A fellow Newtone Forum member managed to score an old Mu-Tron III envelope filter. Because it was in a bad state and broken, he managed to get it for a good price. Most notable are the huge 15mm holes drilled into the enclose, which makes it impossible to restore it in original condition.
The owner got stuck trying to fix it and sent it over to me. The optocoupler had died and was replaced by 2 NSL32's, the transistor was taken out of the circuit and the traces where bridged like it shows on the known Mu-Tron III schematic. (this transistor doesn't show up in the original Mu-Tron schematics, which indicates this unit is from 1979 after ARP had bought Musitronics before going out of business about a year later.) A 9v+ to 9v+/9v- voltage converter was also added.
