H7861 PSU issues

[Rob Jarratt]

Great news, congratulations! I am about to see if my H7826 works tonight
after cleaning it up and replacing a few electrolytic capacitors.

Regards

Rob

> -----Original Message-----
> From: Aaron Jackson [mailto:aaron at aaronsplace.co.uk]
> Sent: 01 November 2017 18:25
> To: rob at jarratt.me.uk
> Cc: 'General Discussion: On-Topic Posts' <cctech at classiccmp.org>
> Subject: Re: H7861 PSU issues
> 
> Picked up a few 555s and sockets and now it works! I am very happy. Going
> from not knowing how switch mode power supplies work, to watching some
> YouTube videos, and then finally being able to debug the problem and fix
it was
> a lot of fun.
> 
> I wonder what will die next.
> 
> Thanks,
> 
> Aaron.
> 
> 
> Rob Jarratt writes:
> 
> > 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
> 
> 
> --
> Aaron Jackson
> PhD Student, Computer Vision Laboratory, Uni of Nottingham
> http://aaronsplace.co.uk