16.3 520STFM PSU

Tips around the 520STF(m) PSU

Problem:

An old Atari 520STFM with the internal supply that has gone bad. Its output voltage is only 3-4 v on the 12 v output and 1.5-2 v on the 5V output. I have replaced mains filter cap, all caps on the outputs, the opto-isolator and a 3-leg IC HA17431P. There is a potentiometer in the voltage-sensing circuit, but adjustment doesn't affect the output voltage. I would be grateful if someone could point me in the right direction to get this old gem running again.

Repairs
Try to disconnect the PSU from machine and then measure its ouptut with possibly some light load, which must be on 5V side (something like 20 Ohm resistor). If your output voltage become O.K. then it's probably only main transistor too "tired".

Usually it's BU508. Replace it & try again. HA17431 is only some kind of gated zener diode which is used for feaadback loop for regulation of 5V side.

Optocopler also plays that role...

If at lighter load everything seems fine, then regulation works, only BU508 can't pump enough power into load to achieve desired voltages....

That potentiometer is for voltage finetuning and you should not touch it if voltages are already way out of reach (it means that failure is somewhere alse)., but you can use it later, when PSU starts behaving sensibly to tune optimal values on all outputs with load connected (=rest of computer).