> -----Original Message-----
> From: Rob Jarratt via cctalk <[email protected]>
> Sent: 14 October 2025 21:42
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> <[email protected]>
> Cc: Rob Jarratt <[email protected]>
> 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
Thanks
Rob
