7 May
2023
7 May
'23
1:54 a.m.
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
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?
Regards
Rob
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
That’s a good idea, I will try that
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
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