> -----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?
>
> 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
> >
> >
