Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

söndag 19 april 2020 skrev Chris Zach via cctalk :

> Well, let's see: I took the bottom off the VT52 and checked the
> connections to the high voltage board on the back. Cleaned up both
> connectors, plugged in, same problem.
>
> I do hear a medium to high pitched squeal coming from the back board.
> Can't quite triangulate it (I need a hose and some time) but it might be
> coming from those center transistors. Any idea what these boards are doing,
> and if there is a schematic for these circuits?


The left is the video /CRT part. The right is the power supply part. The
transistors are mostly regulator transistors.

I have linked to both maintenance manual and schematic from the page I
referred to in one of my previous replies to this thread.
http://www.datormuseum.se/home/dec-vt5x

Take a look there!

/Mattis



> Given that it went weird after being powered on for awhile I would suspect
> it has something to do with a component that generates heat. I could also
> take an IR picture of the back board and see if anything is unusually
> hot/not hot.
>
> Audio of the noise: https://i.imgur.com/ps4Ovkj.mp4
> Pic of the back: https://i.imgur.com/HgNoTYK.jpg
>
> C
>

RE: Great, my VT52 is shot.

[Wayne Sudol via cctalk]

Here's a link to the vt52 maintenance manual. Has testing and service info. 
Hope it helps.

 
http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/terminal/vt52/EK-VT52-MM-002_maint_Jul78.pdf

Wayne

-Original Message-
From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Chris Zach via 
cctalk
Sent: Saturday, April 18, 2020 3:56 PM
Cc: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Great, my VT52 is shot.

Well, let's see: I took the bottom off the VT52 and checked the connections to 
the high voltage board on the back. Cleaned up both connectors, plugged in, 
same problem.

I do hear a medium to high pitched squeal coming from the back board. 
Can't quite triangulate it (I need a hose and some time) but it might be coming 
from those center transistors. Any idea what these boards are doing, and if 
there is a schematic for these circuits?

Given that it went weird after being powered on for awhile I would suspect it 
has something to do with a component that generates heat. I could also take an 
IR picture of the back board and see if anything is unusually hot/not hot.

Audio of the noise: https://i.imgur.com/ps4Ovkj.mp4 Pic of the back: 
https://i.imgur.com/HgNoTYK.jpg

C

Re: Unusual Punched Cards

[Paul Koning via cctalk]

> On Apr 18, 2020, at 12:51 PM, Peter Van Peborgh via cctech 
>  wrote:
> 
> 
> Guys,
> 
> I have been collecting all types of computer punched cards for a few years
> now. In the distant past I actually used them!
> 
> I am desperately looking for two types I do not have been able to find a
> sample of:
> * Jacquard fabric/carpet loom cards. OK, not computer but the
> ancestor, for completeness of the history. There may be several formats of
> these.

Did you already get a street organ "book"?  Similar to punched cards in a way, 
though semi-analog (the time element is analog).

paul

Re: Unusual Punched Cards

[William Donzelli via cctalk]

> I am desperately looking for two types I do not have been able to find a
> sample of:
> *   Jacquard fabric/carpet loom cards. OK, not computer but the
> ancestor, for completeness of the history. There may be several formats of
> these.

I donated my Jacquard punch to Techworks in Binghamton NY. I forget
what the format is. You might ask them. It is likely I will not be
there for a while

I did a video (on Youtube). I think it is called "A Most Unusual Card
Punch" or something like that.

--
Will


--
Will

Re: Unusual Punched Cards (Peter Van Peborgh)

[brianb1224 via cctalk]

There is a lady who collects them also plus the wooden blooms usually at the 
renaissance festivals. Sorry I forgot her name.Sent from my Verizon, Samsung 
Galaxy smartphone
 Original message From: Michael Thompson via cctech 
 Date: 4/18/20  2:14 PM  (GMT-06:00) To: cctech 
 Subject: Re: Unusual Punched Cards (Peter Van Peborgh) 
>> Guys,>> I have been collecting all types of computer punched cards for a few 
years> now. In the distant past I actually used them!>> I am desperately 
looking for two types I do not have been able to find a> sample of:> *   
Jacquard fabric/carpet loom cards. OK, not computer but the> ancestor, for 
completeness of the history. There may be several formats of> these.>>Something 
like 
this?http://www.ricomputermuseum.org/Home/interesting_computer_items/jacquard-loom-cards--
 Michael Thompson

Re: Unusual Punched Cards (Peter Van Peborgh)

[Michael Thompson via cctalk]

>
> Guys,
>
> I have been collecting all types of computer punched cards for a few years
> now. In the distant past I actually used them!
>
> I am desperately looking for two types I do not have been able to find a
> sample of:
> *   Jacquard fabric/carpet loom cards. OK, not computer but the
> ancestor, for completeness of the history. There may be several formats of
> these.
>
>
Something like this?
http://www.ricomputermuseum.org/Home/interesting_computer_items/jacquard-loom-cards


-- 
Michael Thompson

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Well, let's see: I took the bottom off the VT52 and checked the 
connections to the high voltage board on the back. Cleaned up both 
connectors, plugged in, same problem.


I do hear a medium to high pitched squeal coming from the back board. 
Can't quite triangulate it (I need a hose and some time) but it might be 
coming from those center transistors. Any idea what these boards are 
doing, and if there is a schematic for these circuits?


Given that it went weird after being powered on for awhile I would 
suspect it has something to do with a component that generates heat. I 
could also take an IR picture of the back board and see if anything is 
unusually hot/not hot.


Audio of the noise: https://i.imgur.com/ps4Ovkj.mp4
Pic of the back: https://i.imgur.com/HgNoTYK.jpg

C

RE: Another Unrelated PSU Question

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Brent Hilpert
via
> cctalk
> Sent: 18 April 2020 18:58
> To: General Discussion: On-Topic and Off-Topic Posts

> Subject: Re: Another Unrelated PSU Question
> 
> On 2020-Apr-18, at 10:03 AM, Rob Jarratt via cctalk wrote:
> > I have another PSU I have been meaning to look at for a long time.
> > This one has fairly high output ripple and some of the voltages do not
> > appear to be where they should be. I have checked all the capacitors
> > for ESR and they appear to be OK, with the exception of the two big
> > smoothing capacitors on the primary side. One of them appears to be
> > slightly bulging, but has low-ish ESR, the other has a much higher
> > ESR. Is it possible that these capacitors could be the cause of the
out-of-spec
> outputs?
> 
> What frequency is the ripple - mains frequency (50/60Hz) or switching
> frequency?

It looked like switching frequency to me. I think I should replace the two
primary capacitors anyway though.

> 
> If the ripple is at mains frequency then, yes, the problem could be the
> mains/primary filtering. (Primary supply drooping far enough across mains
> cycles that the switcher is affected and unable to maintain drive.)
> 
> Either way, you could scope the primary supply under operation (some
load), to
> see what the ripple there looks like.

I daren't put it under load in the machine itself because some of the
outputs are too far out of tolerance and it has damaged some memory modules.
I can use the load module I have from a MicroVAX 2000, but for the other
outputs I may try to lash up some resistors perhaps.

If anyone has a MicroVAX 3100 Model 95, I would really like to know which
wires on the connector are which voltage, because some seem to be too far
out to identify the nominal voltage.

