Tech Forum

You didn't say, but I'll presume you have the classic, small (8"x6"x4") model 7-100 that I have. Mine sits for long periods unused and always is scratchy.  But, so far, it always cleans up by rotating the control shaft through its entire range for a minute or so. I usually do this before I turn it on. If that doesn't do it, cleaning is the next step. DeoxIT D5 is a product most used now in the amp repair business. I think RadioShack even carries it. The pot on mine is sealed in the normal opening at the terminals, but I saw a small hole in the side or you might be able to get some cleaner around the back side of the shaft. I don't think I have ever cleaned mine.

I saw  a couple of schematics floating around at Activity Stream - Music Electronics Forum.  But you don't need one to replace the pot. Mine is 50K ohm, but I also saw 10K ohm on a schematic. There are several versions of this amp, updated over the years. I presume these are logarithmic/audio taper. You can check the value and taper with an ohmmeter. The pot is really small and has the power switch included. I haven't seen any replacement parts listed or mentioned. One of the best and cheapest parts sources is Mouser Electronics - Electronic Components Distributor. You would have to remove the battery holders and metal shield to use any larger pot. You could get by without the power switch, since power is removed when the cable is removed from the input jack, like is done on many guitar FX pedals.

You can SAFELY clean a pot with a spray cleaner like "Deoxit D5" (item #64-249 @ RadioShack). Just spray a small amount of it into the pot (typically in the case opening where the solder tabs are), rotate the pot through its' range a few times, then repeat once or twice more.  This should clean up your pot nicely.  NOTE - This stuff is also good for cleaning noisy/intermittent switches (make sure there's NO POWER APPLIED to the switch!).

However, if it doesn't help (a possibility, depending on the age of the pot and its overall use), you can replace it with a similar unit. Just unsolder ONE LEAD from one of the end tabs and use a DMM to measure the resistance between the end tabs - this will give you the resistance value of the pot. In many cases, your local RadioShack will have a similar-value pot in stock - if not, get the next larger value pot that's closest to what you read (i.e., your pot read 7.5K, the next closest value would be 10K). If The Shack doesn't have anything close, order one through DigiKey (www.digikey.com) or Jameco (www.jameco.com).  IMPORTANT: make sure you replace it with an audio taper pot, not a linear taper pot.

RadioShack may still sell an aerosol can of potentiometer/contact cleaner, just use it sparingly!

If the sounds still cuts out at certain spots, the control is probably bad and needs to be replaced. If you can use a soldering iron this is a simple job.

Connect an ohmmeter across the two outer tabs, you will not get a reading if the control is broken. Now take a reading from the center tab to each of the outer tabs and add them together to get the value of the new one to buy.

The best fix is to replace the pot. If you don't want to do that, then a spray on cleaner is available at most RadioShack or electronic parts supplies that will supposedly clean these pots. I've never had much success with the spray stuff though. It's supposed to wash away the carbon dust and leave a protective slippery film behind to prevent further ware. My preferred method is to disassemble the pot, clean it thoroughly with alcohol, re-lube it with pot grease, then reassemble it. Even all this doesn't permanently fix the problem, it will return.

The problem is usually caused by the carbon resistance material breaking down. Most pots have their value stamped somewhere on the case, generic replacements are available and are  cheap and easy to install.