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  1. 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 on the internal photo transistor. The studies showed that the deterioration varied between 10% to 30%. Other suspects in a Mac Classic include the TDA4605 (IP1) IC and the 5.6V zener diode (DP11). All components are quite cheap so it might pay to just replace them all.
  2. Joe

    Classic II 1.44MB Floppy Drive

    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 stretched, which is the default mode when no disk is being present and the head is pushed into its topmost position. I noticed that the base the spring hooks up to has three little notches that allows for adjustment of tension, but admittedly I have never quite got any consistent result with that.
  3. 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 access to a benchtop power supply, you can inject 12V directly on the analog board (with the Mac disconnected from the mains) and check current draw and troubleshoot the feedback circuit (see below). Like bibilit mentioned, the opto-coupler is usually a prime suspect, and on rare occassions I have also had to change the LM324 op amp. Here is the feedback circuit that controls the switching transistor: And the explanation to the above: The feedback control circuit monitors the +12V supply voltage through a voltage divider comprising R35, R56 and R38. This divided-down voltage is compared with a 6.2V zener diode-derived reference voltage (from R34 and CR19). The system tries to maintain equality of the voltages presented to the input terminals of the op-amp by driving the switching converter more or less hard as necessary. An optoisolator (U3) couples the control circuitry, which is all referenced to digital ground, to the core switching converter, which is referenced to primary ground. Resistor R56 (located just above the speaker) allows fine adjustment of the output voltage. Stability of the feedback system is assured by C21, R37, C27 and C28. If the +12V supply voltage is too low, the LED inside the optoisolator is driven with less than normal current. This, in turn, reduces the current through the optoisolator’s transistor, which ultimately commands the switching core to increase its output. The opposite happens if the supply voltage is too high.
  4. 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 on the power and see if the fuse pops or not. If it does, then it is likely you have a fault somewhere from the AC input to the 170V rail. If not, then most likely the switching transistor (yellow circled in photo above) and/or associated circuitry is shorted out.
  5. Joe

    Macintosh Plus (M0001AP) boot chime loop

    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 sound upon power-on as the high-voltage circuit comes and goes too.
  6. Joe

    Compact Mac Repair Guide

    Links above to the repair guide all seem broken but I found a copy here: https://cdn.hackaday.io/files/1681007191990400/classicmac2.pdf
  7. I think what you want is a 10 nF/250 Volt capacitor, if it look similar to this one.