[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

I have done a little more probing around. I have found that the 7812 regulator that drives Vstart on sheet 1 of Tony Duell’s schematic is shorted, so I will have to replace this too. I have not found anything else that looks obviously suspicious. I can’t test the output rectifiers for shorts without desoldering them, which I would rather avoid. I guess the next step is to replace the broken parts and use the light bulb current-limiter method to power on the PSU. Regards Rob

From: Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt; Sent: 24 November 2022 21:45 To: 'Mattis Lind' &lt;mattislind(a)gmail.com&gt;om>; rob(a)jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts' &lt;cctalk(a)classiccmp.org&gt; Subject: RE: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a suggestion that the PIV should be 200V. Not sure what rating I should be going for here? Regards Rob From: Mattis Lind &lt;mattislind(a)gmail.com <mailto:mattislind@gmail.com> > Sent: 22 November 2022 07:54 To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk> ; General Discussion: On-Topic and Off-Topic Posts &lt;cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply 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 <https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-failure/> lure/ /Mattis Thanks Rob