MVH - MV1T Player 1 Button 3 stuck

[hardyhell]

Hy.

I just got a MVH - MV1T mvs motherboard.
It has the problem that button 3 of player 1 is always on. Can somebody perhaps help me to find the problem?
Perhaps point me to the chip that handles the input signal or help me trace the route of the singnal.
It’s not the input from the cab. That I made sure already.

Thanks