Fender Pro Junior - Weird Volume Pot Behavior

[acorkos]

The volume pot on this Pro Junior is acting funny. There is low volume when the knob is on zero....turning the knob clockwise, from 0-2, the volume diminishes slightly, and the output is distorted. As the knob is turned clockwise beyond 2, the volume starts to increase normally. Any ideas what could be the problem? The pot appears to be intact and the PCB contacts are good. Thanks for your help!

http://www.ampwares.com/schematics/pro_jr.pdf

[Enzo]

The sound at zero is crosstalk between stages of the amp. I happens that this crosstalk is out of phase with the signal at the volume control. As you turn up from zero, the rising main signal starts to cancel out the crosstalk - opposite phase combines to nothing. Then as you turn past the minimum point, the ever increasing main signal drowns out the little crosstalk.

I don't consider it a problem, and a solution might be difficult.

[km6xz]

Is the tone correct when at the minimum position? If the tone changes as it gets about the "2" point, check the volume control ground connection or the pot itself for a an open trace. If you have an ohm meter, and the unit off, measure the resistance between the center pin of the pot to ground. When rotating the pot shaft the resistance should rise or fall smoothly. If there is an abrupt change as you reach about "2" you probably have an pot with the resistance element cracked inside the pot just before the "2" position. The signal path under the "2" can be through the tone filter until passing the 2 point where the connection inside the pot is above the break in the resistance element.

[acorkos]

tone control has no effect until '3' is reached on the volume control. resistance between the center pin of the volume pot to ground increases smoothly, and then decreases at the upper range, 10-11-12. I pulled the knobs off the pot shafts...the volume pot shaft is a lot more 'wiggly' than the tone pot. no cracks or anything on the pot back, but i'm still suspicious that the pot might be bad. i'm gonna replace it and see what happens.

[Gtr_tech]

The pot shaft may have been pushed in causing the back of the pot casing to be displaced. Its just held together with bent over tabs. Have a look at it.....it may just need to be "put back together" so it fits tight and makes proper mechanical connection.

[acorkos]

the crosstalk was caused by a bad preamp tube in v1. new tube, everything works like it should

[pdf64]

I've got a mazda ECC83 that does the same thing - it's pretty weird.