Testing H7864 (MicroVAX II) PSU With No Load

Robert Jarratt robert.jarratt at ntlworld.com
Mon Nov 2 17:17:50 CST 2015

> -----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...



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