Fixing an MXR bass d.i.+ pedal

2021-11-05

In 2016 I bought the MXR bass d.i.+ pedal to use as a DI box for bass guitar. The pedal is great and suits my taste in bass sounds really well. Unfortunately, I accidentally dropped the pedal off my table one day and broke the volume potentiometer for the clean channel.

Back then I tried to find a replacement part for the potentiometer, but as with so many things guitar electronics related; the hardest part is not finding something that works, but finding something that also fits in the design.

The potentiometer is specifically made for horizontal PCB mounting and I could not find other manufacturers besides Alpha (Taiwan) that make this exact model. As I do not use Ebay, finding a retailer that sells this model of potentiometer was harder than I expected.

Finding the part

At first I checked Alpha's datasheet to figure out the exact model of the potentiometer. This one appears to be a RV16AF-41-B2-15K-B100K-3. That is 16mm body, horizontal PCB mount, M7 6.5mm long shaft, metal shaft knurled, linear 100kOhm, no center click. Unfortunately Digikey or Mouser don't seem to sell this exact model, so I had to look elsewhere.

https://www.mouser.fi/datasheet/2/13/RV16AF-2303884.pdf

I remembered that StewMac sells all sort of guitar related tools and parts (although they are bit more expensive of course). I saw this potentiometer on their website and it looked exactly like the potentiometer Alpha manufactures, although with a solid shaft. So I ordered one. I also ordered a knob since the old one was made for a knurled shaft.

https://www.stewmac.com/electronics/pedals/pedal-components-and-parts/pots-for-pc-boards/

Installing the new potentiometer

After two weeks the parts arrive from StewMac. On the bottom of the potentiometer it says Alpha, so I assume this is the correct model. Testing the fit proves my assumptions correct.

There's a plastic cover on the bottom of the potentiometer. I try to install the pedal with the cover on but it does not quite fit so I decide to take it off. I think this cover is to protect the back of the pot so it's easier to solder on to it, as this is something that I have seen done in electric guitars.

After taking the cover off, I get the potentiometer inline with the other components and it is time to finally put the pedal back together.

Reassembling the pedal

As it has been more than five years since I took the pedal apart, I struggled to get everything back inside.

I start with the main PCB. Once I have that in place, I place the XLR connector in its cutout. After that, putting back the footswitches should not be too hard.

Finally, I put the rest of the pedal back together, and here is the result:

The new knob is a bit shorter than the old one but otherwise it fits perfectly. I could probably just round of the threads from the old knob and glue it on, but if I ever need to fix this pedal again taking the knob off would be more difficult.

Sound test

Of course a proper repair is not complete without a test so here are some sound samples. For the bass I used a Marcus Miller V3 with flatwound strings, both pickups on and the pre amp off.

Signal chain is Marcus Miller V3 -> bass d.i.+ -> Scarlet 2i2. There's no post processing or Cab/Amp simulations.

Clean sound

Distorted sound

The pedal has a really powerful gate that reduces the amount of hiss without chopping the signal like a regular gate.

Bass boosted

If I turn on the preamp on the bass, I can push the distortion to really nasty levels with the bass EQ control. It sustains forever without any noticeable change in the noise level.