On the workbench: Mu-Tron III envelope filter

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.

The LED side of the optocouplers where wired in series, which caused an uneven response, so I rewired them in parallel. The PCB already took using either a singe or dual optocoupler into account.
I also put the transistor back into the circuit, restored the traces and used socket pins so it can easily be swapped out for a different one. The connections are as follows; IC A6 pin 1 to base, collector to 9v+ and emitter to LED+ of the optocoupler.

Optocouplers and driver transistor.

Now I got some sound out of it, but it sounded way off. I couldn't find anything wrong with the voltages and connections but then remembered the very similar EHX Q-Tron has a trimmer in place of the 3k9 current limiter going from optocoupler LED- to ground. My Q-Tron also happened to use the exact same NSL32's so I was thinking the LED side might be getting too much or too little current. So I replaced the 3k9 resistor with a 50k trimmer, and voila, the pedal now sounded right in all modes. I placed the trimmer on the underside of the PCB for easy access.

F I X I N G  A  H O L E . . .

I thought of a solution to cover up the big holes in the enclosure and decided on placing an aluminium plate over them, but this didn't look right, so I tried putting it underneath instead and although the holes where still visible this looked much better. This also gave me the idea of adding LED's for the on/off state and response indicator just like the Q-Tron has. Instead of having two ugly holes in the enclosure it now somewhat look like it was originally build this way.

5mm On/Off LED in the midde and 3mm overload LED at the left.

A D D I T I O N A L  C I R C U I T R Y

Adding an On/Off LED without needing to replace the original switch would need a relay, which also makes true bypass conversion possible. Along with the driver for the overload-LED and the voltage-converter this would need an additional circuit-board. I made a new board that I fixed to the original board using metal-standoffs in place of the standoffs that where already there to make all the mods completely reversible.

Because I couldn't decide on whether to keep the original buffered bypass or change it to true-bypass I made 2 plugins, one for buffered- and one for true-bypass. These plugins are made with different connections fitted on an IC-socket encased in epoxy. These plugins change way the relay connects the circuit to the input and output.

The circuitry for the 'overload' or 'response' LED
comes from the Q-Tron+ schematic, which is virtually identical to that of the Mu-tron, which is no coincidence since they are both designed by Mike Beigel.

B A S S  A N D  G U I T A R  D E M O