1979 Stern Hot Hand, it's a M-100 MPU. About the only thing functional is the GI. The guy I got it from said it was working well, then everything went out at once. He thought it had to do with the tiny red push button with a short section of attached white wire, breaking off something. I believe the button is the diagnostics button?
At first the LED flashed 5 times at power on. I switched U10 & U11, then it went to 6 blinks. There doesn't appear to be any battery damage. There's an external battery holder mounted to the inside of the cab, but there were no batteries in it.
I've left the machine on & a couple of times the displays will later for no apparent reason light. First time I rebooted, the GI came back on, but not the displays. The next morning the displays were on, and stayed that way for a couple of days until I turned the machine off.
All fuses test good, voltages on rectifier board TP1-4 are as they should be, but TP5 is 0. It should be +43. Seat unseat connectors, nothing. Push on the corner of the board and it acts like a bad connection. Remove the rectifier board from the metal backing plate and everything lights up, GI, feature lights, even the giant rotating flipper bat at the top of the playfield. I touch a drop target, it registers 500. Drop a quarter in the slot, push the start button and the ball pops out. Everything is looking the way it should. Plunger back and we're playing. Hit the right flipper button, it starts to flip, but stops and sadly returns to the down position. Now it's back to just GI, the way we began. All fuses still check good. The coil isn't binding, it moves freely. Reboot and now I'm getting just 1 flash from the LED. Any ideas?