Regards

Rob

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

> We had a VT52 do something akin to that once back in the day... The tech
> we had that worked on this stuff said he had to fix something in the
> vertical refresh circuit to achieve vertical hold... My vague memories
> likely won't help you much, but this is quite distinctive and others of the
> group will be able to suggest things :)
>

Vertical hold implies that there is an oscillator that need to get into
hold or lock with an incoming signal.  There is none in a VT52 so that is
not possible. There is simply no way to adjust vertical hold in a VT52.

/Mattis


> Warner
>
>
>>
>> C
>>
>> On 4/18/2020 4:45 AM, Mattis Lind wrote:
>> > Hello Chris!
>> >
>> > Sorry that your trusty VT52 failed. But it shouldn't be too hard to
>> > repair it!
>> >
>> > I have successfully restored a VT52 (actually a VT50 with a brain
>> > transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is
>> > that the capacitors were in good shape. Especially the bigger filter
>> > capacitor. I think that DEC used great quality capacitors.
>> >
>> > It is always a good idea to check the voltages so that they are within
>> > the spec. But it should not be necessary to replace all the capacitors
>> > in my opinion.
>> >
>> > The VT52 is a quite interesting design with a very simplistic CPU built
>> > from TTL components and a microprogram that has 1024 by 8 bits. Most of
>> > the operations inside it is controlled from this program. In mine the
>> > sockets for the mikroprogram were bad which meant that it failed to do
>> > anything. Then I had a couple of gates that were bad.
>> >
>> > Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk
>> > mailto:cctalk@classiccmp.org>>:
>> >
>> > Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
>> > running for a day or two. Now the screen is filled with snow and it
>> > looks like the text is all over the place horozontally.
>> >
>> >
>> > It would be great to have a picture since it could give some hints on
>> > what is wrong with it. My understanding is that the scan is working
>> both
>> > horizontally and vertically. Otherwise you would only have a straight
>> > line. Unlike many common CRTs this CRT circuitry has no oscillator in
>> it
>> > that creates a picture without input. And since the horisontal
>> > deflection circuit also creates the high voltage there would be no
>> > picture at all unless the horisontal signal is there.
>> >
>> > This means that the two control boards in the bottom creates some kind
>> > of horisontal and vertical signals. These signals comes from a simple
>> > divider chain. There could be some kind of problem in the divider
>> chain.
>> > But then it would be more likely to not work at all if one of the
>> > counters are bad. But it is a good idea to check the H and V signal som
>> > they are inline with the spec. Around 15kHz and 60 or 50 Hz
>> respectively.
>> >
>> > Since I haven't seen how the output looks like I speculating a bit. It
>> > could be the case that the divider chain is correct but the video
>> signal
>> > is not generated in sync with the divider chain. Then the characters
>> > would end up all over the place on screen.
>> >
>> > There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is
>> > the video flip-flop. It controls when the screen is rendered. It might
>> > be the case it could cause your problem. I think it is a  good idea to
>> > check the signals around this flip-flop.
>> >
>> > I am always a bit skeptical towards 74Hxx. My impression is that they
>> > fail more often than standard 74xx.
>> >
>> > When I repaired my VT52 I made a quick circuit that created a composite
>> > video signal from the control board. The signal was fed into a standard
>> > CRT monitor which I had around.  I could then run the control boards
>> > flat on the bench to take measurements rather than assembling prior to
>> > each test. I thought that it was quite handy.
>> >
>> >
>> > Any tips or thoughts on where to start looking to fix? The keyboard
>> > seems to be working as does the RS232 input (the snow on the screen
>> > changes when the pdp11 talks to it)
>> >
>> >
>> > This indicate that since you are using RS232 you have at least some
>> kind
>> > of voltage level on +12 V and -12 V since otherwise there would be no
>> > communication. And +5V is probably not that bad either. Since not much
>> > would operate in that case. Likewise the -5V is probably ok since the
>> > char rom would not work very well without the bias.
>> >
>> > Good luck with your repair!
>> >
>> > /Mattis
>> >
>> >
>> > Thanks!
>> > Chris
>> >
>>
>

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

lördag 18 april 2020 skrev Chris Zach :

> Here's a video of it with sound on.
>
> https://i.imgur.com/X1qVYGP.mp4


You have video even in the horisontal retrace. I still think you have a
problem somewhere around the video flip flop. Check that part of the
circuit.

/Mattis


>
> C
>
> On 4/18/2020 4:45 AM, Mattis Lind wrote:
>
>> Hello Chris!
>>
>> Sorry that your trusty VT52 failed. But it shouldn't be too hard to
>> repair it!
>>
>> I have successfully restored a VT52 (actually a VT50 with a brain
>> transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is
>> that the capacitors were in good shape. Especially the bigger filter
>> capacitor. I think that DEC used great quality capacitors.
>>
>> It is always a good idea to check the voltages so that they are within
>> the spec. But it should not be necessary to replace all the capacitors in
>> my opinion.
>>
>> The VT52 is a quite interesting design with a very simplistic CPU built
>> from TTL components and a microprogram that has 1024 by 8 bits. Most of the
>> operations inside it is controlled from this program. In mine the sockets
>> for the mikroprogram were bad which meant that it failed to do anything.
>> Then I had a couple of gates that were bad.
>>
>> Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk <
>> cctalk@classiccmp.org >:
>>
>> Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
>> running for a day or two. Now the screen is filled with snow and it
>> looks like the text is all over the place horozontally.
>>
>>
>> It would be great to have a picture since it could give some hints on
>> what is wrong with it. My understanding is that the scan is working both
>> horizontally and vertically. Otherwise you would only have a straight line.
>> Unlike many common CRTs this CRT circuitry has no oscillator in it that
>> creates a picture without input. And since the horisontal
>> deflection circuit also creates the high voltage there would be no picture
>> at all unless the horisontal signal is there.
>>
>> This means that the two control boards in the bottom creates some kind of
>> horisontal and vertical signals. These signals comes from a simple divider
>> chain. There could be some kind of problem in the divider chain. But then
>> it would be more likely to not work at all if one of the counters are bad.
>> But it is a good idea to check the H and V signal som they are inline with
>> the spec. Around 15kHz and 60 or 50 Hz respectively.
>>
>> Since I haven't seen how the output looks like I speculating a bit. It
>> could be the case that the divider chain is correct but the video signal is
>> not generated in sync with the divider chain. Then the characters would end
>> up all over the place on screen.
>>
>> There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is
>> the video flip-flop. It controls when the screen is rendered. It might be
>> the case it could cause your problem. I think it is a  good idea to check
>> the signals around this flip-flop.
>>
>> I am always a bit skeptical towards 74Hxx. My impression is that they
>> fail more often than standard 74xx.
>>
>> When I repaired my VT52 I made a quick circuit that created a composite
>> video signal from the control board. The signal was fed into a standard CRT
>> monitor which I had around.  I could then run the control boards flat on
>> the bench to take measurements rather than assembling prior to each test. I
>> thought that it was quite handy.
>>
>>
>> Any tips or thoughts on where to start looking to fix? The keyboard
>> seems to be working as does the RS232 input (the snow on the screen
>> changes when the pdp11 talks to it)
>>
>>
>> This indicate that since you are using RS232 you have at least some kind
>> of voltage level on +12 V and -12 V since otherwise there would be no
>> communication. And +5V is probably not that bad either. Since not much
>> would operate in that case. Likewise the -5V is probably ok since the char
>> rom would not work very well without the bias.
>>
>> Good luck with your repair!
>>
>> /Mattis
>>
>>
>> Thanks!
>> Chris
>>
>>

Unusual Punched Cards

[Peter Van Peborgh via cctalk]

Guys,

I have been collecting all types of computer punched cards for a few years
now. In the distant past I actually used them!

