Today i decided to modify an old ATX PSU i had into a lead acid battery charger.
It's some very old power supply but it has nichicon caps in it so it should be good quality.
It also has a 2kBit EEPROM chip in it with other weird stuff, but i digress.
I disconnected the voltage feedback opto and hooked my circuit to it.
It's basically to bypass all the complicated voltage feedback circuitry to increase the voltage to ~14V and to add current limiting, it's a flyback SMPS so increasing the voltage is fine.
It works great, i get 14.2V and it limits current at just over 3A but it squeals like mad at the most annoying frequency possible and i can't seem to figure out a way to stop it.

I tried connecting capacitor/resistor combos between pin 1 and 2 but didn't get far.
Schematic of my voltage feedback circuit included, it was drawn with LM393 in mind, but after testing and trying to silence the noise i replaced it with a LM358 with two diodes added to outputs to only sink current.
With both ICs i get noise but i had better success silencing it with the LM393.