Are there any tricks to quiet down a scratchy carbon pot. Alot of these older pots cant be replaced with modern pots because of their many center tap connections.

Open the pot. Clean it with soap and water. Dry it. Get some di-electric grease from an automotive parts store (it's used on spark plug cables). Rub a fine film of the grease onto the carbon of your control. If the track appears to be badly worn or has some bad spots, you can move the wipers so that they contact an unused portion of the carbon track if the assembly is conventional like most modern pots, where the carbon is on a horizontal plane. The Centralab controls where a fibre wiper wipes over a band of stainless steel are not adjustable. You can sometimes repair dead spots with rear window defogger repair paint, as someone else on here has suggested before (not my idea...can't take credit for it).

Try to never use WD-40 in a control. Though it does not conduct, carbon can collect in it and form a leakage path to the metal housing. This can cause any number of problems, depending on the construction of the radio, television, or other device. If you do not wish to remove the control from the device at this time (too difficult, or you are afraid that you might damage something if the area is congested), you can hit it with some WD-40. Usually this corrects things. Keep in mind, though, that it may cause troubles later in some radios. If the chassis is floating, as in some AC-DC sets, leakage can cause hum. If in an AC set, tonal shifting can occur. With television circuits, it may be impossible to adjust the picture if leakage currents alter the voltages which are normally supposed to be present in the particular control.

Thomas

:Are there any tricks to quiet down a scratchy carbon pot. Alot of these older pots cant be replaced with modern pots because of their many center tap connections.

Be extremely careful what potentiometer you wish to clean. Water or any other solvant (yes, water IS a solvent) may instatntly destroy some early pots. You may safely presume anything made after 1930 is safe to clean. Best way I found so far to clean controls (some of my repairs have been out in the field > 5years now) is first to use WD-40 or plain rubbing alcohol (if you can open the control) to remove gunk and grease.

Grease attracks and accumulate gunk. You want to clean thouroughly first then use a light film of long lasting lubricant. The lubricant I am now using is an industrial one (Zep Preserve). If you can find it.

Otherwise, DeOxIt is good although somewhat expensive. Some late controls have damping grease on the shaft for a smoother control. This will be difficult to reinstate unless you buy a special grease pump for controls (which fits over the control shaft and inject grease along the internal shaft. Never use any grease or heavy lubricant on the resistive part of a control, that would be looking for problems. You should grease _only_ the mechanical parts. Not doing so may shorten the life of the control. There is not a universal way to clean potentiometers and control. Everything I said here will instantly kill a studio console's fader, permanently. These require a totally different attention but I doubt that is what you wish to repair.

Z-

I read somewhere that soft pencil lead might work in the resistance part to smooth it out. What do you think? I agree with not using grease or oil. They would attact dirt.

:I read somewhere that soft pencil lead might work in the resistance part to smooth it out. What do you think? I agree with not using grease or oil. They would attact dirt.

I do my own repairs on worn pot. Last one I fixed was an Italian Magnadyne. Impossible to find the right pot (it has a weird tap value for tone shaping) as the track was badly worn on the first third of it's course (On-Off switch mounted on pot, see the picture ?). The control kept cutting so I mixed clear nail polish with carbon from an HB pencil. I scraped the pencil lead and saturated a nail polish drop (wich is really lacquer with a lot of solid). I then used a drop of lacquer thinner and spread a very fine coat on the damaged area. When dry, I used 1000 sandpaper to feather the repair. Result is a perfectly working control with maybe 10% difference in value >850K from 1Meg initially. It's been over a year now and it's holding fine. In fact, a buddy has an original control repair kit fro maybe the late 40ies with different values of resistance "per inch". It is nothing else than lacquer saturated with carbon. That is where I took the idea.

Z-

Very true about water for early pots. Also, when I first started working on radios a long time ago (maybe 15 years ago), I tried grease. It really doesn't work at all, and makes a mess. I stuck to WD-40 until trouble started showing up. More recently I tried di-electric grease (a fine film), and found that it works perfectly, and doesn't get everything wet. If it was to cover everything, carbon could get in it like it got into the WD-40, and cause leakage paths.

Regarding lubricating the shaft, it is somewhat difficult to do, but all you need to do is remove the C clip. This requires two screw drivers to pry it apart. Putting it back on is simple. A pliers will bend it back to shape once it is in place. I have removed the C clip from just about every control I have ever taken apart, though. It is a crutial part of every cleaning and lubrication job.

T.

Ron,
Sometimes a cleaning or wiping will remove some of the carbon element, making the pot higher in value. If disassembling the pot is an absolute necessity, try lightly dusting the carbon element with powdered graphite from the hardware store, vigorously shake off the excess before rotating the wiper. Now keep in mind that the extra graphite will decrease the R value, so practicing on an unused pot is advised until you get your technique perfected. I once replaced an entire graphite rheostat element using this technique. The various taps can be duplicated with external resistors and some serious calculations
with Ohm's Law

marv

:Are there any tricks to quiet down a scratchy carbon pot. Alot of these older pots cant be replaced with modern pots because of their many center tap connections.

The problem with using external resistors for a tap is that the slider doesn't come near the tap then. When the tap is part of the volume control, then the slider comes near it during its rotation, which affects the tone one way or another, depending on the circuit design.

Thomas