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

Tony Duell has reverse engineered the following schematic. http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf I will go with the 1000V as you suggest anyway. Thanks Rob From: Mattis Lind &lt;mattislind(a)gmail.com&gt; Sent: 25 November 2022 07:12 To: rob(a)jarratt.me.uk Cc: 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 On 24 Nov 2022, at 22:45, Rob Jarratt &lt;robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> > wrote:  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? Given that I didn’t have a schematic and this is on the primary side I went for the recommendation of 1000V. 200V may a bit low on the primary side depending on the application of the diode. On the primary there can be sustained voltages up to 400V and peaks that go even higher. Using a diode with higher PIV almost never affects the operation as long as other parameters stay the same. In this case the most important parameter is the trr. It has to be a fast recovery diode. In this case the UF4007 is slightly slower than the UF4004. But I doubt it has a big significance. Actually the A114 is much slower. 200 ns. :Mattis 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