[cctalk] Re: Rainbow H7842 PSU

-----Original Message----- From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org> Sent: 21 October 2025 06:44 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 carefully. 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.

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.