Quick comment w/o looking at the schematic yet, is that zeners don’t do rectification, but do do regulation. If you suspect bad a bad rectifier you can easily check with a scope.
Sent from my iPhone > On May 7, 2023, at 03:33, Peter Coghlan via cctalk <cctalk@classiccmp.org> > wrote: > > >> >> >> The comments about the tolerance of the 7812 were right, it doesn’t >> appear to be an issue with the replacement 7812 regulator because when >> I tried using the bench PSU to feed exactly 12V to the circuit from the >> output of the 7812 the comparator still gave the wrong result. It was >> still wrong if I applied only 11V >> > > 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. > >> >> I then looked at the value of Vz on the good and bad PSUs, when applying >> 12V to the 7812 output. That was 5.4V in both the good and bad PSUs. Where >> I saw a difference was on the -12V output, it was +0.4V on the good PSU >> and 0.56V on the bad one (the voltage varied so this was an average). I >> checked the voltage drop across the current sense resistor. It is 0.01V >> on the good PSU and 0.08V on the bad PSU, which would explain the higher >> positive voltage on the -12V output and the comparator being turned on. >> >> >> >> I am wondering if there could be a problem on the -12V output circuit >> (PSU Sheet 3). I am struggling to understand the purpose of the two >> transistors and the Zener diode there, but maybe one of them should be >> switched on and isn’t. I am also unsure now as to which diode is doing >> the rectification (to -12V). Someone said it was the one attached to >> pin 6 of the transformer, but is that right? Isn’t it the Zener diode >> half way across the page? >> > > 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. > >> >> Regards >> >> >> >> Rob >> >> >> >> From: Rob Jarratt <robert.jarr...@ntlworld.com> >> Sent: 02 May 2023 08:19 >> To: 'Mattis Lind' <mattisl...@gmail.com>; r...@jarratt.me.uk; 'General >> Discussion: On-Topic and Off-Topic Posts' <cctalk@classiccmp.org> >> Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault >> >> >> >> That’s a good idea, I will try that >> >> >> >> From: Mattis Lind < <mailto:mattisl...@gmail.com> mattisl...@gmail.com> >> Sent: Tuesday, May 2, 2023 7:55 AM >> To: <mailto:r...@jarratt.me.uk> r...@jarratt.me.uk; General Discussion: >> On-Topic and Off-Topic Posts < <mailto:cctalk@classiccmp.org> >> cctalk@classiccmp.org> >> Cc: Rob Jarratt < <mailto:robert.jarr...@ntlworld.com> >> robert.jarr...@ntlworld.com> >> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault >> >> >> >> >> >> >> >> >> >> >> Not quite sure what you mean here. I had advice from a friend to bench test >> the control module by providing 14V to the input of the 7812. On the good >> PSU I can see the PWM operate, on the bad one the PWM is shutdown. >> >> >> >> And what happens if you feed in exactly 12 V on the output of the 7812? >> Would the PWM work then? By using a lab supply you could check if the >> circuit is sensitive to variation in the 12V supply. >> >> >> >> >> >> /Mattis >> >> >> >> >> Regards >> >> Rob >> >>
