Get a Digital Multimeter and set to ohms (resistance) range. Turn the tone to brightest, volume to max and plug a lead in. Measure the resistance between the tip and sleeve of the other end of the lead using the meter. Check in all switch postions. You should get under 10K ohms in all positions. If higher than this then open 'er up and we'll need to do some testing inside. Also, turn the tone fully the other way and check you get the same readings.