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.

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