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 two of the base of the problem is 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

-----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

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

> > 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

> > 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

anything. -- Aaron Jackson PhD Student, Computer Vision Laboratory, Uni of Nottingham http://aaronsplace.co.uk