There's only one pot on the sound board. The noise happens with the volume down completely and doesn't have any bearing on the noises volume except adding more noise static when you turn it loud.Have you twisted all the sound pots back and forth powered off? Same with speaker volume.
If there is only one pot on the sound board twiddle that back and forth a few times same with the volume. Machine off.
Let me know when you have done this.
It's all a process of elimination. You start with the basics first.
Sounds more and more like random noise
lots of things it could be,
There are effectively 3 separate sound gen circuits in on the board - all feed into the 741 Ic - you can isolated each in turn by cutting each of the resistors, r24, r51, r58 to isolate different parts of the board and see if the issue disappears with one part removed (obviously you will lose some sounds too):
View attachment 166945
You said you tried R58, so try temporarily cutting each of the other 2 in turn - leave 58 in.
if that yields nothing or the sound still there will all 3 circuits out of action, then we can try other stuff:
maybe the capacitors around the power output:
View attachment 166943
Maybe a dodgy -12v supply, check diodes and caps after checking voltage:
View attachment 166944
It literally could be anything, the symptoms are intermittent which means finding the fault is trickier. Seems to clear on a reboot which maybe is a component failing with heat.
I've tried different connectors on the same ribbon but not a brand new oneHave the ribbons been swapped over to eliminate this?
The right pin on 7805 is 5.08, on AC its zerochecked diodes d2 and d3?
they help generate the -12v circuit. that needs to be good.
also check the 5v line is good, the sound board creates its own 5v regulated from the 12v unregulated, so check the 7805 is giving a nice clean 5v dc out and not too much ac.
then its maybe try replace the the 741 ic or the TD1010 amplifier
Swap the ribbons that goto the sound board and the CPU.I've tried different connectors on the same ribbon but not a brand new one
yep should be actually -12v when your black lead is grounded and the red lead on the point where those 2 resistors meet. no ac is a good sign.
try a different ribbon cable.
Finding another ribbon seems difficult. It's a long old thing.
Any ideas where I can get o
Only the tiny one between CPU and interface boardisn’t there one in the machine somewhere