> > I don't think we can draw any conclusions from voltage measurements > around the TIP121 without the -12V line being approximately correct or at > the very least being negative. > > It's collector voltage being 0V on the good PSU suggests there is no current > flowing through the parallel 20R resistors and therefore no emitter-collector > current through the TIP121 so it's not being switched on meaning that it's > driver doesn't feel the need to pull the -12V line down nearer 0V because it's > already low enough. (Maybe having a load is enough to keep it at the correct > level and the shunt regulator only starts doing stuff if the load is less than > expected or removed altogether? Or maybe the -12V line is not present when > the test was done?) > > The collector voltage of the TIP121 on the bad PSU being slightly positive is > probably due to the voltage on it's emitter (ie the -12V line) being positive > instead of negative which makes it's bias conditions all wrong and any > measurements around it mostly meaningless.
I am going to read your answer more carefully later. But I wanted to check one thing. I measured the base-emitter voltage as negative in both cases, and yet the TIP121 appears to be conducting on the bad PSU. Surely that means that the TIP121 is not working correctly? > > How about applying -12V or slightly less from a bench supply current limited > to about 150mA to the -12V line and seeing what happens? If the PSU > and/or -12V load tries to draw more than that there would seem to be a real > overload and further investigation is needed to find out what is drawing the > current. > > Measure the voltage across the zener diode in the -12V regulator and > compare with the good PSU. Also compare the voltage across the parallel > 20R resistors. > A larger voltage here would indicate more current being drawn through the > TIP121. > > Under these conditions, there shouldn't be any voltage across the 51R > current sensing resistor for the -12V line. If there is, it suggests there > could > be reverse leakage through the -12V rectifier diode. > > > However, Brent's calculations show that the current trip value for the -12V > line is as low as 1.3mA and I can't see any reason to disagree with his > calculations or his conclusion that this seems very low (except that another > tiny smidgen of current is available from the negative startup supply but this > won't really have any bearing on things). If this is really the case, then > placing > something like a 5k6 resistor across the -12V line on the good PSU should > cause enough current to flow for the trip to operate. Finding this level of > leakage in the failed PSU is not going to be easy. > > On the other hand, if this test doesn't trip it, then please look very > closely at > the resistors and connections to the inputs of E3d and verify that they are as > described on the circuit diagram. > > It seems very strange indeed to have a trip value as low as 1.3mA combined > with a shunt regulator whose method of regulation is to pull the voltage > down by drawing current from the supply line. Perhaps the shunt regulator > might be able to pull enough current to cause the trip to operate if the -12V > line was too high (in the negative sense) or if the shunt regulator was under > the mistaken impression that the -12V line was too high? > > (This is a bit unlikely but the 115V/230V switch is set correctly, isn't it?) > > On the other other hand, if the manual says that the -12V line is supposed to > be able to supply 150mA, then it doesn't make sense for the current trip to > operate at 1.3mA and we must be going wrong somewhere. > > Regards, > Peter. > > > > > > > > > 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 > > > > > > > >
