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:aa...@aaronsplace.co.uk] > Sent: 01 November 2017 18:25 > To: r...@jarratt.me.uk > Cc: 'General Discussion: On-Topic Posts' <cct...@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-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 > > > -- > Aaron Jackson > PhD Student, Computer Vision Laboratory, Uni of Nottingham > http://aaronsplace.co.uk