Just trying to decide what to replace the failed diode with, and looking at the UF400x series, as suggested by Mattis. It seems to me that as long as the PIV is 200V or higher it should be fine from that point of view, the switching speed is never higher than 70ns, while the original A114x (assuming it *is* an A114x) has a switching speed of 200ns (possibly even 200us from the datasheet).
However, I am wondering about the forward voltage drop. The datasheets suggest that the A114x parts have a 1.3V forward voltage drop. I have a spare H7842 that was working (until I messed it up today, another story), so I tested the diode in that, its forward voltage appears to be 0.5V, using a little tester I have. The UF400x have ratings of either 1.0V or 1.7V. How sensitive is the circuit going to be to the forward voltage on the diode? Given that the forward voltage of the suggested replacement is higher, would it slow down the speed with which the transistor is switched off too much and cause it to be overloaded and fail? Thanks Rob > -----Original Message----- > From: Peter Coghlan via cctalk <cctalk@classiccmp.org> > Sent: 25 November 2022 09:44 > To: General Discussion: On-Topic and Off-Topic Posts > <cctalk@classiccmp.org> > Cc: Peter Coghlan <cct...@beyondthepale.ie> > Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power > Supply > > It is often possible to infer the component ratings needed from the other > components around them. A component in the base circuit of a transistor is > likely to experience lower currents and voltages than one in the collector > circuit. > > In this case, we can see from Tony's diagram that there is a 2.7 Ohm resistor > in parallel with the diode. Suppose it is a 1W resistor. This means that the > from P = I squared R, the average current the resistor is likely to pass is > less > than 1A. From Ohm's law, V/I = R, this means the average voltage across the > resistor is likely to be no more than 2.7 Volts. > > It is possible that the peak current / voltage involved could be higher than > the average for short periods of time but we have plenty of margin for error > here so we don't need to think about that too much. A diode with a PIV of > 200V should be fine here. > > Regards, > Peter Coghlan > > > > > 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 <mattisl...@gmail.com> > > Sent: 25 November 2022 07:12 > > To: r...@jarratt.me.uk > > Cc: General Discussion: On-Topic and Off-Topic Posts > > <cctalk@classiccmp.org> > > Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 > > Power Supply > > > > > > > > > > > > > > > > > > > > On 24 Nov 2022, at 22:45, Rob Jarratt <robert.jarr...@ntlworld.com > <mailto:robert.jarr...@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 <mattisl...@gmail.com <mailto:mattisl...@gmail.com> > > > > > Sent: 22 November 2022 07:54 > > To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ; > > General Discussion: On-Topic and Off-Topic Posts > > <cctalk@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.ht > > ml > > > > > > > > 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- > supp > > ly-failure/ > > <https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power- > sup > > ply-failure/> > > > > > > > > > > > > > > /Mattis > > > > > > Thanks > > > > > > > > Rob > >
