[cctalk] Re: Rainbow H7842 PSU Fault

20 May 2023

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 -----Original Message-----
 From: Peter Coghlan via cctalk &lt;cctalk(a)classiccmp.org&gt;
 Sent: 10 May 2023 12:04
 To: 'General Discussion: On-Topic and Off-Topic Posts'
 &lt;cctalk(a)classiccmp.org&gt;
 Cc: Peter Coghlan &lt;cctalk(a)beyondthepale.ie&gt;
 Subject: [cctalk] Re: Rainbow H7842 PSU Fault

 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.

 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? 
Ok, it looks like there is not a severe leak from the -12V line to ground then.

I am puzzled by the extra current draw on Vstart by the bad PSU but I'm not
sure that tracking this down would lead us to the real problem.

On the other hand, did you mention at one point that Vstart was varying?
If this is the case, the reason for this would probably need to be found and
fixed independent of whether it leads to finding the main problem as this is
supposed to be a stable supply.

I don't think there is likely to be any serious leakage via E1b because the
link to the -12V line is via a 75K resistor which would limit any leakage
current to roughly 160uA.  Of course this applies if the resistor really
is 75K and doesn't have carbon deposits bridging the tracks and connections
around it to somewhere else.

I would suggest looking carefully at the resistors around E3d to make sure
they have the correct values, especially the 360K resistor and making sure
there is no debris etc around these components that could be bridging any
connections associated with them to somewhere else, also that no connections
have been severed.  Problems here could be leading to E3d falsely triggering
when there is no real overload.

It might be useful to check the voltages and resistor values in the -12V
regulator and compare with same in the good power supply, especially the
voltage across the zener diode.

...

 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.

This capacitor is probably there to ensure the 7812 doesn't oscillate.  Looking
at Vstart with an oscilloscope should confirm that this is not an issue.  If
it doesn't have excessive leakage current and has approximately the correct
capacitance, it is probably ok.  However, if there is gunk trapped underneath
it around the leads, this might account for the extra current draw on Vstart.

The explosion could have had other bad effects.  Maybe E3 got damaged by a
surge in its power supply when the transistor blew up?  Maybe the -12V
rectifier was affected?  It is probably not as robust as the rectifiers for
the other lines and the chopper transistor shorting would have likely caused
a big current pulse in the chopper transformer primary, leading in turn to
surges at it's secondaries.  Also the diode in parallel with the 51R sense
resistor might be suspect.

I'm not sure how to test these components comprehensively without trying
replacements for them.

If the 7812 was damaged at the time of the explosion, other components
powered from Vstart could have experienced surges as well.  Maybe stuff on
the input side of the 7812 too?

Regards,
Peter. 

...

 Regards

 Rob

2024

2023

2022

[cctalk] Re: Rainbow H7842 PSU Fault