19 May
2023
19 May
'23
9:24 p.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 10 May 2023 12:04
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
So, I did the following test. I used my bench PSU to apply +12V to the regulator output
and -12V to the actual -12V output. Under these conditions the PWM operated correctly. I
repeated the test on the good PSU and the result was the same.
The good PSU drew 13mA on the -12V line and 92mA from the regulator side. The bad PSU was
16mA and 86mA respectively. When I test only applying +12V to the regulator output, the
good PSU draws about 90mA and the bad one 140mA.
It is worth repeating that the PWM operates correctly on the good PSU when I only apply
+12V to the regulator, but not on the bad one. I guess there must be some kind of short
somewhere, but really not sure where it might be. It has to be something that is pulling
the -12V line slightly higher. The only place where this seems like it could happen is
around E1b I think. Perhaps there is a path through the positive input to E1b to Vcc on
the LM393 (Control Module Sheet 1). Does that seem logical?
I will do all the suggested checks, but I won't be able to do this for
a number of days. However, I wanted to understand something in the
meantime.
The conditions I am applying are (I think!) what
would happen during
startup, and during startup the control board has to make the PWM run,
otherwise the main switching transistor won't operate and the
transformer won't operate to produce the -12V in the first place. So
surely in the startup condition I am applying (which is to supply
Vstart from a bench PSU) is valid? Where is the flaw in my reasoning here?
The PSU also gives a kick start to the -12V line by applying some negative
voltage probably around -15V to it via a 2k7 resistor (on sheet 1). If you are
not providing this, perhaps this is why the -12V line is able to swing slightly
positive and upset conditions in the -12V current sense circuit enough to
prevent the PWM from starting?
Even if you are providing this resistor limited -12V startup supply, could it be
insufficient due to a faulty component leaking current from the -12V line to
ground meaning that the -12V line does not become sufficiently negative to
fulfil the startup conditions?
Could the PWM be managing to start up and run for a cycle or two and then
stop due to a fault in the PWM circuit somehow attempting to draw too much
current from the -12V line? I don't see any connections from the -12V line to
the PWM so this is probably not the case.
Is this the same PSU whose chopper transistor exploded
a while back? Could
there be any carbon deposits remaining on the board or conductive
remnants wedged under components etc causing leakage from the -12V line
to ground?
The component nearest to the exploded transistor is the 10uF capacitor on the output of
the 12V regulator. There are some carbon deposits on it. I did a cursory check for
resistance and ESR and it seemed OK.
Regards
Rob