my brother has an old mac computer that didnt work, so he asked me to fix it, its one of those macs from the early 90s, a Macintosh Classic II.
So i opened it up and i found electrolyte all over the power supply, some of the secondary caps leaked it, i also discovered that the MOSFET was burned up, the shotky diodes looked "warmed" because even the part number wiped out, but they tested ok, i tested every other component with my multimeter and the rest tested ok (besides mosfet and fuse).
I replaced every electrolytic with low ESR caps, including the main reservoir cap, and i also replaced the MOSFET, the mosfet wasnt exactly the same part IRFBC40A (original) vs IRFBC40APBF (replacement) but i compared both datasheets and they seem to be the same thing on paper.
Anyway, after replacing those parts (electrolytics and MOSFET) the powersupply lid up, the computer turns on, the OS loads and it seems to be working properly, however theres a very loud tone that appears to come from somewhere between the FET and the transformer, i cant exactly tell which one is vibrating since both parts are very close together.
This tone starts at a very very high frequency, im guessing above the human hearing threshold (say 25KHz), and starts sweeping down slowly, first you can barely hear it, but as the frequency decreases, you can hear it very loud, the tone is always sweeping down, i can compare it to the sound of a piezoelectric buzzer on old computers, i havent actually waited for it to end, i've turned it off when the sound is around 1KHz, but the sound is so intimidating that i can imagine the SMPS completely melting down at some point.
Could it be that the transformer's primary is not completely dead, but somehow damaged? or that the electrolyte somehow leaked into the transformer?, could it also be that the MOSFET may not be the correct one?
Any ideas of what might be the culprit? like i said the PSU works but its unbearable to hear that tone.