Today, while repairing an analog board for a Macintosh Plus, I stumbled upon a phenomenon: when the 12V is overloaded, the 5V voltage rapidly increases until it burns out the logic board. This phenomenon was reproduced on this Mac.

My analysis of the possible causes is as follows: The negative feedback regulator (comparator and optocoupler) monitors the 12V voltage in real time. When the 12V is overloaded, the voltage tends to decrease. The negative feedback regulator attempts to increase the PWM duty cycle, at which point the 5V voltage also increases simultaneously. Since the 5V load is mainly the chips on the logic board, which is relatively fixed, the voltage increase causes a quadratic increase in power consumption, leading to a sharp increase in the load on the power transformer. This further pulls down the 12V voltage, causing the PWM duty cycle to increase further, creating positive feedback. This continues until the 5V voltage reaches a terrifying 8-10V, partially burning out the logic board. Then, the load changes, and the power supply enters a "flup-flup fault mode" and stops.

Therefore, I will add a 5.6V Crowbar circuit to this Mac while repairing it to prevent further burnout.

Wow. This is the first I've heard of that problem.

The 128K through the Plus use essentially the same Analog Board, so it would potentially have implications for all of those machines.

Even so, your report is the first I've heard of such a problem. And I've been in this hobby since about 2004, participating in a lot of online forums. Then again, people don't always describe their troubles in great detail like you did, so it could be others have seen this before but didn't have the means to describe it properly. Very interesting find.

>> wskjinfen said:

Therefore, I will add a 5.6V Crowbar circuit to this Mac while repairing it to prevent further burnout. Click to expand...

It would be great if you could document this here for the rest of us that might need it someday. At least you've found this issue.