> -----Original Message-----
> From: Rob Jarratt via cctalk <[email protected]>
> Sent: 21 October 2025 06:44
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> <[email protected]>
> Cc: Rob Jarratt <[email protected]>
> Subject: [cctalk] Re: Rainbow H7842 PSU
>
>
>
> > -----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.
I have removed it. The markings are:
MK (the brand I assume)
058
71
00
426
It measures with a Zener voltage of 5V according to my DMM.
Given the "426" marking, is it reasonable to assume this should be a 6V Zener?
Thanks
Rob
> >
> > 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.