I am desperately looking for two types I do not have been able to find a
sample of:
*   Jacquard fabric/carpet loom cards. OK, not computer but the
ancestor, for completeness of the history. There may be several formats of
these.
*   Original Hollerith card, 12 rows by 24 columns. Once again, this is
a long shot.

Well if you don't ask, you never get! Thank you for your interest. Peter

Re: VAXmate PSU fixed, but no video

[Peter Coghlan via cctalk]

>
> There were two VT220 designs (I think), using either an onboard or
> offboard flyback. The part number of the onboard flyback is 16-26299-01,
> and there are some available on eBay if you are feeling rich.
>

I don't want to hijack Rob's thread too much :-)

I actually have one of each design and the one with the offboard transformer
(I think) works. The board layouts are different but the components seem
to be the same.  It is hard to be sure whether the only differences between
the transformers relate to their mounting arrangements.  I tried swapped some
of the HV components back and forth between them to eliminate them from
suspicion.  I didn't dare try swapping the transformers in case they are
not electrically identical and because I didn't want a fault elsewhere in
the bad terminal damaging what is probably my only good transformer.

I have the part number for the other type of transformer somewhere but
I can't locate it right now...

Regards,
Peter Coghlan

>
> Cheers,
> Aaron
> 

Re: Another Unrelated PSU Question

[Brent Hilpert via cctalk]

On 2020-Apr-18, at 10:03 AM, Rob Jarratt via cctalk wrote:
> I have another PSU I have been meaning to look at for a long time. This one
> has fairly high output ripple and some of the voltages do not appear to be
> where they should be. I have checked all the capacitors for ESR and they
> appear to be OK, with the exception of the two big smoothing capacitors on
> the primary side. One of them appears to be slightly bulging, but has
> low-ish ESR, the other has a much higher ESR. Is it possible that these
> capacitors could be the cause of the out-of-spec outputs?

What frequency is the ripple - mains frequency (50/60Hz) or switching frequency?

If the ripple is at mains frequency then, yes, the problem could be the 
mains/primary filtering. (Primary supply drooping far enough across mains 
cycles that the switcher is affected and unable to maintain drive.)

Either way, you could scope the primary supply under operation (some load), to 
see what the ripple there looks like.

Re: Another Unrelated PSU Question

[Peter Coghlan via cctalk]

>
> I have another PSU I have been meaning to look at for a long time. This one
> has fairly high output ripple and some of the voltages do not appear to be
> where they should be. I have checked all the capacitors for ESR and they
> appear to be OK, with the exception of the two big smoothing capacitors on
> the primary side. One of them appears to be slightly bulging, but has
> low-ish ESR, the other has a much higher ESR. Is it possible that these
> capacitors could be the cause of the out-of-spec outputs?
>

In my experience, the next step after bulging is either leaking and a sticky
mess on whatever is underneath or a big bang and sticky mess everywhere.
I'd get rid of the bulging one even if it seems otherwise good, epecially if
leaving it powered for anything other than very short periods.  It will
probably heat up over time and build more pressure.

The high ESR capacitor can't really be contributing much to smoothing so
I think it needs to be replaced before any further investigating can be done.
With any luck, that might sort out all the issues but even if it doesn't, it
would have to have been eliminated anyway.

Regards,
Peter Coghlan

> 
> Thanks
> 
>  
> 
> Rob
> 

Re: Great, my VT52 is shot.

[Warner Losh via cctalk]

On Sat, Apr 18, 2020 at 10:47 AM Chris Zach via cctalk <
cctalk@classiccmp.org> wrote:

> Here's a video of it with sound on.
>
> https://i.imgur.com/X1qVYGP.mp4


We had a VT52 do something akin to that once back in the day... The tech we
had that worked on this stuff said he had to fix something in the vertical
refresh circuit to achieve vertical hold... My vague memories likely won't
help you much, but this is quite distinctive and others of the group will
be able to suggest things :)

Warner


>
> C
>
> On 4/18/2020 4:45 AM, Mattis Lind wrote:
> > Hello Chris!
> >
> > Sorry that your trusty VT52 failed. But it shouldn't be too hard to
> > repair it!
> >
> > I have successfully restored a VT52 (actually a VT50 with a brain
> > transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is
> > that the capacitors were in good shape. Especially the bigger filter
> > capacitor. I think that DEC used great quality capacitors.
> >
> > It is always a good idea to check the voltages so that they are within
> > the spec. But it should not be necessary to replace all the capacitors
> > in my opinion.
> >
> > The VT52 is a quite interesting design with a very simplistic CPU built
> > from TTL components and a microprogram that has 1024 by 8 bits. Most of
> > the operations inside it is controlled from this program. In mine the
> > sockets for the mikroprogram were bad which meant that it failed to do
> > anything. Then I had a couple of gates that were bad.
> >
> > Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk
> > mailto:cctalk@classiccmp.org>>:
> >
> > Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
> > running for a day or two. Now the screen is filled with snow and it
> > looks like the text is all over the place horozontally.
> >
> >
> > It would be great to have a picture since it could give some hints on
> > what is wrong with it. My understanding is that the scan is working both
> > horizontally and vertically. Otherwise you would only have a straight
> > line. Unlike many common CRTs this CRT circuitry has no oscillator in it
> > that creates a picture without input. And since the horisontal
> > deflection circuit also creates the high voltage there would be no
> > picture at all unless the horisontal signal is there.
> >
> > This means that the two control boards in the bottom creates some kind
> > of horisontal and vertical signals. These signals comes from a simple
> > divider chain. There could be some kind of problem in the divider chain.
> > But then it would be more likely to not work at all if one of the
> > counters are bad. But it is a good idea to check the H and V signal som
> > they are inline with the spec. Around 15kHz and 60 or 50 Hz respectively.
> >
> > Since I haven't seen how the output looks like I speculating a bit. It
> > could be the case that the divider chain is correct but the video signal
> > is not generated in sync with the divider chain. Then the characters
> > would end up all over the place on screen.
> >
> > There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is
> > the video flip-flop. It controls when the screen is rendered. It might
> > be the case it could cause your problem. I think it is a  good idea to
> > check the signals around this flip-flop.
> >
> > I am always a bit skeptical towards 74Hxx. My impression is that they
> > fail more often than standard 74xx.
> >
> > When I repaired my VT52 I made a quick circuit that created a composite
> > video signal from the control board. The signal was fed into a standard
> > CRT monitor which I had around.  I could then run the control boards
> > flat on the bench to take measurements rather than assembling prior to
> > each test. I thought that it was quite handy.
> >
> >
> > Any tips or thoughts on where to start looking to fix? The keyboard
> > seems to be working as does the RS232 input (the snow on the screen
> > changes when the pdp11 talks to it)
> >
> >
> > This indicate that since you are using RS232 you have at least some kind
> > of voltage level on +12 V and -12 V since otherwise there would be no
> > communication. And +5V is probably not that bad either. Since not much
> > would operate in that case. Likewise the -5V is probably ok since the
> > char rom would not work very well without the bias.
> >
> > Good luck with your repair!
> >
> > /Mattis
> >
> >
> > Thanks!
> > Chris
> >
>

