5E7 Volume doesn't shut all the way off!

[Frenster]

It does on the mic channel but not the instrument channel. Before I tear it apart, does that sound like a grounding issue on the vol pot?

[Bruce / Mission Amps]

A bad ground on the pot could do that but it sounds like it could be a bad pot too.

Bruce

[Frenster]

Bruce, is there another way for me to find out before taking it apart?

[Tom Phillips]

Try this to check the pot without disassembling anything. Turn the amp off and drain the filter caps: (Assumes you have an Ohm meter)
1) Verify that you get a zero Ohm reading with the ohm meter leads shorted together. This is just calibration check.
2) Measure the resistance from the grounded lug on your misbehaving channel volume pot to chassis ground. The reading should be zero Ohms. If it is then the volume pot’s ground is good. If that's OK, go on to #3.
3) Measure the resistance from the center lug of the volume pot to chassis ground. The reading will vary as you turn the knob but it should go to (or close to) zero when you turn the volume pot all the way down. You can take a reading on the good pot to find out how close to zero is good enough. Even a "good" pot may still read 25 to 50 Ohms. If the minimum reading is much higher then you have a not so good pot that will always let some signal through. If you decide to change it, check the replacement first.

Let me know what reading you get.
Good luck,
Tom

[Frenster]

Thanks Tom, just what I was looking for. I'll let you know how it goes.

Fren

[Frenster]

Both volumes let a little signal through, but more on the Instrument vol.

My digital meter zero's, then settles on .4 ohms. Ground lug on both volumes zero to ground, then also settle to about .4.

Middle lug on Ins. settles about 4 ohms, Mic. vol settles at 3.4. Still getting bleed through when the volume is off.

[Tom Phillips]

Your tests indicate that the Pots are OK. If the signal with the volume all the way down is excessive then it could be a wiring mistake. If you lift the bright cap both channels should act the same. It's hard for me to tell without hearing the amp but you may just have a normal amount of bleed through that most people never worry about. Hi Fi music amp designers take special pains to eliminate all the bleed through and cross talk but guitar amp designs usually don't. It doesn't take much signal to be noticeable. Sometime the signal can even sneek around through the power supply.
Tom

[Frenster]

That sounds about right. It's not excessive, but it's noticable at a higher volume. The rest of it sounds great. Thanks for the input about the output.