> -----Original Message-----
> From: Peter Coghlan via cctalk <[email protected]>
> Sent: 21 October 2025 01:01
> 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: 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.
> >
>
> Could it be a 1N4626 ? According to the datasheet, this would be a 5.6V
> Zener which seems plausable.
>
> A 4.3V unit would be a 1N4623 according to the same datasheet. 4.3V seems
> like an odd value though.
>
> You mentioned measuring Vz at 6.4V in a previous mail in this thread so there
> is a possibility the voltage might be varying which would not be good news.
Indeed I think I have had different results at different times, I guess this
means the Zener diode could be bad. I will lift it and try to get a better look
at the markings.
Thanks
Rob
>
> Regards,
> Peter.
>
> >
> > 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.
