My Stone SBA-20 Bass Guitar Amp Volume control pot broke. It is marked on the back with b100k[ohm symbol]. The shaft is knurled. This is a standard pot except that it has a fourth lug. Needless to say, I could not find this specialty part. I learned that this Stone amp was a private label product and likely not intended for repair. But I hate to throw anything away and was willing to invest up to $10 to fix it. So I did the next best thing -- faked it.

I took the broken pot apart to figure out the reason for the fourth lug. It appears that it was a cheap way to avoid adding a resister or two to the PC board as the fourth lug is just a tap off of the pot. It is 36k ohms from the lug directly below the fourth lug and 56k ohms from the other outside lug. The fourth lug does not involve the middle of the lower three lugs.

So, here is the repair.
1. Buy a b1010k audio pot -- pcmount. Something like mouser's 313-1530F-100K Manufacturer Part #: RV16AF-20-15K-A100K-3t This is on page 538 of the current catalog, configuration H. Mouser Page 538 http://www.mouser.com/catalog/629/538.pdf
2. Buy two resisters. each 1/4 o 1/2 watt capacity. One is 36k ohms and the other is 56k ohms. Mouser 30BJ250-36K; Manufacturer Part #: 30BJ250-36K Mouser page 511 http://www.mouser.com/catalog/629/511.pdf AND Mouser 30BJ250-56K.
3. Unplug the amp.
4. Remove the amp from the cabinet by:
a. Remove the four screws from the top of the cabinet that are not associated with the corners or the handle.
b. Lay the cabinet on its face and remove the board (held by four screws) that is across the back of the cabinet.
c. Unplug the two wires fromt the speaker. Make a note where each colored wire is connected.
d. Stand the cabinet back up on its feet and push the face plate of the amp toward the back of the cabinet. It should push out. At this point the amp should be completely separate from the cabinet.
5. Pull the knobs off the five control pots.
6. Remove the nuts that hold the pots and the guitar input jack to the front of the amp.
7. Unscrew the two screws that hold the PC board to the chassis. These screws are in an aluminum clip (about the size of a playing card) and remove the screws that hold the clip to the chassis, not the clip to the board.
8. At this point you should be able to move the board so as to disengage the pots from the face plate and twist the board sufficiently to gain access to the underside of the board.
9. Desolder the volume pot. This includes the wire from the fourth lug.
10. Solder the resisters to the correct lugs. I did this by actually soldering the resister above the lug. There was hollow rivets that held each lug in place. I shoved the resister through this hole and soldered it in place.
11. Solder the pot in place.
12. Adjust the resisters so that their wires are not touching anything, twist the other wire ends together and solder the wire that was attached to the fourth lug to this twisted end. Tidy things up so nothing will touch the resister wires or the fourth wire (lug) connection
13. Reverse steps to reinstall the board taking care to make sure all of the pots are fully though the faceplate.
13. When finished. Test and play.
I am a complete amateur so of course this is all at your own risk.

Jim