Gorf - P2 stuck closed

[scottz29]

Player 2 button stuck closed, so when I coin up twice, 2P game starts immediately. Test mode shows it closed also. P1 button is working fine.

Pulled control panel expecting to see some wiring mishap, or a bad leaf switch, but everything is working fine. Checked continuity of the P2 line and common ground line all the way back to the board connectors, so I'm guessing something is amiss on the board.

Anybody had this happen? I haven't had a chance to glean over the schematics, but if anyone could start me in the right direction, it would be much appreciated.

Thanks!

 

[scottz29]

Update:

Had continuity between common ground (orange/red) and P2 button (red/yellow) lines all the way back to the I/O board.

Player 2 button: Pin 19 on J1 connector. Runs thru R16, R32 which tested good. Traced P2 line back to U3 (a MC14539B multiplexer IC). Logic probe showed pin 6 (Player 1 button) correctly cycling between low and high when button was pressed, pin 10 (Player 2 button) was stuck high.

Waiting on replacement chip to verify fix.

 

[shred]

Good info, let us know how it works out..

Thanks
Dave

 

[scottz29]

Fixed.

C16, a .1uf between the 2 pullup resistors (R16/R32), was causing the high input I saw on the multiplexer IC. Replaced, and Player 2 line no longer stuck closed.