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

This is the circuit: 
https://rjarratt.files.wordpress.com/2020/04/transformerringingtester.png

Regards

Rob

> -Original Message-
> From: Rob Jarratt 
> Sent: 18 April 2020 17:25
> To: 'Toby Thain' ; r...@jarratt.me.uk; 'General
> Discussion: On-Topic and Off-Topic Posts' ; 'Mattis
> Lind' 
> Subject: RE: VAXmate PSU fixed, but no video
> 
> It is not my design, Tony Duell sent it to me and I think he got it from a
> magazine many years ago. I will try to find it.
> 
> Regards
> 
> Rob
> 
> > -Original Message-
> > From: Toby Thain 
> > Sent: 18 April 2020 16:51
> > To: r...@jarratt.me.uk; Rob Jarratt ;
> > General
> > Discussion: On-Topic and Off-Topic Posts ;
> > 'Mattis Lind' 
> > Subject: Re: VAXmate PSU fixed, but no video
> >
> > On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
> > > I checked Q6 (in circuit), it is not shorted. I have already checked
> > > all the
> > electrolytic capacitors for a bad ESR and replaced those where it was high.
> > There are no shorts on the electrolytic capacitors.
> > >
> > >
> > >
> > > I will have to desolder the transformer and do a ring test on it (I
> > > built a ring
> > test circuit some years ago). I don’t think you can do ring tests in
> > circuit can you?
> > >
> >
> > I think more than one of us would be curious about your ring test circuit
> design.
> >
> > I've been slowly learning to design a pwm flyback controller which is
> > more or less the same thing in principle?
> >
> > --Toby
> >
> > >
> > >
> > > Regards
> > >
> > >
> > >
> > > Rob
> > >
> > >
> > >
> > > From: Mattis Lind 
> > > Sent: 18 April 2020 13:31
> > > To: r...@jarratt.me.uk; Rob Jarratt ;
> > > General Discussion: On-Topic and Off-Topic Posts
> > > 
> > > Subject: Re: VAXmate PSU fixed, but no video
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk
> > mailto:cctalk@classiccmp.org> >:
> > >
> > > Some of you may recall seeing me post about the VAXmate PSU failure.
> > > Thanks to members of this list I found the failed part in the PSU
> > > and the PSU is now working again. However, it looks like the PSU
> > > failed because of a failure on the monitor board. There is a burning
> > > smell coming from it, possibly the flyback transformer, but I am not
> > > 100% sure. I don't see physical damage, but of course that doesn't
> > > mean there isn't a problem. When I took the monitor board out again
> > > after this, I wasn't sure if the EHT lead was making good contact
> > > with the CRT anode. The monitor board is described in section 4.4 of this
> document:
> > > http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_D
> > > es
> > > cripti
> > >  > > De
> > > scription_1987.pdf>
> > > on_1987.pdf
> > >
> > >
> > >
> > >
> > >
> > > I need some advice on diagnosing the problem, I have a few questions:
> > >
> > >
> > >
> > > 1.  If the EHT lead was not properly connected to the CRT anode, could
> > > that cause problems?
> > >
> > >
> > >
> > > No, that is unlikely in my opinion
> > >
> > >
> > >
> > > 2.  Is there anything I can safely do with a bench power supply to
> > > isolate the problem?
> > >
> > > 3.  Any other suggestions for diagnosing the problem?
> > >
> > >
> > >
> > > Check Q6 transistor. Looking at the picture 4-14, I think it is
> > > wrong. But
> > nevertheless check the capacitors C47 and C53.
> > >
> > > It happens that the EHT transformer get short-circuit turns. But it
> > > has only
> > occurred to me once. More often a capacitor is bad, a rectifier or the
> > switch transistor.
> > >
> > >
> > >
> > > If the EHT transformer is bad it performs bad when doing a ringing test.
> > >
> > >
> > >
> > > Try to find out what is smelling. Check all power semiconductors.
> > >
> > >
> > >
> > > 4.  There is an outline spec of the flyback transformer in the section
> > > 4.4.3.2 of the VAXmate technical description, what chance of finding
> > > a "modern" replacement?
> > >
> > >
> > >
> > > Usually very low. There are companies selling replacements but there
> > > are so
> > many variants so the chances to find one is very low. And there are
> > less and less of these on the market. But it is worth a try to check
> > what they have. It might be the case that they used the same monitor
> > design as some other DEC product. VT420? Then it could possible a
> > better chance finding one. donberg.ie   seem to have
> VT420 LOPT.
> > >
> > >
> > >
> > > /Mattis
> > >
> > >
> > >
> > >
> > >
> > >
> > > I have posted about the PSU repair here:
> > >
> > >
> > >
> > > https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-bu
> > > t-
> > > no-vid
> > >  > > ut
> > > -no-video/>
> > > eo/
> > >
> > >
> > >
> > > Thanks
> > >
> > >
> > >
> > > Rob
> > >
> > >
> > >
> > >

Another Unrelated PSU Question

[Rob Jarratt via cctalk]

I have another PSU I have been meaning to look at for a long time. This one
has fairly high output ripple and some of the voltages do not appear to be
where they should be. I have checked all the capacitors for ESR and they
appear to be OK, with the exception of the two big smoothing capacitors on
the primary side. One of them appears to be slightly bulging, but has
low-ish ESR, the other has a much higher ESR. Is it possible that these
capacitors could be the cause of the out-of-spec outputs?

 

Thanks

 

Rob

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Here's a video of it with sound on.

https://i.imgur.com/X1qVYGP.mp4

C

On 4/18/2020 4:45 AM, Mattis Lind wrote:

Hello Chris!

Sorry that your trusty VT52 failed. But it shouldn't be too hard to 
repair it!


I have successfully restored a VT52 (actually a VT50 with a brain 
transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is 
that the capacitors were in good shape. Especially the bigger filter 
capacitor. I think that DEC used great quality capacitors.


It is always a good idea to check the voltages so that they are within 
the spec. But it should not be necessary to replace all the capacitors 
in my opinion.


The VT52 is a quite interesting design with a very simplistic CPU built 
from TTL components and a microprogram that has 1024 by 8 bits. Most of 
the operations inside it is controlled from this program. In mine the 
sockets for the mikroprogram were bad which meant that it failed to do 
anything. Then I had a couple of gates that were bad.


Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk 
mailto:cctalk@classiccmp.org>>:


Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
running for a day or two. Now the screen is filled with snow and it
looks like the text is all over the place horozontally.


It would be great to have a picture since it could give some hints on 
what is wrong with it. My understanding is that the scan is working both 
horizontally and vertically. Otherwise you would only have a straight 
line. Unlike many common CRTs this CRT circuitry has no oscillator in it 
that creates a picture without input. And since the horisontal 
deflection circuit also creates the high voltage there would be no 
picture at all unless the horisontal signal is there.


This means that the two control boards in the bottom creates some kind 
of horisontal and vertical signals. These signals comes from a simple 
divider chain. There could be some kind of problem in the divider chain. 
But then it would be more likely to not work at all if one of the 
counters are bad. But it is a good idea to check the H and V signal som 
they are inline with the spec. Around 15kHz and 60 or 50 Hz respectively.


