> -----Original Message-----
> From: Rob Jarratt <[email protected]>
> Sent: 19 October 2025 14:04
> To: [email protected]; 'General Discussion: On-Topic and Off-Topic Posts'
> <[email protected]>
> Subject: RE: [cctalk] Re: Rainbow H7842 PSU
>
>
>
> > -----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
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.
>
> Thanks
>
> Rob
