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 <mattisl...@gmail.com> > Sent: 18 April 2020 13:31 > To: r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; General > Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> > Subject: Re: VAXmate PSU fixed, but no video > > > > > > > > Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk > <cctalk@classiccmp.org <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 > <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 > > > >