Since I haven't seen how the output looks like I speculating a bit. It 
could be the case that the divider chain is correct but the video signal 
is not generated in sync with the divider chain. Then the characters 
would end up all over the place on screen.


There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is 
the video flip-flop. It controls when the screen is rendered. It might 
be the case it could cause your problem. I think it is a  good idea to 
check the signals around this flip-flop.


I am always a bit skeptical towards 74Hxx. My impression is that they 
fail more often than standard 74xx.


When I repaired my VT52 I made a quick circuit that created a composite 
video signal from the control board. The signal was fed into a standard 
CRT monitor which I had around.  I could then run the control boards 
flat on the bench to take measurements rather than assembling prior to 
each test. I thought that it was quite handy.



Any tips or thoughts on where to start looking to fix? The keyboard
seems to be working as does the RS232 input (the snow on the screen
changes when the pdp11 talks to it)


This indicate that since you are using RS232 you have at least some kind 
of voltage level on +12 V and -12 V since otherwise there would be no 
communication. And +5V is probably not that bad either. Since not much 
would operate in that case. Likewise the -5V is probably ok since the 
char rom would not work very well without the bias.


Good luck with your repair!

/Mattis


Thanks!
Chris

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

It is not my design, Tony Duell sent it to me and I think he got it from a 
magazine many years ago. I will try to find it.

Regards

Rob

> -Original Message-
> From: Toby Thain 
> Sent: 18 April 2020 16:51
> To: r...@jarratt.me.uk; Rob Jarratt ; General
> Discussion: On-Topic and Off-Topic Posts ; 'Mattis
> Lind' 
> Subject: Re: VAXmate PSU fixed, but no video
> 
> On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
> > I checked Q6 (in circuit), it is not shorted. I have already checked all the
> electrolytic capacitors for a bad ESR and replaced those where it was high.
> There are no shorts on the electrolytic capacitors.
> >
> >
> >
> > I will have to desolder the transformer and do a ring test on it (I built a 
> > ring
> test circuit some years ago). I don’t think you can do ring tests in circuit 
> can
> you?
> >
> 
> I think more than one of us would be curious about your ring test circuit 
> design.
> 
> I've been slowly learning to design a pwm flyback controller which is more or
> less the same thing in principle?
> 
> --Toby
> 
> >
> >
> > Regards
> >
> >
> >
> > Rob
> >
> >
> >
> > From: Mattis Lind 
> > Sent: 18 April 2020 13:31
> > To: r...@jarratt.me.uk; Rob Jarratt ;
> > General Discussion: On-Topic and Off-Topic Posts
> > 
> > Subject: Re: VAXmate PSU fixed, but no video
> >
> >
> >
> >
> >
> >
> >
> > Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk
> mailto:cctalk@classiccmp.org> >:
> >
> > Some of you may recall seeing me post about the VAXmate PSU failure.
> > Thanks to members of this list I found the failed part in the PSU and
> > the PSU is now working again. However, it looks like the PSU failed
> > because of a failure on the monitor board. There is a burning smell
> > coming from it, possibly the flyback transformer, but I am not 100%
> > sure. I don't see physical damage, but of course that doesn't mean
> > there isn't a problem. When I took the monitor board out again after
> > this, I wasn't sure if the EHT lead was making good contact with the
> > CRT anode. The monitor board is described in section 4.4 of this document:
> > http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Des
> > cripti
> >  > scription_1987.pdf>
> > on_1987.pdf
> >
> >
> >
> >
> >
> > I need some advice on diagnosing the problem, I have a few questions:
> >
> >
> >
> > 1.  If the EHT lead was not properly connected to the CRT anode, could
> > that cause problems?
> >
> >
> >
> > No, that is unlikely in my opinion
> >
> >
> >
> > 2.  Is there anything I can safely do with a bench power supply to
> > isolate the problem?
> >
> > 3.  Any other suggestions for diagnosing the problem?
> >
> >
> >
> > Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But
> nevertheless check the capacitors C47 and C53.
> >
> > It happens that the EHT transformer get short-circuit turns. But it has only
> occurred to me once. More often a capacitor is bad, a rectifier or the switch
> transistor.
> >
> >
> >
> > If the EHT transformer is bad it performs bad when doing a ringing test.
> >
> >
> >
> > Try to find out what is smelling. Check all power semiconductors.
> >
> >
> >
> > 4.  There is an outline spec of the flyback transformer in the section
> > 4.4.3.2 of the VAXmate technical description, what chance of finding a
> > "modern" replacement?
> >
> >
> >
> > Usually very low. There are companies selling replacements but there are so
> many variants so the chances to find one is very low. And there are less and
> less of these on the market. But it is worth a try to check what they have. It
> might be the case that they used the same monitor design as some other DEC
> product. VT420? Then it could possible a better chance finding one. donberg.ie
>   seem to have VT420 LOPT.
> >
> >
> >
> > /Mattis
> >
> >
> >
> >
> >
> >
> > I have posted about the PSU repair here:
> >
> >
> >
> > https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-
> > no-vid
> >  > -no-video/>
> > eo/
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >
> >
> >

Re: Fwd: ROLM CBX 8000 System Service Manual Vol. 2 Scans Released!

[rgalliett--- via cctalk]

Hello. I have both, Rolm CBX System Service Manuals l and ll as well as a 
Release 8/8001 Student Maintenance Course binder. Are you interested in these? 
Thank youSent from my LG Phoenix 3, an AT 4G LTE smartphone

Re: VAXmate PSU fixed, but no video

[Toby Thain via cctalk]

On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
> I checked Q6 (in circuit), it is not shorted. I have already checked all the 
> electrolytic capacitors for a bad ESR and replaced those where it was high. 
> There are no shorts on the electrolytic capacitors.
> 
>  
> 
> I will have to desolder the transformer and do a ring test on it (I built a 
> ring test circuit some years ago). I don’t think you can do ring tests in 
> circuit can you?
> 

I think more than one of us would be curious about your ring test
circuit design.

I've been slowly learning to design a pwm flyback controller which is
more or less the same thing in principle?

