Hello Rob!
> > Given that before the transistor blew up there had clearly been another > failure somewhere else, I tried to find the original failure. There were no > obviously damaged parts, so I just probed around near the transistor for > any > parts that were open circuit or short circuit. I found a diode connected to > the base of the transistor that appeared to be short circuit. So, I decided > to lift one end to check it. As I de-soldered one of the leads, the diode > broke in two. So clearly the diode was either damaged by the failure of the > transistor, or it was the cause of the failure. This is the diode: > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg. > > > DEC used a lot of A114x diodes in their PSUs. They looked exactly like that one. Those are fast recovery diodes. https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html I would replace it with a UF4007 or something similar. https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf > > I can't quite make out the markings on the diode to know what to replace it > with. I think it says "D610". Would that be the right designation? If so, > can anyone suggest a suitable replacement please? > > > > The diode seems to connect an inductor to the base of the switching > transistor and the collector of the transistor is connected to a > transformer. Should I be looking for other failed parts? Not sure if the > diode failed first and then caused the transistor to fail? Or if something > else has failed which caused these parts to fail? > Also check all other semiconductors. Also on the outputs. If there is a 1 ohm fusible resistor in the base drive circuit check that one as well. In the VT100 PSUs it happens that it blows. > > > > I do know that there are no shorts in the Rainbow itself, because I have a > spare PSU that still works fine in the same machine. > > > > I blogged this here (it repeats most of that I have said above): > > https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai > lure/ > <https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-failure/> > > > /Mattis > > Thanks > > > > Rob > >
