Nemesis Repair Logs
Problem: Stuck input
After starting game, it would go into "auto-fire" mode. This board had stuck input when I bought it, so during previous repair, about 8 years ago, I installed sockets for optocouplers and LS253 IC's.
DIP switch 3, dip 3 is test-mode. You need to press 1p start to advance test screens until you get to the IO test. I/O test screen reveals that 1P shoot3 and 2p shoot3 were stuck.
Close to edge connector, there are 19 diodes. Function of these are:
D1 2P SHOOT 2 -> Opto 2e Pin 1 D2 2p SHOOT 1 -> Opto 2e Pin 3 D3 2p right -> Opto 2e Pin 5 D4 2p left -> Opto 2e Pin 7 D5 2p start -> Opto 2f Pin 1 D6 1p start -> Opto 2f Pin 3 D7 2p UP -> Opto 2f Pin 5 D8 2p shoot 2 -> Opto 2f Pin 7 D9 Service -> Opto 2g Pin 1 D10 1p shoot 1 -> Opto 2g Pin 3 D11 1p left -> Opto 2g Pin 5 D12 1p right -> Opto 2g Pin 7 D13 2p down -> Opto 2h Pin 1 D14 1p up -> Opto 2h Pin 3 D15 coin 2 -> Opto 2h Pin 5 D16 coin 1 -> Opto 2h Pin 7 D17 1p down -> Opto 2j Pin 1 D18 1p shoot 3 -> Opto 2j Pin 3 D19 2p shoot 3 -> Opto 2j Pin 5
Close to the diodes, there are 5 IC's mareked PS2401 which are quad optocouplers which also deal with input. These are marked with 2E, 2F, 2G, 2H, 2J. The output of the optocouplers are furter routed to 4 74LS253 logic ic's.
When tapping the corresponding 74ls253, 1p shoot cleared to 0.
Swapped LS253 at 3J and 3G, effectively re-seating them.