--Toby

>  
> 
> Regards
> 
>  
> 
> Rob
> 
>  
> 
> From: Mattis Lind  
> Sent: 18 April 2020 13:31
> To: r...@jarratt.me.uk; Rob Jarratt ; General 
> Discussion: On-Topic and Off-Topic Posts 
> Subject: Re: VAXmate PSU fixed, but no video
> 
>  
> 
>  
> 
>  
> 
> Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk 
> mailto:cctalk@classiccmp.org> >:
> 
> Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
> to members of this list I found the failed part in the PSU and the PSU is
> now working again. However, it looks like the PSU failed because of a
> failure on the monitor board. There is a burning smell coming from it,
> possibly the flyback transformer, but I am not 100% sure. I don't see
> physical damage, but of course that doesn't mean there isn't a problem. When
> I took the monitor board out again after this, I wasn't sure if the EHT lead
> was making good contact with the CRT anode. The monitor board is described
> in section 4.4 of this document:
> http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti 
> 
>  
> on_1987.pdf
> 
> 
> 
> 
> 
> I need some advice on diagnosing the problem, I have a few questions:
> 
> 
> 
> 1.  If the EHT lead was not properly connected to the CRT anode, could
> that cause problems?
> 
>  
> 
> No, that is unlikely in my opinion 
> 
>  
> 
> 2.  Is there anything I can safely do with a bench power supply to
> isolate the problem? 
> 
> 3.  Any other suggestions for diagnosing the problem?
> 
>  
> 
> Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But 
> nevertheless check the capacitors C47 and C53.
> 
> It happens that the EHT transformer get short-circuit turns. But it has only 
> occurred to me once. More often a capacitor is bad, a rectifier or the switch 
> transistor.
> 
>  
> 
> If the EHT transformer is bad it performs bad when doing a ringing test.
> 
>  
> 
> Try to find out what is smelling. Check all power semiconductors.
> 
>  
> 
> 4.  There is an outline spec of the flyback transformer in the section
> 4.4.3.2 of the VAXmate technical description, what chance of finding a
> "modern" replacement?
> 
>  
> 
> Usually very low. There are companies selling replacements but there are so 
> many variants so the chances to find one is very low. And there are less and 
> less of these on the market. But it is worth a try to check what they have. 
> It might be the case that they used the same monitor design as some other DEC 
> product. VT420? Then it could possible a better chance finding one. 
> donberg.ie   seem to have VT420 LOPT.
> 
>  
> 
> /Mattis
> 
>  
> 
> 
> 
> 
> I have posted about the PSU repair here:
> 
> 
> 
> https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-vid 
> 
>  
> eo/
> 
> 
> 
> Thanks
> 
> 
> 
> Rob
> 
> 
> 
> 

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

I don't have an LCR meter but I think it might be a good idea to get one.

Regards

Rob

> -Original Message-
> From: cctalk  On Behalf Of Aaron Jackson
via
> cctalk
> Sent: 18 April 2020 15:13
> To: Peter Coghlan ; General Discussion: On-Topic
> and Off-Topic Posts 
> Subject: Re: VAXmate PSU fixed, but no video
> 
> >> 1. If the EHT lead was not properly connected to the CRT anode, could
> >> that cause problems?
> >
> > Possibly.  I have VT220 terminal which was making a smell of ozone
> > when it was running which I should have done something about but never
got
> around to.
> > This could have been due to corona discharge around the CRT anode
> > connection or around the flyback transformer but I never found out.
> > Eventually, it stopped working, drawing excess current from the 12V
> > power supply.  The flyback transformer appears to have been damaged.
> 
> I recently decided to take another look at a VT220 I've got which appears
to
> have a bad FBT. Q202 switching transistor has been replaced and is
outputting
> a 14.7KHz signal but the flyback also seems to draw too much current and
> causes the terminal to hiccup.
> 
> It appears to be a problem with the primary winding, which has an
inductance
> of 5.6uH, although has now increased to 6.4uH after smoking again (I left
it on
> while hiccuping to check that the transistor was still switching)... If
you have an
> LCR meter I'd be curious to know what inductance you measure on the
primary
> winding.
> 
> I did eventually notice a small crack in the plastic on the primary
winding side.
> I'm not sure if it is superficial but I suspect this is where the smoke
escaped.
> 
> >> 2. Is there anything I can safely do with a bench power supply to
> >> isolate the problem?
> >> 3. Any other suggestions for diagnosing the problem?
> >
> > One approach to testing flyback transformers seems to be to use a
> > circuit that causes them to ring and observing whether the ringing is
> > damped by shorted turns.  I've never got around to trying this myself.
> 
> If you want to check the secondary winding, there is a diode which has a
high
> forward bias voltage. If you pass 20v or so through the secondary and look
at it
> with a volt meter you should see the voltage drop. It won't conduct at all
if the
> voltage is too low.
> 
> >> 4. There is an outline spec of the flyback transformer in the section
> >> 4.4.3.2 of the VAXmate technical description, what chance of finding
> >> a "modern" replacement?
> >
> > I wish you good luck with this.  I never had any luck locating one for
> > my
> > VT220 :-(
> 
> There were two VT220 designs (I think), using either an onboard or
offboard
> flyback. The part number of the onboard flyback is 16-26299-01, and there
are
> some available on eBay if you are feeling rich.
> 
> Cheers,
> Aaron
> 
> (Sorry about the ridiculous footer that will appear below.)
> 
> 
> This message and any attachment are intended solely for the addressee and
> may contain confidential information. If you have received this message in
> error, please contact the sender and delete the email and attachment.
> 
> Any views or opinions expressed by the author of this email do not
necessarily
> reflect the views of the University of Nottingham. Email communications
with
> the University of Nottingham may be monitored where permitted by law.
> 
> 

Re: Looking for a document regarding PDP8

[Bob Smith via cctalk]

thanks!

On Fri, Apr 17, 2020 at 5:48 PM Len Shustek via cctalk
 wrote:
>
> At 10:00 AM 4/17/2020, Bob Smith  wrote:
> >...I believe sometime in the late 70s, maybe as late as 1980, a prof
> >associated with UMass wrote a paper describing an extension of the
> >PDP8 called 8/X or 8X.
> >...I believe, my memory is fuzzy, that it was a prof nnmed Stone or
> >Stoner (perhaps Harold S) who lead the effort and had his name on the
> >paper.
>
> That is probably Harold S. Stone, whom I knew as a Stanford prof in
> the early 1970s and did some corporate consulting with. Brilliant
> guy. From 1974 to 1984 he was at UMass Amherst, so your memory isn't
> fuzzy at all. He also worked at the IBM Yorktown Heights research
> center. He was the author of several books and many papers about
> computer architecture, algorithms, and interfaces.
>
> Unfortunately I don't remember him working on a PDP-8 extension, and
> a quick search of the ACM Digital Library turned up nothing. As far
> as I know he's still alive at the age of 82.
>

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

I checked Q6 (in circuit), it is not shorted. I have already checked all the 
electrolytic capacitors for a bad ESR and replaced those where it was high. 
There are no shorts on the electrolytic capacitors.

 

I will have to desolder the transformer and do a ring test on it (I built a 
ring test circuit some years ago). I don’t think you can do ring tests in 
circuit can you?

 

Regards

 

Rob

 

From: Mattis Lind  
Sent: 18 April 2020 13:31
To: r...@jarratt.me.uk; Rob Jarratt ; General 
Discussion: On-Topic and Off-Topic Posts 
Subject: Re: VAXmate PSU fixed, but no video

 

 

 

Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk 
mailto:cctalk@classiccmp.org> >:

Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
to members of this list I found the failed part in the PSU and the PSU is
now working again. However, it looks like the PSU failed because of a
failure on the monitor board. There is a burning smell coming from it,
possibly the flyback transformer, but I am not 100% sure. I don't see
physical damage, but of course that doesn't mean there isn't a problem. When
I took the monitor board out again after this, I wasn't sure if the EHT lead
was making good contact with the CRT anode. The monitor board is described
in section 4.4 of this document:
http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti 

 
on_1987.pdf





I need some advice on diagnosing the problem, I have a few questions:



1.  If the EHT lead was not properly connected to the CRT anode, could
that cause problems?

 

No, that is unlikely in my opinion 

 

2.  Is there anything I can safely do with a bench power supply to
isolate the problem? 

3.  Any other suggestions for diagnosing the problem?

 

Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But 
nevertheless check the capacitors C47 and C53.

