[cctalk] Re: Rainbow H7842 PSU

> -----Original Message----- > From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org> > Sent: 19 October 2025 18:32 > To: General Discussion: On-Topic and Off-Topic Posts > <cctalk(a)classiccmp.org> > Cc: Peter Coghlan <cctalk(a)beyondthepale.ie> > Subject: [cctalk] Re: Rainbow H7842 PSU > > > > > >> -----Original Message----- > >> From: Rob Jarratt <robert.jarratt(a)ntlworld.com> > >> Sent: 19 October 2025 14:04 > >> To: rob(a)jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic

> >> <cctalk(a)classiccmp.org> > >> Subject: RE: [cctalk] Re: Rainbow H7842 PSU > >> > >> > >> > >> > -----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 > > > > 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

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