Macintosh Plus (Mac 6)

This computer belongs to a customer, when I received it there was an issue with the power supply and the over-current protection kept turning on.

Work done:
Planned work:

Front 1

Front 2

Front 3

Back

Analog board

There was a RIFA paper capacitors on the board, I decided to give the computer a quick test before replacing it, which was a very smelly mistake:

Blown capacitor

After this the board still didn't work, whenever it was powered on it would make the characteristic flup-flup noise of the crow bar protection circuit, meaning that either there was an over-current or over-voltage event.

I spent a considerable amount of time troubleshooting this analog board, testing the capacitors, resistors and diodes, but I wasn't able to identify the fault. It was only when I tested the flyback transformer in another Mac that I realised that it was drawing way too much current.

After replacing the flyback the problem still appeared the same, but now it was pretty straight forward to trace the problem back from the flyback's primary side to the most likely component, and one of the few I hadn't tested yet: Q3

Schematic