will have to think of something else.

 As a matter of interest what capacitance/voltage are those?  Are they of 
the Chemi-con/SXF type too?

  Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

"Maciej W. Rozycki" wrote:
>On Mon, 2 May 2022, Peter Coghlan via cctalk wrote:
>
>> Thankfully, it doesn't seem to have been there long and didn't get a chance
>> to spread around the board.  Bizarrely this capacitor has it's legs pointing
>> upwards and managed to leak while there are similar capacitors on the other
>> board with their legs pointing downwards which don't seem to have leaked 
>> (yet?).
>
> Would the system have been possibly stored upside down sometime?
>

I don't think so.  It may have spent some time lying on it's side due to
deteriorating rubber feet and for ease of access and but I can't see any
reason for it ever being upside down.  Once I discovered this issue a few
years ago, I checked all my power supplies, removed any leaking capacitors
and changed to storing the machines the right way up in a vertical stack,
with newspapers between them in place of the rubber feet.  I hoped this would
prevent any capacitors which hadn't shown any signs of leaking from starting
to leak.  (Now whichever machine I want to use always seems to be at the
bottom of the stack...)

>
>  These nasty caps do leak even while in storage (and even if never used, 
> not even soldered ever, according to one source, a repair professional).
>

In my case, the ones that have been stored in any orientation but not used
much seem to have fared better.  I only started using this machine with the
H7822 for extended periods for the first time a few months ago.  One of the
reasons I started using it more is because I thought it was immune to the
leaky capacitor problem!  It never saw any serious use before that, even
when it was new.

I have at least two machines with H7822 power supplies.  Even though they
have capacitors that look the same as the ones in the H7821, the ones in
the H7822 power supplies didn't seem to be showing any signs of leaking
when I examined them some time ago so I thought they might be from a batch
that was unaffected by the problem.  It seems that this was not the case :-(

I unsoldered the eight other similar capacitors (four one each board) from
the H7822 yesterday evening.  I found a small amount of leakage under most
of them but it was was only evident after they were removed from the board.
In general, there was less damage visible under the ones on the lower board
with leads facing down oddly enough.  I thought one of the capacitors from
the upper board had not leaked at all but I left them standing on the bench
overnight with their leads upwards and they all have some signs of leakage
visible on them today.

The capacitor from the upper board that leaked enough for me to notice it
might be under greater stress when operating than the others.  I think the
same capacitor in the H7821 power supplies seems to leak more in those too.
Maybe it carries greater ripple current than the others or something like
that?  I can see that the other four similar capacitors make up PI filters
for the +5V and +12V outputs along with a pair of smoothing chokes, however
I can't quite figure out what the more leaky one is responsible for.  It
could be associated with the -12V output which might also be the supply
for the fans.  This part of the circuit is very hard to trace.

I don't have enough spare capacitors to replace the four on the lower
board.  I am going to leave those out, leave the input lead to that board
unplugged and plug the green LED into the upper board.  I hope it will then
behave just like a H7821.  There won't be any power to the front disk drive
connector but I am not using that so it doesn't matter unless the "power good"
output is affected, in which case I will have to think of something else.

I am hoping that the leaky capacitors were responsible for the odd
intermittent failures I experienced with this power supply recently.

Regards,
Peter Coghlan.

>  Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

"Maciej W. Rozycki" wrote:
>On Mon, 2 May 2022, Peter Coghlan via cctalk wrote:
>
>> Thankfully, it doesn't seem to have been there long and didn't get a chance
>> to spread around the board.  Bizarrely this capacitor has it's legs pointing
>> upwards and managed to leak while there are similar capacitors on the other
>> board with their legs pointing downwards which don't seem to have leaked 
>> (yet?).
>
> Would the system have been possibly stored upside down sometime?
>

I don't think so.  It may have spent some time lying on it's side due to
deteriorating rubber feet and for ease of access and but I can't see any
reason for it ever being upside down.  Once I discovered this issue a few
years ago, I checked all my power supplies, removed any leaking capacitors
and changed to storing the machines the right way up in a vertical stack,
with newspapers between them in place of the rubber feet.  I hoped this would
prevent any capacitors which hadn't shown any signs of leaking from starting
to leak.  (Now whichever machine I want to work on always seems to be at the
bottom of the stack...)

