Macintosh Plus Analog Board Woes

[Nepnep]

Hello everyone, this is an issue with a computer in my collection that’s been on my nerves for months, and that’s a Macintosh Plus with a dead analog board. It makes the usual audio queue of a dead analog board (a fast, high pitched chirping) and the flyback transformer has seen better days as well. But the odd thing is, all 3 voltage rails (5, -5, and 12) all fluctuate rapidly and don’t even reach their specified voltage. 12v, for example, fluctuates from 0.3 to 2 volts. Plus, since I’ve had it, I’ve replaced literally every part that’s that’s known to fail and are still readily available. It’s been recapped, it has a new Zener diode at CR19, and the LM324 chip. I’ve been trying to also track down the SCR and the Opical Isolator IC. Has anyone had this issue before or knows if even replacing the SCR and opti-isolator IC would even do anything?

 

[bibilit]

to be honest, those are hard to troubleshoot.

You can have a try with a new optoisolator, and those are easy to swap and cheap.

 

[Nepnep]

Okay, I’m guessing it doesn’t have to be the exact optoisolator? Those exact Motorola ones apple used are very difficult to find, at least in my research.

 

[bibilit]

As close as possible…you can use the same one the Classic did, CNY17G-3 (IIRC)