18 Apr
2020
18 Apr
'20
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?