>
>  These nasty caps do leak even while in storage (and even if never used, 
> not even soldered ever, according to one source, a repair professional).
>

They are nasty and devious.  In my case, the ones that have been stored in
any orientation but not used much seem to have fared better.  I only started
using this machine with the H7822 for extended periods for the first time a
few months ago.  One of the reasons I started using it more is because I
thought it was immune to the leaky capacitor problem!  It never saw any
serious use before that, even when it was new.

I have at least two machines with H7822 power supplies.  Even though they
have capacitors that look the same as the ones in the H7821, the ones in
the H7822 power supplies didn't seem to be showing any signs of leaking
when I examined them some time ago so I thought they might be from a batch
that was unaffected by the problem.  It seems that this was not true :-(

I unsoldered the other eight similar capacitors (four one each board) from
the H7822 yesterday evening.  I found a small amount of leakage under most
of them but it was was only evident after they were removed from the board.
In general, there was less damage visible under the ones on the lower board
with leads facing down oddly enough.  I thought one of the capacitors from
the upper board had not leaked at all.  I left the removed capacitors
standing on the bench overnight with their leads upwards and they all have
some signs of leakage visible on them today.  It's hard to draw any
conclusions.

The capacitor from the upper board that leaked enough for me to notice it
might be under greater stress when operating than the others.  I think the
same capacitor in the H7821 power supplies seems to leak more in those too.
Maybe it carries greater ripple current than the others or something like
that?  I can see that the other four similar capacitors make up PI filters
for the +5V and +12V outputs along with a pair of smoothing chokes, however
I can't quite figure out what the more leaky one is responsible for.  It
could be associated with the -12V output which might also be the supply
for the fans.  This part of the circuit is very hard to trace.

I don't have enough spare capacitors to replace the four on the lower
board.  I am going to leave those out, leave the input lead to that board
unplugged and plug the green LED into the upper board.  I hope it will then
behave just like a H7821.  There won't be any power to the front disk drive
connector but I am not using that so it doesn't matter unless the "power good"
output is affected, in which case I will have to think of something else.

I am hoping that the leaky capacitors were responsible for the odd
intermittent failures I experienced with this power supply recently.

Regards,
Peter Coghlan.

>  Maciej

Re: DEC H7822 power supply

[Maciej W. Rozycki via cctalk]

On Mon, 2 May 2022, Peter Coghlan via cctalk wrote:

> Thankfully, it doesn't seem to have been there long and didn't get a chance
> to spread around the board.  Bizarrely this capacitor has it's legs pointing
> upwards and managed to leak while there are similar capacitors on the other
> board with their legs pointing downwards which don't seem to have leaked 
> (yet?).

 Would the system have been possibly stored upside down sometime?

 These nasty caps do leak even while in storage (and even if never used, 
not even soldered ever, according to one source, a repair professional).

  Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

I wrote:
>
> At least the H7822 does not seem to suffer from the leaky capacitor problem
> like the H7821 does.
>

Well, it's made a liar of me already.  While checking over my previous work
replacing the zener diode to make sure I didn't break something else, my
eye was drawn to a ring of sticky brown goo around the base of one of
the capacitors that looks the same as the ones that leak in the H7821.
Thankfully, it doesn't seem to have been there long and didn't get a chance
to spread around the board.  Bizarrely this capacitor has it's legs pointing
upwards and managed to leak while there are similar capacitors on the other
board with their legs pointing downwards which don't seem to have leaked (yet?).

When I compare them side by side, I can now see that the upper board in
the H7822 is identical to the single board in the H7821.  It seems that
the lower board in the H7822 is only responsible for powering the front
disk drive connector which I found to be independent of the other supplies.

Regards,
Peter Coghlan.