Testing H7864 (MicroVAX II) PSU With No Load

Mattis Lind mattislind at gmail.com
Mon Nov 2 07:20:12 CST 2015

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.

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

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.

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!


> Regards
> Rob

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