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I have had a large number of Mac Classics and Mac Pluses where I needed to replace the optocoupler (QP1) in order to reach the required voltages. After Googling around I learned that the LED light output inside the optocoupler deteriorates over decades and no longer delivers the required light...

I have seen the top head dangling over the disk surface on both 800 kB and 1.44 MB drives, but never tried the solution you suggested which I most certainly will now. In my case I had not cleaned any of the drives so I wonder if the spring perhaps just weakens over the years/decades from being...

Are you measuring the 12V rail with or without load? Maybe something is pulling too much current of this rail, causing the voltage drop. I usually disconnect logic board, FBT and the vertical drive circuit, which all feed of the 12V rail, when troubleshooting this scenario. Also, if you have...

The photo seems to show a black mark on top of LF1, if that is a burn mark then maybe LF1 has a short. Also, is the hightlighted resistor burnt or just black in colour? If all good then my next step would be to desolder the switching transistor (and remove it from the heat sink), then switch...

Check the 12V overvoltage trip circuit. If the 12V rail exceeds 12V and a bit, a zener diode fires a thyristor that shortens the 12V rail to ground, the power supply shuts down momentarily, autorecovers, only to shutdown again, ad infinitum. You can sometimes hear this as a continous tick-tick...

Links above to the repair guide all seem broken but I found a copy here:  https://cdn.hackaday.io/files/1681007191990400/classicmac2.pdf

I think what you want is a 10 nF/250 Volt capacitor, if it look similar to this one.