Outrun sound problem

Hi all,

I have a problem with the sound on my Outrun.

The engine sound suddenly is bad, now it's just garbage.

I already pushed down all ic's.. but all other sounds play well (background music, crashes, ..). In tests all samples play well, all eproms have correct checksums.

I noticed the sound is not stereo anymore, comes only out of 1 speaker.

Searching for information I found there are 2 ic's involved (don't know which ones) for sound.. so I think 1 to play samples from eproms, and 1 to play engine sound (as it's controlled by cpu and increases with speed).
So maybe one of these ic's is bad.. or maybe where the sound is mixed or the amplifier ?

Does anyone have schematics and can tell me what ic's are involved in this sound part ?
Or anyone who can repair the board so I can send it to ?

Thanks,

Aeneas.