Expression mod for Keeley BubbleTron

[bleachboy]

Hi everyone, this is my first post here !

The Keeley BubbleTron is a great pedal that begs to be modded, a lot. I've already put external on/on switches for the internal bank dip switches.
I would like to add an expression control jack for the speed potentiometer.

Here's what I have done so far :
- I have disconnected the speed potentiometer from the PCB.
- I have soldered leads from the PCB to a 1/8'' switched TRS jack plug on the « fixed » legs of the TRS plug.
- I have soldered leads from the potentiometer lugs to the « switched » legs of the TRS plug.

All of my connections go this way : 3=sleeve, 2=ring, 1=tip. The speed potentiometer has the same value as the one in my exp pedal.

Here's what's working : the speed knob works as normal, when no jack is inserted into the 1/8'' plug. However, when a TRS cable is plugged into it, going to a Dunlop mini expression/volume pedal, although the speed of the effect changes (different speed than when no cable is inserted), rocking the exp pedal has no effect on the speed at all. It just stays fixed where it is. I have tried to reverse lugs 1 and 3 but it did not make any difference, except that the pot was backwards (full speed CCW).

Here's a quick video I've just made

Does anybody know what I might be doing wrong ?

Thanks a lot !

[ElectricDruid]

I'm guessing since we can't see the expression pedal wiring, but my bet is that it has the wiper taken to the tip connection.

You should buzz the expression pedal out with your multimeter and then make sure you've wired up the jack the way that the expression pedal requires.

You'll also find out like that what value pot is inside the expression pedal. It sounds like it's not 10K.

HTH

[bleachboy]

Thank you very much, I'll try that. The manual of the Dunlop DVP-4 pedal states that the pot for the expression is 10k ohms.



I should add that when the 1/8'' TRS cable is plugged into the switched connector, it only lifts the sleeve tab, not the ring nor the tip. I don't know if it's standard or not...

[bleachboy]

There's a ring/tip polarity switch inside the expression pedal, but that didn't seem to make any difference.

Here are some MM readings that I've done :

On the internal pot :
- when measuring pin 1 with pin 2, I get nothing, no matter the orientation of the potentiometer. I just have the number 1 showing on the meter.
- when measuring pin 2 with pin 3, I get 10k ohms when the pot is fully CCW, and 0 when it's fully CW.

On the exp pedal pins :
- when measuring pin 1 with pin 2, I get 10k ohms on the toe down position, and I get 10k ohms on the heel down position.
- when measuring pin 2 with pin 3, I get 0 in toe down position, and 10k on the heel down position. Seems like it's the same than with pins 2 and 3 on the potentiometer of the Keeley pedal.

Plugging a TRS cable into the exp pedal,
- when measuring the tip with the ring, I get 10k in the down position, and 0 in toe down
position.
- when measuring ring with sleeve, I get 10k both on the heel down and toe down position, and when I flip the internal switch on the exp pedal (tip/ring) to the ring position, I get 0 on heel
down position, 10k on the toe down position.
- when I flip the internal switch back to the tip position, measuring tip with sleeve I get 0 on heel down position, 10k in toe down.

What seems a little off to me is that I don't get any readings when measuring pin 1 of the internal pot, contrary to when I do the same thing on the exp pedal, with or without TRS cable.

I'd love some help ! Thanks a lot.