It happens that the EHT transformer get short-circuit turns. But it has only 
occurred to me once. More often a capacitor is bad, a rectifier or the switch 
transistor.

 

If the EHT transformer is bad it performs bad when doing a ringing test.

 

Try to find out what is smelling. Check all power semiconductors.

 

4.  There is an outline spec of the flyback transformer in the section
4.4.3.2 of the VAXmate technical description, what chance of finding a
"modern" replacement?

 

Usually very low. There are companies selling replacements but there are so 
many variants so the chances to find one is very low. And there are less and 
less of these on the market. But it is worth a try to check what they have. It 
might be the case that they used the same monitor design as some other DEC 
product. VT420? Then it could possible a better chance finding one. donberg.ie 
  seem to have VT420 LOPT.

 

/Mattis

 




I have posted about the PSU repair here:



https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-vid 

 
eo/



Thanks



Rob

RE: VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

> -Original Message-
> From: cctalk  On Behalf Of Peter Coghlan
via
> cctalk
> Sent: 18 April 2020 12:57
> To: General Discussion: On-Topic and Off-Topic Posts

> Subject: Re: VAXmate PSU fixed, but no video
> 
> >
> > Some of you may recall seeing me post about the VAXmate PSU failure.
> > Thanks to members of this list I found the failed part in the PSU and
> > the PSU is now working again. However, it looks like the PSU failed
> > because of a failure on the monitor board. There is a burning smell
> > coming from it, possibly the flyback transformer, but I am not 100%
> > sure. I don't see physical damage, but of course that doesn't mean
> > there isn't a problem. When I took the monitor board out again after
> > this, I wasn't sure if the EHT lead was making good contact with the
> > CRT anode. The monitor board is described in section 4.4 of this
document:
> > http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Des
> > cripti
> > on_1987.pdf
> >
> 
> Congratulations on getting your PSU failure sorted out.  I suppose you
realise
> this means there will be a line of people wanting you to look at our PSU
> problems the next time we meet up :-)

Given how long it took me to fix this one, I don't think anyone will want to
ask me!


> 
> >
> > I need some advice on diagnosing the problem, I have a few questions:
> >
> > 1.  If the EHT lead was not properly connected to the CRT anode, could
> > that cause problems?
> >
> 
> Possibly.  I have VT220 terminal which was making a smell of ozone when it
> was running which I should have done something about but never got around
> to.
> This could have been due to corona discharge around the CRT anode
> connection or around the flyback transformer but I never found out.
> Eventually, it stopped working, drawing excess current from the 12V power
> supply.  The flyback transformer appears to have been damaged.
> 
> >
> > 2.  Is there anything I can safely do with a bench power supply to
> > isolate the problem?
> > 3.  Any other suggestions for diagnosing the problem?
> >
> 
> One approach to testing flyback transformers seems to be to use a circuit
that
> causes them to ring and observing whether the ringing is damped by shorted
> turns.  I've never got around to trying this myself.
> 
> >
> > 4.  There is an outline spec of the flyback transformer in the section
> > 4.4.3.2 of the VAXmate technical description, what chance of finding a
> > "modern" replacement?
> >
> 
> I wish you good luck with this.  I never had any luck locating one for my
> VT220 :-(
> 
> Regards,
> Peter Coghlan.
> 
> >
> > I have posted about the PSU repair here:
> >
> >
> >
> > https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-
> > no-video/
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >

Re: VAXmate PSU fixed, but no video

[Mattis Lind via cctalk]

>
>
> One approach to testing flyback transformers seems to be to use a circuit
> that causes them to ring and observing whether the ringing is damped by
> shorted turns.  I've never got around to trying this myself.
>
>
I have a VT100 that failed with some smoke coming out of the center of the
transformer coil. I tried the ringing test. Here are my results:
http://www.datormuseum.se/peripherals/terminals/vt100

/Mattis


>
>

Re: VAXmate PSU fixed, but no video

[Aaron Jackson via cctalk]

>> 1.   If the EHT lead was not properly connected to the CRT anode, could
>> that cause problems?
>
> Possibly.  I have VT220 terminal which was making a smell of ozone when it was
> running which I should have done something about but never got around to.
> This could have been due to corona discharge around the CRT anode connection
> or around the flyback transformer but I never found out.  Eventually, it
> stopped working, drawing excess current from the 12V power supply.  The
> flyback transformer appears to have been damaged.

I recently decided to take another look at a VT220 I've got which
appears to have a bad FBT. Q202 switching transistor has been replaced
and is outputting a 14.7KHz signal but the flyback also seems to draw
too much current and causes the terminal to hiccup.

It appears to be a problem with the primary winding, which has an
inductance of 5.6uH, although has now increased to 6.4uH after smoking
again (I left it on while hiccuping to check that the transistor was
still switching)... If you have an LCR meter I'd be curious to know what
inductance you measure on the primary winding.

I did eventually notice a small crack in the plastic on the primary
winding side. I'm not sure if it is superficial but I suspect this is
where the smoke escaped.

>> 2.   Is there anything I can safely do with a bench power supply to
>> isolate the problem?
>> 3.   Any other suggestions for diagnosing the problem?
>
> One approach to testing flyback transformers seems to be to use a circuit
> that causes them to ring and observing whether the ringing is damped by
> shorted turns.  I've never got around to trying this myself.

If you want to check the secondary winding, there is a diode which has a
high forward bias voltage. If you pass 20v or so through the secondary
and look at it with a volt meter you should see the voltage drop. It
won't conduct at all if the voltage is too low.

>> 4.   There is an outline spec of the flyback transformer in the section
>> 4.4.3.2 of the VAXmate technical description, what chance of finding a
>> "modern" replacement?
>
> I wish you good luck with this.  I never had any luck locating one for my
> VT220 :-(

There were two VT220 designs (I think), using either an onboard or
offboard flyback. The part number of the onboard flyback is 16-26299-01,
and there are some available on eBay if you are feeling rich.

Cheers,
Aaron

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Re: VAXmate PSU fixed, but no video

[Mattis Lind via cctalk]

Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk <
cctalk@classiccmp.org>:

> Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
> to members of this list I found the failed part in the PSU and the PSU is
> now working again. However, it looks like the PSU failed because of a
> failure on the monitor board. There is a burning smell coming from it,
> possibly the flyback transformer, but I am not 100% sure. I don't see
> physical damage, but of course that doesn't mean there isn't a problem.
> When
> I took the monitor board out again after this, I wasn't sure if the EHT
> lead
> was making good contact with the CRT anode. The monitor board is described
> in section 4.4 of this document:
>
> http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti
> on_1987.pdf
> 
>
>
>
>
>
> I need some advice on diagnosing the problem, I have a few questions:
>
>
>
> 1.  If the EHT lead was not properly connected to the CRT anode, could
> that cause problems?
>

No, that is unlikely in my opinion


> 2.  Is there anything I can safely do with a bench power supply to
> isolate the problem?

3.  Any other suggestions for diagnosing the problem?
>

Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But
nevertheless check the capacitors C47 and C53.
It happens that the EHT transformer get short-circuit turns. But it has
only occurred to me once. More often a capacitor is bad, a rectifier or the
switch transistor.

If the EHT transformer is bad it performs bad when doing a ringing test.

