Fixing a Half Functioning MXR Pedal

Somewhere on the America west coast, in a bar or venue somewhere, lays my original missing MXR Phase 90 potentiometer knob. It went missing at some point on tour maybe 6 years ago. It bothered me ascetically and I could no longer tweak the large knob with my toe on the fly. I replaced the knob at some point a year later. However, up until a few weeks ago I didn't realized the damaged that had been done while not having that knob there. I should preface that I love MXR pedals and the Phase 90 is my all time favorite phaser. It does however have a small design flaw that people should know about.

While recording a few weeks ago I pulled out the Phase 90 for a slow moving phase effect. My pedal somehow didn't seem to modulate below 12 o'clock. It worked perfectly at a faster rate; how I usually have it set on my board. I was perplexed. I couldn't understand how only half the pot functioned correctly. After taking the pedal apart and poking around a bit, I noticed the pot shaft was able to be pushed down. I then see the bottom of the pot split and bulging downward every time I pushed the pot shaft. The bottom of the pot was FR-4 PCB type board material . The shaft of the pot had at some point been stepped on and pushed through the bottom. An easy thing to happen with any MXR pedal or “stomp box” missing the knobs. Without the knob covers, anyone could easily accidentally push the pot shaft down and destroy the pot function.

Here you can see the crack in the carbon element

The strange part about how my pot broke, is it cracked the PCB board at a point where the wiper was still able to make contact with carbon composition resistance element. The crack only disabled the pot for half of it's function (modulation). Crazy it took 6 years for me to notice it. The original pot is an odd one. It's a reverse audio 470K. I didn't have any of those on hand, so I swapped a 500K linear pot instead. The sweep of the effect is slightly different, but not enough for me to special order of seek out the 470K reverse audio. The new pot has a metal encasing which should be sturdier.

Bottom of the pot. FR-4 Board (easily broken)

So this is a PSA. If you buy or have an MXR pedal that's missing its knobs, replace them as soon as possible. It's the only line of defense from that pot shaft going through your pedal.

My new roadie approves of my repair job!