Nemesis Repair Logs

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Repairer: InvZim

Nemesis Schematics

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.

Diagnostics

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.

Investigation

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.

Fix

Swapped LS253 at 3J and 3G, effectively re-seating them.