18 Apr
2020
18 Apr
'20
4:26 a.m.
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
18 Apr
18 Apr
4:57 a.m.
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-vi…
Thanks
Rob
7:13 a.m.
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 :-(
8:13 a.m.
I don't have an LCR meter but I think it might be a good idea to get one.
Regards
Rob
If you want to check the secondary winding, there is a diode which has a high
-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Aaron Jackson
via
cctalk
Sent: 18 April 2020 15:13
To: Peter Coghlan <cctalk at beyondthepale.ie>; General Discussion: On-Topic
and Off-Topic Posts <cctalk at classiccmp.org>
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.
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 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. 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.
There were two VT220 designs (I think), using either an onboard or
offboard
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 :-( 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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permitted by law.
10:54 a.m.
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
7:36 a.m.
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
7:40 a.m.
-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Peter Coghlan
via
cctalk
Sent: 18 April 2020 12:57
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at
classiccmp.org>
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 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 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
>
>
5:30 a.m.
Den l?r 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk <
cctalk at classiccmp.org>:
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/
>
<https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-video/
> Thanks
> Rob
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
<http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Description_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?
8:09 a.m.
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 <mattislind at gmail.com>
Sent: 18 April 2020 13:31
To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>; General
Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: VAXmate PSU fixed, but no video
Den l?r 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk <cctalk at classiccmp.org
<mailto:cctalk at 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
<http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Description_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 <http://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
<https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-video/>
eo/
Thanks
Rob
8:50 a.m.
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 <mattislind at gmail.com>
Sent: 18 April 2020 13:31
To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>; General
Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: VAXmate PSU fixed, but no video
Den l?r 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk <cctalk at classiccmp.org
<mailto:cctalk at 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
<http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_Description_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 <http://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
<https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-no-video/>
eo/
Thanks
Rob
9:24 a.m.
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 <toby at telegraphics.com.au>
Sent: 18 April 2020 16:51
To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>; General
Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>; 'Mattis
Lind' <mattislind at gmail.com>
Subject: Re: VAXmate PSU fixed, but no video
On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
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
<mattislind at gmail.com>
> Sent: 18 April 2020 13:31
> To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>;
> General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> Subject: Re: VAXmate PSU fixed, but no video
> Den l?r 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk
<cctalk at classiccmp.org <mailto:cctalk at 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
> <http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_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
<http://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
> <https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but
> -no-video/>
> eo/
> Thanks
> Rob
>
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?
10:05 a.m.
This is the circuit:
https://rjarratt.files.wordpress.com/2020/04/transformerringingtester.png
Regards
Rob
-----Original Message-----
From: Rob Jarratt <robert.jarratt at ntlworld.com>
Sent: 18 April 2020 17:25
To: 'Toby Thain' <toby at telegraphics.com.au>; rob at jarratt.me.uk;
'General
Discussion: On-Topic and Off-Topic Posts' <cctalk at classiccmp.org>;
'Mattis
Lind' <mattislind at gmail.com>
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
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
> > <https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-b
> > ut
> > -no-video/>
> > eo/
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >
> >
> >
-----Original Message-----
From: Toby Thain <toby at telegraphics.com.au>
Sent: 18 April 2020 16:51
To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>;
General
Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>;
'Mattis Lind' <mattislind at gmail.com>
Subject: Re: VAXmate PSU fixed, but no video
On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
I think more than one of us would be curious about your ring test circuit
design.
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've been slowly learning to design a pwm flyback controller which is
more or less the same thing in principle?
--Toby
document:
Regards
Rob
From: Mattis Lind <mattislind at gmail.com>
Sent: 18 April 2020 13:31
To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>;
General Discussion: On-Topic and Off-Topic Posts
<cctalk at classiccmp.org>
Subject: Re: VAXmate PSU fixed, but no video
Den l?r 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk
<cctalk at
classiccmp.org <mailto:cctalk at 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
http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_D
es
cripti
<http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_
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 <http://donberg.ie> seem to have
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11 comments
5 participants
participants (5)
-
Aaron.Jackson＠nottingham.ac.uk -
cctalk＠beyondthepale.ie -
mattislind＠gmail.com -
robert.jarratt＠ntlworld.com -
toby＠telegraphics.com.au