[cctalk] Re: Rainbow H7842 PSU Fault

What do you mean by "gave the wrong result"? If the power supply to the comparator is "reasonable" and the comparator's output does not reflect what is happening at it's inputs, the comparator is faulty. If the comparator is giving the "wrong result" because it's inputs are telling it to, it is behaving correctly. With no mains supply connected and a positive startup voltage applied to Vstart and a negative startup voltage applied to the -12V line via a 2k7 resistor, you could try shorting the inputs of the comparator together and see whether this changes the comparator's output. I want to emphasise doing this without power going to the mains rectifier feeding the chopper so that if there is real overload, the magic smoke will not be released. If the output if the comparator is then "correct" and this output being "wrong" was the source of the PSU not working, then the PWM should start up like it does in the good power supply. This would confirm that there is a problem in the components providing and/or mmonitoring the -12V line. If the PWM does not start up, this suggests the problem is elsewhere. The TIP121 darlington transistor is a shunt regulator for the -12V line. The voltage at the base of the MPSA55 transistor varies with the -12V line because it is connected to the -12V line via the potential divider formed by the 1k24 and 1k10 resistors. If the voltage on the -12V line increases in the negative direction, the voltage at the base of the MPSA55 increases in proportion to it via the potential divider. It's emitter voltage is fixed by the zener diode so the MPSA55 is turned on more and it pulls more current through the base of the TIP121 which results in the TIP121 conducting more and pulling the -12V line down closer to it's correct voltage. The opposite happens if the -12V goes lower than it should be, the TIP121 is turned on less and this allows the -12V line to increase negatively to it's correct value. None of this circuitry should be doing very much until the chopper transformer is producing the source for the -12V line. I suppose if the TIP121 is sborted or the zener diode is shorted, it could be causing problems, however, from the test results etc we have been given so far, I am not completely convinced there is a problem with the -12V line. If a negative startup voltage is applied to the -12V line as well as a positive voltage to Vstart to better simultate startup conditions, it may reveal more about what is happening with the -12V line. It might then be possible to compare the voltages across the zener diodes in the working power supply and the non-working power supply for example. Regards, Peter.