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.
Thanks. I'll find a video on the suction cap... I've seen some 8-bit Guy videos where does it but it's been a while. I'm mostly concerned about the neck board, given that the neck is the weakest part of the tube and thus the most prone to breaking.
I recall seeing what appears to be hot glue around the area the neck board connects to the tube. Would I need to do anything with that prior to attempting to remove the board?
Also, how much force does it take to remove the neck board? This computer (as far as I know) has never been serviced before I purchased it, so it's likely the neck board has never been removed from the CRT since the computer was manufactured.
For me, it’s just a matter of minutes.
If you never had done it before, the tricky moment is removing the suction cap.
The board and neck board are easy to remove with just one or two screws for the AB and nothing for the neck one.