Buzzard (Gyrodine) - obscure sound fault

Solder

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I have a Buzzard (Gyrodine) boardset with a very unusual sound AND coin up fault - two things happen if the volume level is turned up:

a) When coining up, the counter increases at a rapid rate, along with the coin up sound - this continues until it gets to 99 where it stops

b) The sound level does NOT increase when the volume pot is turned up, it stays at a low to medium level

However, if the volume pot is turned down then not only does the sound level decrease to nothing, BUT the 'phantom' coin ups also stop!

The music is also a bit distorted at the start of a game

Have replaced the AY-3-8910, checked the relevant coin inputs, replaced the volume pot and electrolytic caps, replaced the C1181 amp, etc but nothing seems to help.

Why on earth is the volume pot NOT increasing the volume while it IS madly increasing the coin up input?

All voltages are correct, wiring is fine, connections are fine.

Any ideas please?

Thanks
 
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