VAXmate PSU fixed, but no video

[Toby Thain]

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.
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> 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?
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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

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> Regards
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> Rob
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> 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
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> Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk <cctalk at classiccmp.org <mailto:cctalk at classiccmp.org> >:
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> 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
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> I need some advice on diagnosing the problem, I have a few questions:
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> 1.      If the EHT lead was not properly connected to the CRT anode, could
> that cause problems?
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> No, that is unlikely in my opinion 
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> 2.      Is there anything I can safely do with a bench power supply to
> isolate the problem? 
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> 3.      Any other suggestions for diagnosing the problem?
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> Check Q6 transistor. Looking at the picture 4-14, I think it is wrong. But nevertheless check the capacitors C47 and C53.
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> 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.
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> If the EHT transformer is bad it performs bad when doing a ringing test.
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> Try to find out what is smelling. Check all power semiconductors.
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> 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?
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> 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.
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> /Mattis
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> I have posted about the PSU repair here:
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> 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/
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> Thanks
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> Rob
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