Disclaimer: I'm still a bit of a pinball newbie but I know my way around electronics. I specialize in arcade/CRT repair and have done the odd pinball repair.
A friend of mine picked up a non working Space Shuttle for a decent price. The previous owner bought it from a vendor "when the game was 8 years old" but apparently has been sitting in their basement not working "for at least 20 years". The aftermarket mylar is in horrible condition but the cabinet, backbox, playfield (besides mylar), and electronics are in good physical condition. No oxidation around transformers, no battery rot, no sketchy/hacked/disconnected wiring. The few repair jobs to the MPU appear to be done well (RAM replaced, small transistor, all pins reflowed).
First try we get playfield GI and backbox lights but nothing else (no playfield/gameplay lights, no attract, nothing on all displays, no coinup). The MPU was reporting Error code 7. After checking all fuses (nothing blown), removing/inspecting MPU, reseating all connectors and chips in sockets, the MPU boots (code 0). The game passes the Sound/Voice test with SW2 on the MPU. However, upon reboot the game still appears in it's frozen state (only GI and backbox lights) and won't coinup until SW1 is pressed on the MPU. After SW1 is pressed the game does a quick audible blip/chirp and then the playfield lights comes alive with coinup, gameplay & sound working (couple faulty switches, nothing major) but without displays. The displays never show anything at any time.
Checking schematics and testing displayboard power input while disconnected I get -93v (both lines) and +5v, but the positive HV line is only at +55v. I was running low on time so took the main Power board home for further inspection (didn't have proper tools with me) and will look at it tomorrow. Everything after F1 on the Power board appears display related so I'm hoping to find a problem there.
My main concerns are; why does SW1 need to be pressed for the game to work, is there a known issue regarding the displays not working, and what is the best way to go about removing a mylar overlay? The mylar is cracked, bubbling, peeling/flaking, but luckily doesn't appear to have damaged the playfield artwork. I can see the faint outline where the factory mylar once was and the replacement is smaller/shorter at most edges. Once removed, what is the best approach to protecting the old playfield?