19 Oct
2025
19 Oct
'25
8:03 a.m.
-----Original Message-----
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: 14 October 2025 21:42
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
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