> -----Original Message-----
> From: Peter Coghlan via cctalk <[email protected]>
> Sent: 19 October 2025 18:32
> To: General Discussion: On-Topic and Off-Topic Posts <[email protected]>
> Cc: Peter Coghlan <[email protected]>
> Subject: [cctalk] Re: Rainbow H7842 PSU
>
>
> >
> >> -----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.
> >
>
> 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.
I measured Vz at 4.3V. In my notes I have previously measured 5V or 5.4V,
possibly on different H7842 PSUs (my notes are not clear). I have found the
Zener diode, it is hard to make out the markings without lifting it, but I
*think* the markings are
00
426
Not sure if this means the Zener voltage is supposed to be 6V? I will lift it
when I get a bit of time, probably in a couple of days, so I can look at it
properly and check it out of circuit with my DMM.
Thanks
Rob
>
> 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.
