Testing H7864 (MicroVAX II) PSU With No Load

Robert Jarratt robert.jarratt at ntlworld.com
Sun Nov 8 16:08:12 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Robert
> Jarratt
> Sent: 02 November 2015 23:18
> 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 21:44
> > To: General Discussion: On-Topic and Off-Topic Posts
> > Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load
> >
> > 2015-11-02 22:05 GMT+01:00 Robert Jarratt <robert.jarratt at ntlworld.com>:
> >
> > >
> > >
> > >
> > > 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
> > >
> > >
> > Checked the new one. There are still something weird. The base of Q301
> > it is short circuited to the emitter (which if that is true would of
> > cause your problem). But I think this is a drawing mistake. The base
> > signals goes to the
> > D405 diode which is connected to emitter Q405 and emitter Q406 which
> > is then connected to emitter Q301...  Please check this again. Maybe
> > there are a short circuit component that makes you trace it wrong (if
> > you do it by ohm-meter)
> >
> > /Mattis
> Thanks for spotting the drawing mistake. I have noted an apparent short
> before, and I have some notes I made at the time, but that was so long ago
> (almost a year!), and the notes don't seem to conclude whether I found the
> short or not. I think I may have put it down to some of the transformers at the
> time, but I can't be sure. I will investigate this further as this is quite promising,
> but it looks like I will need to desolder some components...
> Regards
> Rob

I have been doing some further checking of the schematic for the primary control section, because that is where the failure was, and I have found some significant errors. I have been over it as carefully as I can and hopefully it is now much more accurate. I have also made the connections to the primary side of the transformer correct, although as I remark on the schematic, I don't know how the transformer is wired in relation to the secondary outputs. Regarding the apparent short between the base and the emitter of Q301 (as measured, not as drawn - I have corrected the drawing), seems to be just the low value of R410, the resistance I measure b-e is 39R, which is exactly the marking on this resistor. So I think this would explain that. One other thing worth noting is that there is evidence of heat around D408, but it tests OK (in circuit).

The updated Primary Control schematic is still here: http://1drv.ms/1KQkTBp



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