18 Apr
2020
18 Apr
'20
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