Hello Peter, Thanks for the analysis. There doesn't seem to be anything further back from the diode, unless you mean further back behind the inductor?
I will check on the output side as you suggest. The circuit breaker did pop out when it failed. The onboard fuse is intact. The house RCD triggered and cut the power to the whole house when the transistor exploded! Do you, or anyone else, have an idea what the diode could be so that I can find a replacement. Like I said, it seems to be marked D610, and there are some other ones that look to be the same elsewhere in the PSU. Thanks Rob > -----Original Message----- > From: Peter Coghlan via cctalk <[email protected]> > Sent: 20 November 2022 18:50 > To: General Discussion: On-Topic and Off-Topic Posts > <[email protected]> > Cc: Peter Coghlan <[email protected]> > Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power > Supply > > Hi Rob, > > I'm only guessing here. I think the sequence may have been that the main > switching transistor failed first as it would be under more stress than a diode > in the base circuit. If the transistor shorted E-B-C then the HT would become > connected to the circuitry at it's base which would be compelely unable to > cope with voltages and currents involved. This probably resulted in the > failure of the diode. I think it may be worth looking at the components > further back the drive chain from the diode. > The inductor could be ok unless it is a very frail little thing but small signal > semiconductor components and/or resistors further back may not have > fared as well as it. > > It might also be worthwhile checking for shorted rectifiers on the output side > in case this was the cause of the stress on the switching transistor. > However, the power supply might have an overcurrent trip to reduce the > possibility of this sort of damage. If there is an overcurrent trip or thermal > trip, this may have been reset after the power supply was powered off for a > while and when it was powered on again, the already damaged transistor > could have been teed up to fail more spectacularly? Like I said, just guessing > here. > > Were there no fuses failed or cutouts cut out? Does it look like there should > have been? I would think a shorted switching transistor should have caused > some safety device to operate. Or is it the case of the old adage that the > faster acting transistor managed to sacrifice itself in time to protect the quick- > blow fuse from blowing? > > Regards, > Peter Coghlan. > > > > > The H7842 PSU in my Rainbow failed yesterday. At first the machine > > just powered down and there was a slight burning smell, I wasn't next > > to the machine when this happened, so I didn't see or hear anything to > > tell me where the problem might be. Not being sure if there was a > > short in the machine or a problem in the PSU, I disconnected the fans, > > FDD and HDD and, probably foolishly, I applied power again to see if the > machine would work. > > At this point there was a bang and a flash in the PSU. > > > > > > > > On opening up the H7842 power supply I found that one of the > > transistors had completely disintegrated. It looks to be the main > > switching transistor, here is a picture of it: > > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165850.jpg. > > I have identified a source for this transistor, but if anyone can > > suggest a modern replacement that would be useful too. However, that > > is not my main problem. > > > > > > > > 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. > > > > > > > > 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? > > > > > > > > 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- > supp > > ly-fai > > lure/ > > > > > > > > Thanks > > > > > > > > Rob > >
