7 May
2023
7 May
'23
12:03 p.m.
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(a)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.
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> Regards
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> Rob
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> From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
> Sent: 02 May 2023 08:19
> To: 'Mattis Lind' <mattislind(a)gmail.com>om>; rob(a)jarratt.me.uk;
'General Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org>
> Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
>
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> That’s a good idea, I will try that
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> From: Mattis Lind < <mailto:mattislind@gmail.com> mattislind(a)gmail.com>
> Sent: Tuesday, May 2, 2023 7:55 AM
> To: <mailto:rob@jarratt.me.uk> rob(a)jarratt.me.uk; General Discussion: On-Topic
and Off-Topic Posts < <mailto:cctalk@classiccmp.org> cctalk(a)classiccmp.org>
> Cc: Rob Jarratt < <mailto:robert.jarratt@ntlworld.com>
robert.jarratt(a)ntlworld.com>
> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
>
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> 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.
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> 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.
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> /Mattis
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> Regards
>
> Rob
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>