I have a 1977 Stern Pinball machine that randomly resets. The game has the alltek ultimate MPU and SDU boards. I checked 5 volts on the MPU board and got 4.7 which was a little low but I think OK. On the SDU it reads 4.9. So I decided to check other voltages. Checking the voltages at the rectifier board I found 15.6VDC on TP3 instead of 11.9VDC. I checked the voltage from the transformer for that circuit and found 13.2VAC which seemed a little high but not 15.6. I thought I might have a short so I pulled fuse F3 and the TP3 went to 0.
I am baffled how I can have more DC Voltage out than AC voltage in? Not sure if this is the cause of my reset problem but it is a problem. I am surprised that something has not smoked with the voltage that far out of range.
Any thoughts would be much appreciated.