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 <cct...@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-boun...@classiccmp.org] On Behalf Of Aaron > > Jackson via cctech > > Sent: 31 October 2017 21:26 > > To: cct...@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