How to Overhaul 500-C, 800-C, 400 Pot I just finished overhauling a 500C volume control/on-off switch. It's a few hours work, but I had some issues with my left channel and traced it down to that device. To start, you must physically remove the unit. I started by taking pictures and documenting every wire attached to each part of the pot, the center taps, on/off, etc. It's tricking removing and replacing the center tap connections, as they are underneath the pot. Once all leads are disconnected and labeled, undo the brass nut and pull the pot out. The pot comes apart in 3 stages. First unbend the 6 small tabs on the front plate attached to the threaded silver bushing. Once removed, you will lift off the right channel carbon path, which is connected to the phenolic base. Under that, you will also find a brass wiper attached to a keyed nylon bushing . You DO NOT need to remove the sprung ring or bushing/wiper assembly from the shft. I cleaned this part of the pot with rubbing alcohol, if you have isopropynol it's likley better. My pot was filthy black and gummed up with a ton of old Deoxit and dirt. It took about 10 minutes and a ton of Q-tips to remove all the residue. It's really gummy and smells like the fuel I used as a kid flying gas-powered model airplanes. To get at the left channel wiper and carbon path takes 2 steps. First undo the four metal tabs hidden under the first pot housing assembly. I just used a small jewlers screwdriver to carefully pry them up. You also have to desolder two solder points on either side of the pot. To get full access you now have to pull off the back of the pot which is the on/off switch mechanism. It's a keyed friction fit (mine had not solder points here), just cafefully remove it , and it will just pop off. My switch did not have the protective cover you see on some Fisher power switches. To further pull the pot apart from the back you must also carefully remove a small brass locking ring that sits on top of a brass washer. This holds the shaft in place from the rear (that's what she said). Once removed the pot will pull apart allowing access to the left channel wiper and carbon path. On my unit this was not in as good a shape as the right channel (no idea why), and also totally gummed up with old dirt and Deoxit. Many black Q-tips later, it was time for reassembly. I first put just a small bit of Fader Lube on each carbon path with a Q-Tip, I did not flood it with liquid. To reassmble you essentailly work backwards. The 2 metal housings for each phenolic plate holding the carbon paths are keyed in such a way that they only fit in one way. Same with the back nylon bushing holding the left channel wiper - it is keyed but can be put together wrong if you are not careful. Just be sure that both wipers are keyed in the same position and that one is not 180 degrees off-set. Use the wiper still attached to the shaft as the reference. Before you seal everything up, twist the shaft to each end of clockwise and counter-clockwise. Extreme clockwise has both main wiper points at the 11:00 o'clock positon. Going all the way counter-clockwise, you will end up with the main wiper points in the noon position. That is normal, until you snap the back switch mechanism back in place (again keyed), then your stopped position should be 1:00 o'clock. If you need a new switch mechanism "metalbone.net" has them (as of June 2012), and you can pop a new one on the back. The only caveat is that they are the older original switches that were more prone to failure due to current surge. To overcome this, he ships with a thermistor (CL 80 or 90). If you install this on the mains line, you avoid this issue. The last piece is putting on the plate with the bushing on it. A couple drops of fine oil inside the bushing well help things run smoother.