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-supply-fai lure/ Thanks Rob
