bad pot question

[Ron Victoria]

I just got an old lap. The pots were frozen but I got them free with spray. The volume works but it goes off when I turn the tone. I replaced the cap, but that didn't help. I'm sure it's the pot. As I have to remove it anyway, would it be worth a try taking it apart and cleaning, or once they go bad, that's it? Can I put a meter on it while it's still wired in? Thanks, Ron

[Dave Mudgett]

On a vintage instrument, I normally prefer to fix an original pot, if possible. Replaced pots, and even breaks in soldering, can affect both value and integrity of the instrument.

How the can comes off depends on the pot - there are different varieties. Sometimes, like on the old CTS pots often used by Fender and Gibson, it's held on by tabs - just pull the tabs up, the can pulls off. If it's an old carbon-comp pot, there's a wiper which contacts the carbon element - sometimes it just needs cleaning, and sometimes delicately 'fiddling with' the wiper fixes it. If you're going to toss the pot anyway, you've got nothing to lose.

What kind of spray did you use to free up the pot? I like Caig DeOxit D-5, available a lot of places, including Musicians Friend. It cleans and lubricates. Some cleaners don't lubricate, and vice-versa. You need both.

Edit - whether you can accurately measure the resistance 'in-circuit' depends on the circuit and how the pickup and other pots are wired to that pot. For many typical guitar circuits, this is possible, just measure across the soldering lugs. Unless you just want to know the resistance, I'm not sure how critical this measurement is. If the volume or tone control works properly, that's good enough for me.

Another thing - I have successfully opened up and fixed some pots without desoldering the pot. Just remove the pot from wherever it is attached, pull the can off, try to fix it, and if successful, put the can back on and screw back in place. On a vintage instrument, it's better not to break the solder joints if possible. Sometimes a soldering lug is bent back and soldered to the can - in this case, you'll at least have to unsolder this to proceed.<font size="1" color="#8e236b"><p align="center">[This message was edited by Dave Mudgett on 03 September 2005 at 01:02 PM.]</p></FONT>

[John Daugherty]

Ron, a dirty or open tone control pot will not cause the volume to drop to zero if the capacitor is not shorted.
I suspect that one terminal on the tone pot is connected to the ground (shield). Look it over and see if this is the case. one terminal of the tone pot should be connected to the high side of the volume control and one terminal should be connected through a cap to ground. There should be one unused terminal.

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[Ron Victoria]

Here's the schematic. 2 terminals on the tone are grounded to the pot. The third terminal goes thru the cap to the other pot.

[Ron Victoria]

I checked both pots and I got up and down readings w/o a short. I'm going to try different caps. Maybe the one in there was wrong, it's a .5.
Ron