> > > > -----Original Message----- > > From: Peter Coghlan via cctalk <cctalk@classiccmp.org> > > Sent: 07 May 2023 10:34 > > To: General Discussion: On-Topic and Off-Topic Posts > > <cctalk@classiccmp.org> > > Cc: Peter Coghlan <cct...@beyondthepale.ie> > > Subject: [cctalk] Re: Rainbow H7842 PSU Fault > > > > > > > > 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. > > Sorry for the loose language. I mean that the comparator is working correctly > but the output is not what should be expected, because its inputs are wrong. > > > > > 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 can tell you that if I remove the diode on the output of the -12V > comparator then the PWM operates normally. > > > > > > > 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. > > Thanks for the explanation. I am getting a better understanding now. I have > noticed a difference here between the good and the bad PSU. On the good PSU > I found the collector on TIP121 at 0V. On the bad PSU it was at +0.016V. > The base-emitter voltage drop is -0.02V on the good PSU and -0.09V on the > bad one. This is making it feel like the TIP121 may be faulty, would you > agree? >
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. 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 > > > > > >
