The purpose of this post is to detail how I managed to finally repair my FC-7 expression pedal. It also shows what the inside of the Yamaha FC-3 potentiometer looks like and hopefully, this will clarify how the pedal works. First, a little bit of history about mine.
I've owned an FC-7 from the late 1980's when I also bought a Yamaha DX21 and it initially gave me great service. However, over the years it began to get crackly and I occasionally opened it up and sprayed the pot with contact cleaner. Later on when spraying no longer seemed to help, I would open up the pot a few times to adjust the tension of the wiper half-ring, which had a small black cone-shaped tip making contact with the track. I'm not sure what this was made of, possibly a hard carbon but my attempts to adjust the tension eventually caused the tip to be badly mis-alligned so, back around 2012, I changed the pot for a different linear 50K one but never could get it to work as well. I kept the original Yamaha pot for a future repair project.
Recently I took up the challenge to try and fix the existing pot, as a new Yamaha one would have cost me around $30 with shipping and tax added on (a brand-new pedal can be bought for $39). This meant having to fully dismantle it so after (yet again) prying back the 4 tabs of the metal case, I separated the wiper assembly from the track, by carefully drilling the flared-out end of the shaft and prying off the nylon disk.
Here is a picture of the track surface of the original Alps pot from the FC-7 on the right, compared to another 'standard' Alps 50k linear pot. As has been previously suggested by others, the carbon track does not cover the full extent of travel of the wiper. Mine is in quite a poor state with a broken solder tab at one end and if you look closely, you can see the remnants of a pink and yellow wire on the other. They are for a polarity switch which I wired up but that's another story.
The next picture shows the differences in the wiper arm arrangement and the hole in the one on the right is where the cone-shaped tip had been attached.
After separation of the original wiper, the contact tip eventually became dislodged, fell on the floor of my garage and was nowhere to be found. However, I managed to replace it by inserting a 1/16 inch ball bearing, after ensuring the wiper arm was well tensioned. This had always been difficult to do earlier, before separation and although using a pair of ex-British Telecom specialised tension adjusting pliers, my previous attempts probably contributed to the damaged of the original tip.
After spraying the track with Deoxit D5 and F5, I re-assembled the pot by carefully using a punch on the end of the shaft and installed it in the pedal. It's now working as good as new and hopefully, will continue like that for a good few years. Perhaps this will encourage others to at least attempt a repair if you are having serious issues with the potentiometer in your FC-7.
