Moin Electopionier,
I included the primary side in the schematic. The pdf has 4 pages. Also the current monitoring. The current monitoring is global to the board. Digital and analog supplies current monitors are all tied together, likely to turn of via the power on/of optocopler path.
I began probing when it was connected, im the meantime I have it externally, yes with isolation transformer and proper precautions (when I repaired my 1st smps about 25 years ago, I made that mistake touching the cooling plate :-)). I hooked up loads to the supply.
So... Digital sipplies are all ok, including the +3v3, for that reason I focus on the analog supply path. OTP OVP OCP is not triggering, this globally turns off the suplly.
Focus now is the primary side of the analog transformer.
There is a voltage feedback loop over an optocouplet and +-5V are the ones, that are taken to close the loop.
What I notice here is, that the optocoupler works. After a short amount of turn on and off, it goes to permanent off state.
Still, ths VFB of uc3844 is 2.51V, which is, because the COMP pin has 4.2V and only due to the resistive divider path with the 68K1 resistor you get that voltage.
This looks a bit tight dimensioned, considering, that 2.5V is the nominal threshold here.
Replacing the uc3844 resulted in no change of this behaviour.
Oh... The ISense path shows a slowly changing DC voltage of 300..500mV, which is funny, will look in that domain a bit further. Would not expect DC here, especially not, if the OUT pin is continously LOW.
Edit: The sounds are mainly some clicking and sometimes a (sorry for german: Pfeifen, with about 400Hz frequency). I tried long time to find the location of that sound, but failed (Another incidence of where a "sound camera" would be cool to have).
Beeing more an RF guy, SMPS are a bit of miracle, allthough I understand the principle well, lacking a bit of experience how it exactly works to the latest details (like the current sensing path), allthough I repaired already quite a few, so any comments welcome.
I attached the primary side of the schematic + notes of some voltages I measured on the UC3844. Whenever I measure a DC voltage with a multimeter, I also checked with the scope, if it is really DC or not. So.. UC3844 is pretty stable in the sense of, that nothing is toggling there.
Best regards,
Kai
Ps: REF shows proper 5V on the uc3844! The big capacitors and PFC are ok and show proper voltages (otherwise the digital supply wom't work too).