Testing H7864 (MicroVAX II) PSU With No Load

[Robert Jarratt]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Robert
> Jarratt
> Sent: 02 November 2015 20:36
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Subject: RE: Testing H7864 (MicroVAX II) PSU With No Load
> 
> 
> 
> > -----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
> > >
> > >

I have just uploaded all the corrections. So the connector names on the risers should now correspond. I have also corrected the area on the primary control module around C403 and R407.

Regards

Rob