Modify TRS expression pedal to TS, outside the box

[daviddec]

Hello,

I recently got a line 6 m9 pedal, and need to be able to use the expression pedal jacks on it. The standard line 6 expression pedal is a 10k ohm linear pot, with a mono, or TS, cable coming off it, and this is what the m9 pedal is looking for in it's expression jacks. I currently own (6!) expression pedals and they work great for all my other needs, but they are equipped with a TRS cable, and 4 of them (yamaha fc-7) have 50k pots in them. they don't work with the m9 as is, and I don't think I need to spend $100 on two more expression pedals.

What I want to do is use two of the yamahas on the m9, but without modifying the insides of the pedal(s).  (I can't get into them, plus I don't think I have to - am I wrong?)
I have looked around online and can't find the solution I am looking for, because what I want to do is to make a small box with two TRS input jacks, some resistors inside, and two TS output jacks.

Thus, the Yamahas get plugged into the trs jacks, and mono cables attach to the outputs, and then connect to the m9.  inside the resistors convert the resistance change of the yam exp into the required 0-10k ohm range the m9 is expecting. I read somewhere a 12.5k resistor wired in parallel with the 50k exp potentiometer would cut the range down to as needed, but that doesn't seem right to me...

In fact, I tried wiring a 12k resistor in parallel with the incoming yamaha signal exactly like this fellow did

from http://circuits-from-my-shed.blogspot.com/2012/02/first-build-line-6-ex-1-expression.html but it didn't work. (if he is using a 47k to cut a 20k in half, shouldn't it be like 250k I need?)

I tried putting the resistor between the tip and the ground, and grounding the 'shaft' signal like this image but it didn't work either...

Any ideas? Is my 12k resistor the problem? Is my lack of a local radioshack annoying? (yes. )

[daviddec]

Well, a'int that how it goes.

I get off the forum, and re check my attempts, and the resistor I had been using was not the 12k I thought it was. I go, and try the 12k resistor I do have, and lo and behold it works fine.

Problem solved!

[scstowaway]

could you just explain or share a diagram of what actually worked?  thanks

[Kevin Mitchell]

could you just explain or share a diagram of what actually worked?  thanks

This thread is almost 5 years old and the poster hasn't been active since then.
They had a 50k ohm expression pedal and wanted to modify it to make it 10k and use a mono cable for their M9 pedal. Very simple stuff.

It's about a common taper modification. Lots of info here;
The Secret Life of Pots

There are also cheat calculators around if you're trying to do this yourself.