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.
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.
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
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
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,
> believe is correct.
> 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
> > 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