H7861 PSU issues

[Aaron Jackson]

That's good to know. I'll head over to Maplin after work and pick up
some timers and sockets.

I removed one of the suspicious ceramic caps and tested it and it came
out fine. The other cap is shared with the other oscillator so I assume
it is fine. 

Thanks!

Aaron.

Rob Jarratt via cctech <cctech at classiccmp.org> wrote:

> 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.
> 
> Regards
> 
> Rob
> 
> > -----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
> two
> > 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