H7861 PSU issues

Aaron Jackson aaron at aaronsplace.co.uk
Tue Oct 31 16:26:10 CDT 2017

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.



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

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