Big Muff tone control Expression Jack mod problems

[M.Spencer]

Hi forum, a friend asked me to modify his NYC Big Muff so that he could control the tone pot with an expression pedal (more specifically, using the EXH CNTL Knob expression pedal which switches between two 10k pots). I added the 6 terminal stereo switching jack, and everything seems to be hooked up correctly but the expression pedal gives no control over the tone.

It is wired like this: disconnected the tone pot from the PCB; PCB pot pad 2 to non-switching jack tip, pad 3 to non-switching jack ring, pad 1 to non-switching jack sleeve; switching side of the jack is wired to the stock B100k tone pot accordingly. When the TRS cable is unplugged the tone pot works as normal. With the TRS plugged into the CNTL Knob it's as if the tone pot is full CCW and no tone adjustment can be made by moving the expression pedal's pots.   

Notes: I assumed there will be a sweep difference switching between the 10k and 100k pots. There is absolutely no change in the tone with the exp pedal plugged in; when I alligator clamped a b10k pot to a stereo jack and plugged in the TRS cable there was a noticeable tone sweep. I have continuity tested all connections with my DMM - when connected the pots on the CNTL Knob connect to the PCB pads as you would expect.

Can somebody please help me trouble shoot this? What am I doing wrong here? Thank you! 

EDIT:
I just wired up two b10k pots and a stereo jack to a 3PDT stomp switch and it works as intended, switching between two pots on the muff. I tried the CNTL Knob with a Boss pitch shifter and it functions correctly. So.....why does my caveman analog expression pedal solution work with the Big Muff but the CNTL Knob does not?

[M.Spencer]

"why does my caveman analog expression pedal solution work with the Big Muff but the CNTL Knob does not?"

Answer: (and please correct me if I'm wrong) The CNTL Knob switches between two pots that are set up as variable resistors, e.g. pot lugs 1&2 or lugs 2&3 are wired together. Most Expression Pedals are set up as variable resistors with one outer pot lug going to ground or tied to the wiper, NOT as voltage dividers with all three lugs in play. 

My passive alligator-clamped CTRL Knob works as both a voltage divider for the tone knob in the Big Muff, OR as a variable resistor for pedals that are looking for a CV in such as my Boss PS-5 (these circuits essentially "ignore" lug 1 and measure resistance between lugs 2 & 3. Again, correct me if I'm wrong).

[antonis]

Could you plz post a draft schematic 'cause my brain hurts..?? 

[iainpunk]

the sweep of a BMP tonestack really loses a lot of control over the bass with only a 10k pot.

i thought CV pedals generally give 5 and 0 volts to the outer lugs, and the wiper creates a voltage in between.

its a bit of a confusing post, but i guess that in your exp. pedal, lug 1 an 2 are connected, which shouldn't be the case.

cheers

[ElectricDruid]

TBH the Big Muff tone control is a worst-case scenario for an external expression pedal control. The tone circuit is very interactive and hugely affected by different pot values so plugging in a different expression pedal (even one that worked) could alter the whole response of the thing completely. If you stick with "You can use it with *this pedal* and no other" then it'd be ok, but you can bet your life that someone will plug something else in at some point.

[M.Spencer]

I appreciate the responses. Sorry my post was hard to understand, I was trying to give as much detail and background as I could and I don't have the experience you folks here have.

I took my DMM and traced the path on the CNTL Knob from the pots to the stomp switch, it seems that pot 1 has lugs 2&3 connected, pot 2 has lugs 1&2 connected, so that won't work in my admittedly strange application.

I had read the expression pedal article on geofx a while back and thought I would take a stab at it. i told my friend ahead of time that the 10k for 100k swap would change the tone sweep, he said go for it. After much Google searching it appearsno one mass produced an expression pedal that would do what I want it to do exists, probably for good reason. I will build him a box that does as he wishes and lets hope he doesnt lose it or sell the pedal without! Thanks for your help.

[M.Spencer]

Could you plz post a draft schematic 'cause my brain hurts..?? 

This is what I came up with.


I thought up the DPDT switch that alters the pot value last night, do you think it will work as intended? That way the switcher may be usable for pedals with different pot values.

Here is what I made for my friend. He seems pleased with it so far.




Quick and dirty graphic: