This is the circuit: https://rjarratt.files.wordpress.com/2020/04/transformerringingtester.png
Regards Rob > -----Original Message----- > From: Rob Jarratt <robert.jarr...@ntlworld.com> > Sent: 18 April 2020 17:25 > To: 'Toby Thain' <t...@telegraphics.com.au>; r...@jarratt.me.uk; 'General > Discussion: On-Topic and Off-Topic Posts' <cctalk@classiccmp.org>; 'Mattis > Lind' <mattisl...@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 > > > -----Original Message----- > > From: Toby Thain <t...@telegraphics.com.au> > > Sent: 18 April 2020 16:51 > > To: r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; > > General > > Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org>; > > 'Mattis Lind' <mattisl...@gmail.com> > > Subject: Re: VAXmate PSU fixed, but no video > > > > 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_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 > 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 > > > > > > > > > > > >