Try to find out what is smelling. Check all power semiconductors.


> 4.  There is an outline spec of the flyback transformer in the section
> 4.4.3.2 of the VAXmate technical description, what chance of finding a
> "modern" replacement?
>

Usually very low. There are companies selling replacements but there are so
many variants so the chances to find one is very low. And there are less
and less of these on the market. But it is worth a try to check what they
have. It might be the case that they used the same monitor design as some
other DEC product. VT420? Then it could possible a better chance finding
one. donberg.ie seem to have VT420 LOPT.

/Mattis


>
>
> I have posted about the PSU repair here:
>
>
>
>
> https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-vid
> eo/
> 
>
>
>
> Thanks
>
>
>
> Rob
>
>
>
>

Re: VAXmate PSU fixed, but no video

[Peter Coghlan via cctalk]

> 
> Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
> to members of this list I found the failed part in the PSU and the PSU is
> now working again. However, it looks like the PSU failed because of a
> failure on the monitor board. There is a burning smell coming from it,
> possibly the flyback transformer, but I am not 100% sure. I don't see
> physical damage, but of course that doesn't mean there isn't a problem. When
> I took the monitor board out again after this, I wasn't sure if the EHT lead
> was making good contact with the CRT anode. The monitor board is described
> in section 4.4 of this document:
> http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti
> on_1987.pdf
>   

Congratulations on getting your PSU failure sorted out.  I suppose you realise
this means there will be a line of people wanting you to look at our PSU
problems the next time we meet up :-)
 
> 
> I need some advice on diagnosing the problem, I have a few questions:  
> 
> 1.If the EHT lead was not properly connected to the CRT anode, could
> that cause problems?
>

Possibly.  I have VT220 terminal which was making a smell of ozone when it was
running which I should have done something about but never got around to.
This could have been due to corona discharge around the CRT anode connection
or around the flyback transformer but I never found out.  Eventually, it
stopped working, drawing excess current from the 12V power supply.  The
flyback transformer appears to have been damaged.

>
> 2.Is there anything I can safely do with a bench power supply to
> isolate the problem?
> 3.Any other suggestions for diagnosing the problem?
>

One approach to testing flyback transformers seems to be to use a circuit
that causes them to ring and observing whether the ringing is damped by
shorted turns.  I've never got around to trying this myself.

>
> 4.There is an outline spec of the flyback transformer in the section
> 4.4.3.2 of the VAXmate technical description, what chance of finding a
> "modern" replacement?
> 

I wish you good luck with this.  I never had any luck locating one for my
VT220 :-(

Regards,
Peter Coghlan.

>
> I have posted about the PSU repair here:
> 
>  
> 
> https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-video/
> 
>  
> 
> Thanks
> 
>  
> 
> Rob
> 
>  

Re: DEC QBUS Backplanes

[Bill Gunshannon via cctalk]

This is an interesting discussion.  I may have to dig thru my
boxes and see if I still have the box this backplane came in.
Lots of time to do  it.

But, on another note, I was planning on sending it to
someone when the thought occurred to me last night.  I
can't.  I don't even know if the Post Office is still
open other than for picking up mail from PO Boxes.
Haven't been anywhere near it in weeks and don't
anticipate going anywhere in that direction for several
more weeks.

But, I have other things that will need to go, too, Some that
can't be practically sent. Like a currently non-functioning
RX02 box.  Might need to find someone close enough to pick
this stuff up when life returns to something closer to
normal.

bill

VAXmate PSU fixed, but no video

[Rob Jarratt via cctalk]

Some of you may recall seeing me post about the VAXmate PSU failure. Thanks
to members of this list I found the failed part in the PSU and the PSU is
now working again. However, it looks like the PSU failed because of a
failure on the monitor board. There is a burning smell coming from it,
possibly the flyback transformer, but I am not 100% sure. I don't see
physical damage, but of course that doesn't mean there isn't a problem. When
I took the monitor board out again after this, I wasn't sure if the EHT lead
was making good contact with the CRT anode. The monitor board is described
in section 4.4 of this document:
http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Descripti
on_1987.pdf

 

 

I need some advice on diagnosing the problem, I have a few questions:

 

1.  If the EHT lead was not properly connected to the CRT anode, could
that cause problems?
2.  Is there anything I can safely do with a bench power supply to
isolate the problem?
3.  Any other suggestions for diagnosing the problem?
4.  There is an outline spec of the flyback transformer in the section
4.4.3.2 of the VAXmate technical description, what chance of finding a
"modern" replacement?

 

I have posted about the PSU repair here:

 

https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-vid
eo/

 

Thanks

 

Rob

 

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

Hello Chris!

Sorry that your trusty VT52 failed. But it shouldn't be too hard to repair
it!

I have successfully restored a VT52 (actually a VT50 with a brain
transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is that
the capacitors were in good shape. Especially the bigger filter capacitor.
I think that DEC used great quality capacitors.

It is always a good idea to check the voltages so that they are within the
spec. But it should not be necessary to replace all the capacitors in my
opinion.

The VT52 is a quite interesting design with a very simplistic CPU built
from TTL components and a microprogram that has 1024 by 8 bits. Most of the
operations inside it is controlled from this program. In mine the sockets
for the mikroprogram were bad which meant that it failed to do anything.
Then I had a couple of gates that were bad.

Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk <
cctalk@classiccmp.org>:

> Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
> running for a day or two. Now the screen is filled with snow and it
> looks like the text is all over the place horozontally.
>

It would be great to have a picture since it could give some hints on what
is wrong with it. My understanding is that the scan is working both
horizontally and vertically. Otherwise you would only have a straight line.
Unlike many common CRTs this CRT circuitry has no oscillator in it that
creates a picture without input. And since the horisontal
deflection circuit also creates the high voltage there would be no picture
at all unless the horisontal signal is there.

This means that the two control boards in the bottom creates some kind of
horisontal and vertical signals. These signals comes from a simple divider
chain. There could be some kind of problem in the divider chain. But then
it would be more likely to not work at all if one of the counters are bad.
But it is a good idea to check the H and V signal som they are inline with
the spec. Around 15kHz and 60 or 50 Hz respectively.

Since I haven't seen how the output looks like I speculating a bit. It
could be the case that the divider chain is correct but the video signal is
not generated in sync with the divider chain. Then the characters would end
up all over the place on screen.

There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is the
video flip-flop. It controls when the screen is rendered. It might be
the case it could cause your problem. I think it is a  good idea to check
the signals around this flip-flop.

I am always a bit skeptical towards 74Hxx. My impression is that they fail
more often than standard 74xx.

When I repaired my VT52 I made a quick circuit that created a composite
video signal from the control board. The signal was fed into a standard CRT
monitor which I had around.  I could then run the control boards flat on
the bench to take measurements rather than assembling prior to each test. I
thought that it was quite handy.


> Any tips or thoughts on where to start looking to fix? The keyboard
> seems to be working as does the RS232 input (the snow on the screen
> changes when the pdp11 talks to it)
>

This indicate that since you are using RS232 you have at least some kind of
voltage level on +12 V and -12 V since otherwise there would be no
communication. And +5V is probably not that bad either. Since not much
would operate in that case. Likewise the -5V is probably ok since the char
rom would not work very well without the bias.

Good luck with your repair!

/Mattis


>
> Thanks!
> Chris
>