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What number did you exactly use? Are you 100% sure the replacement part isn't from new old stock? Anyway, if it works after heating up that area, it must be some of the semiconductors. I'd try replacing the transistors/diodes as well.

Actually the TDA4605 is not the optocoupler, that's just a power supply controller. It can go bad but this fault isn't that common. The component that I meant is the 6-pin OP1 IC, labeled CNYxxxx. I've replaced mine with a CNY75GB which is compatible and quite easy to obtain. Sorry for the...

Exactly, the optocoupler is an important (if not the main) part of the voltage stabilization mechanism in a switching power supply and the LEDs which are inside of it can go bad with time. If the voltages aren't correct after replacing the OP1, you can adjust all of them using the PP1 trimmer...

Re-soldering anything will most likely not help. The OP1 optocoupler is almost certainly at fault. You'll have to replace it.

So I've got an update! This problem is apparently caused by the optoisolator and/or diodes DP3/DP4. I've just replaced these parts and my voltage is now perfectly stable and cold boot finally works. Btw, I used CNY75GB, not CNY17. So in order to eliminate issues with wobbly screen and cold boot...

My mains frequency is totally fine. I just thought maybe the frequency of the short is different because my CRT is wobbling like jelly and the frequency of that is definitely not 50Hz as it is not flickering that fast and the wobble is clearly visible. I don't own a scope though and didn't...

Mine is also 220V. But I think the multimeter got the measurement wrong as it might not necessarily be at 50Hz. Anyway, if you try it, just be sure not to touch any metal parts. I've found this tip in this thread where it was "solved" using a space heater at first.

Did you try powering it without the grounding point of the outlet? I know it's dangerous but I found out that if I do it, the voltages ramp up way faster and it's more stable. I have 120V AC between ground and neutral though. So there must be a short somewhere...

Hey, have you solved it yet? I have the exact same problem. Which part was causing it?

The s/n on the logic board is unfortunately a bit washed out. No s/n, model name or any other info on the case.

Here you go!

In summer, I made a post about my Macintosh LC prototype. The original post is probably lost now. Anyway, I've managed to replace the damaged logic board with a new one and I fixed a leg on the ROM chip and now it's working! So my question is… Is this an ordinary ROM or is it somehow...