I service a Data East LW3 for a friend of mine and according to them the sound keeps gradually getting louder and louder when the machine has been on for a while (i.e. hours), even with the volume knob turned all the way down. The sound itself is fine, and the machine itself works OK too. Something must be heating up and changing resistance...

Now I'll take their word for it because i couldn't stay to listen for a few hours I've taken the sound board home and already replaced some questionable caps. I'm not sure what else could be causing such thing. In another topic about a similar issue it was suggested to apply fresh thermal paste to the amplifiers and tighten the screws. But I haven't found any topics with the exact same issue.

Everything else i could think of looks and measures ok and I can't reproduce/test since I don't have a DE machine near.

Has anyone else ever seen this, or have some insights on how to fix it/what else to look for?

I have done the above and returned the board to service. I have not heard from them since, so I'm assuming the problem is fixed So I have no definite answer, but it cant hurt to try