Sound issue on Tron

[tec9]

Hey guys-
Last night I was repairing a bad SSIO PCB on my Tron machine. I got the PCB up and running (had a bad 74LS244 on the SSIO- which kept the whole stack from booting) Everything now looks good, and the game plays, but the sound is mssing. I can very faintly hear it in the background, and it sounds correct. (all channels can be heard faintly in channel test) I know the problem is on the SSIO PCB, because I have swapped another SSIO PCB in its place, and it works fine. I tried replacing the 3900 on the PCB, but then had to call it a night. I researched a bit today, and read about the mute circuit, so I'll check that also. Anybody else have any ideas on what would cause the sounds to be almost silent? Just looking for advice before I take another look at it tonight.
-Mark

 

[SeanMcL]

Since Tron has a separate sound amp, I think your diagnosis of the mute circuit will prove correct. Sounds like the mute is stuck on.

Chip D3, an LM3900 amp chip, seems to be a prime suspect. It looks to me like this could be/is the muting chip, with outputs to speakers 1 and 2, and one output to ground.