[cctalk] Re: Rainbow H7842 PSU Fault

-----Original Message----- From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org> Sent: 20 May 2023 09:20 To: 'General Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org> Cc: Peter Coghlan <cctalk(a)beyondthepale.ie> Subject: [cctalk] Re: Rainbow H7842 PSU Fault Ok, it looks like there is not a severe leak from the -12V line to ground then. I am puzzled by the extra current draw on Vstart by the bad PSU but I'm not sure that tracking this down would lead us to the real problem. On the other hand, did you mention at one point that Vstart was varying? If this is the case, the reason for this would probably need to be found and fixed independent of whether it leads to finding the main problem as this is supposed to be a stable supply. I don't think there is likely to be any serious leakage via E1b because the link to the -12V line is via a 75K resistor which would limit any leakage current to roughly 160uA. Of course this applies if the resistor really is 75K and doesn't have carbon deposits bridging the tracks and connections around it to somewhere else. I would suggest looking carefully at the resistors around E3d to make sure they have the correct values, especially the 360K resistor and making sure there is no debris etc around these components that could be bridging any connections associated with them to somewhere else, also that no connections have been severed. Problems here could be leading to E3d falsely triggering when there is no real overload. It might be useful to check the voltages and resistor values in the -12V regulator and compare with same in the good power supply, especially the voltage across the zener diode. from This capacitor is probably there to ensure the 7812 doesn't oscillate. Looking at Vstart with an oscilloscope should confirm that this is not an issue. If it doesn't have excessive leakage current and has approximately the correct capacitance, it is probably ok. However, if there is gunk trapped underneath it around the leads, this might account for the extra current draw on Vstart. The explosion could have had other bad effects. Maybe E3 got damaged by a surge in its power supply when the transistor blew up? Maybe the -12V rectifier was affected? It is probably not as robust as the rectifiers for the other lines and the chopper transistor shorting would have likely caused a big current pulse in the chopper transformer primary, leading in turn to surges at it's secondaries. Also the diode in parallel with the 51R sense resistor might be suspect. I'm not sure how to test these components comprehensively without trying replacements for them. If the 7812 was damaged at the time of the explosion, other components powered from Vstart could have experienced surges as well. Maybe stuff on the input side of the 7812 too?