[cctalk] Re: Rainbow H7842 PSU

19 Oct 2025

...

  -----Original Message-----
 From: Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt;
 Sent: 19 October 2025 14:04
 To: rob(a)jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts'
 &lt;cctalk(a)classiccmp.org&gt;
 Subject: RE: [cctalk] Re: Rainbow H7842 PSU
  -----Original Message-----
 From: Rob Jarratt via cctalk &lt;cctalk(a)classiccmp.org&gt;
 Sent: 14 October 2025 21:42
 To: 'General Discussion: On-Topic and Off-Topic Posts'
 &lt;cctalk(a)classiccmp.org&gt;
 Cc: Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt;
 Subject: [cctalk] Re: Rainbow H7842 PSU
 Oh my! I have clearly got my understanding the wrong way around, not
 sure how I did that because I read the datasheet carefully. Somehow, I
 got confused. I re-measured and found 1IN+=5.5V, 1IN-=9.4V, Power
 OK=6.7V, but GND (on the 393) is -13V. So as you say Power OK should
 be -13V and AC OK H would be asserted. I guess this must mean that the
 comparator itself is faulty. I have some 393s, so I will replace it and see what 
happens.

 Today I replaced the LM393P with a new LM393N. The AC OK H signal is now
 working correctly. However, the outputs are a bit on the low side. The +5V
 output is 4.37V, the 6% tolerance quoted in the technical manual would allow
 a minimum of 4.7V. The +12V output is 11.3V, 6% tolerance would allow a
 minimum of 11.3, so only just OK. Any suggestions on what I could look at
 here, maybe the output capacitors? I checked the two capacitors on the +5V
 output and the ESR seems OK when measured in circuit, but perhaps the big
 one before the smoothing choke should be replaced? NB Raw 5V is about
 4.2V 
 Thinking about this a bit more, I am thinking the 5V output capacitors
 because the 5V output is proportionally lower than the 12V output. But they
 are both too low and I guess that implicates the drive control. I seem to
 remember replacing the PWM fairly recently so I would hope its not that, but
 maybe one of its inputs. Vstart is only 11V, which seems a bit suspicious.
 Either the 7812 is bad or something is pulling it down. But it looks like I
 also replaced the 7812. I might try using a bench PSU to provide power to
 the 7812 to see how it behaves.

I still think the first thing to be done is to figure out whether Vz is
correct or not, ideally by reading the markings on the zener diode or otherwise
figuring out what voltage is supposed to be across it and then measuring the
voltage directly across it to confirm or deny that it is correct.
The most likely causes of only 11V coming out of the 7812 are either the input
to it is low (it needs to be at least several volts higher than 12V) or the
current drawn from it is excessive, in which case it will be hot or it is
faulty.   There is also a remote possibility that is oscillating due to a bad
decoupling capacitor.
(It is possible that nothing cares that Vstart is a bit low, some of the
comparator inputs come from it via voltage dividers but the more critical ones
seem to be referenced to Vz.)
There is also the odd 0.7V difference between two ground connections in
different parts of the power supply that needs to be accounted for or fixed
if it turns out to be a fault.
Once these items are dealt with, the low outputs can be addressed if they
are still low after dealing with the other issues.
Regards,
Peter Coghlan.

2025

2024

2023

2022

[cctalk] Re: Rainbow H7842 PSU