H7861 PSU issues

Rob Jarratt robert.jarratt at ntlworld.com
Tue Oct 31 19:02:24 CDT 2017

I had a dead 555 on a completely different PSU, so it could be worth
checking. I socketed it when I replaced it so it was easy to replace again.

Also, just because a capacitor doesn't appear swollen or show signs of
leakage, it seems that this doesn't necessarily mean that it doesn't need
replacing. In yet another PSU that I repaired recently, replacing the
capacitors fixed it, although in the end I think the one that really fixed
it was showing signs of leakage. On a lot of caps that I replace they show
no leakage signs, but I do see a bit of a deposit on the negative terminal,
I am not sure if this is a sign of any kind of problem.



> -----Original Message-----
> From: cctech [mailto:cctech-bounces at classiccmp.org] On Behalf Of Aaron
> Jackson via cctech
> Sent: 31 October 2017 21:26
> To: cctech at classiccmp.org
> Subject: Re: H7861 PSU issues
> Just had another look after watching a video about how switch mode power
> supplies work.... On the small control board connecting to J4, there are
> D44Q1 transistors. As expected, there is about 65KHz going into the base
of the
> transistor for the 5V side. However, there is no signal going into the
base of the
> transistor for the 12V side, from pin 3 of the 555. So, it looks like the
problem is
> coming from around here. I measured the suspicious components around the
> 555 and they seem fine.
> How likely is it that the 555 is dead? There is 10v going into pin 8,
which I
> believe is correct.
> Thanks,
> Aaron.
> Aaron Jackson writes:
> > Hi everyone,
> >
> > I've been trying to figure out what is wrong with the 12V rail on my
> > H7861 (BA11-S) power supply. It's showing about 4.2V. The 5V rail is
> > spot on.
> >
> > Page 39 of the following schematics is the main part board of the PSU.
> > http://bitsavers.org/pdf/dec/qbus/MP01233_BA11-S_schem_Mar81.pdf
> >
> > Going into the collector of Q3 is about 80V coming straight from T2 (I
> > think I measured it at about 100Hz), but the emitter is putting out
> > the 4.2V, which is the same as the base voltage and output voltage. I
> > tried replacing this transistor because the hFE was about 80 and a
> > good one was about 120. Unfortunately it didn't do anything.
> >
> > None of the capacitors look swollen and I don't see any leakage. There
> > is a smaller board which I think goes into J4. The 12V side seems to
> > have a 555 timer and adjusting the pot doesn't change the voltage at
> > all.
> >
> > My understanding of switchmode power supplies is very poor. Does
> > anyone have some pointers on what to check or what might be the possible
> cause?
> >
> > Hopefully I can get my PDP up and running again... Only got about 20
> > minutes use out of it.
> >
> > Thanks!
> >
> > Aaron.
> --
> Aaron Jackson
> PhD Student, Computer Vision Laboratory, Uni of Nottingham
> http://aaronsplace.co.uk

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