Testing H7864 (MicroVAX II) PSU With No Load

[Robert Jarratt]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Mattis Lind
> Sent: 02 November 2015 13:20
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load
> 
> 2015-11-01 17:35 GMT+01:00 Robert Jarratt <robert.jarratt at ntlworld.com>:
> 
> >
> >
> > > > > Maybe you can publish your schematic somwhere. It is much easier
> > > > > to come
> > > > up
> > > > > with ideas to pin point problems if we all view the same schematic.
> > > > > I
> > > > think it
> > > > > would be possible to identify the problem without comparing the
> > > > > two PSUs
> > > > at
> > > > > least in this case where there is some kind of fundamental problem.
> > > >
> > > > I do intend to publish the schematics, but right now I am pretty
> > > > sure they
> > > > are:
> > > >
> > > > a) full of mistakes
> > > > b) not drawn logically
> > > > c) there are a couple of areas I couldn't trace without major
> > > > surgery on the board.
> > > >
> > >
> > > Well. I guess that anything is better than nothing.
> > >
> >
> >
> > Here is a link to what I have now http://1drv.ms/1KQkTBp
> >
> > There are Eagle sch files, and PNG files for those who do not have Eagle.
> >
> 
> I took a quick look at the schematics and I had hard times following the
> schematics, but I guess I asked for it.


I did warn you! :-) :-), but thanks for looking and trying to make sense of them.

> 
> Anyway. I think that the signal is produced somewhere behind the transformer
> T301 and then passes through a number of driver stages (Q405 and Q408)  until
> it gets to the Q301. You could check the signal on T301 and onwards. The
> designation for the connector of the small boards are all
> X1 while there X1, X2, X3 mentioned on one schematic so I am a little bit
> confused. Then there are four small boards which presumably have Xn
> connectors but I can only find three in your schematics. But this is maybe a
> result of the fact the schematics are incomplete or me misunderstanding
> something.

Yes, I wasn't very disciplined with the labelling. Basically on the big schematic, called Secondary I have drawn the connectors for each of the riser boards. The text near the connector identifies which riser board it is the connector for and the connector schematic only has one connector, but it will always be called X1, I didn't make it correspond to the Secondary schematic. I will fix this. Note also that the small riser on the primary side is included in the Primary Control schematic, I didn't do a separate schematic, which was a mistake I think.


> The circuitry C403 and R407 doesn't make sense to me. It has to be
> a drawing mistake.

Thanks, I will check this. I have found lots of mistakes, I am sure this is another one.

> 
> Maybe it is a good idea to to focus on one area and to make that  part of the
> schematic complete. Like the part surrounding the main switch transistor. Then
> try to follow the signal back through T301 and trace that part out.


Yes, I agree, the Primary Control is the one where the incorrect operation seems to be happening, Q301 does not appear to be switching. I think this schematic is as complete as I can make it, although it could still have mistakes....

Some time ago I got a scope on Q301, and I could see the b-e never went high enough to switch it on. There were some very short and very low pulses every 40us, but that was all. When I compared this to the working one, I could see a signal, still every 40us, but definitely switching the transistor on during the cycle.

> 
> If the PWM signal is produced on the secondary side then there has to be some
> small PSU providing power to this circuitry at startup so that the PSU can
> bootstrap. But I cannot see where this is.
> 
> Good Luck!
> 
> /Mattis
> 
> 
> 
> >
> > Regards
> >
> > Rob
> >
> >