Volume potentiometer does not attenuates to ∞ completely

[Karl vd Berg]

Hi all,

I finished recapping an amp. Everything "seems" OK, buuuuuut........ 😕

... but the volume potentiometer does not attenuates completely the sound. When it is on the ∞ mark (total counter-clockwise position) I still hear something whereas it should block any sound, right?

What can it be? 🙁

 

[Mooly]

Look at the circuit. Does the volume pot connect directly to ground (99.9% do) or is there a low value resistor present that stops the pot from attenuating fully.

Other than that and its probably the pot at fault. You can prove that by soldering a wire between the wiper and the earthy end of the pot (do both channels) If that kills the audio then the pot is at fault. If it doesn't then............ check the pot first.

 

[jan.didden]

Agree with Mooly. Also, most pots have some residual resistance between wiper and track as well as between wiper and lower terminal when CCW. The remaining sound probably is very low level, but not zero.

Jan

 

[Karl vd Berg]

It's my "new" AX-900U.

Well, I did a full cleaning in all the pots before I reassemble it back. But I'll have a look back again at this pot.

Thanks!

Circuit attached

 

[Mooly]

That should attenuate to zero. Check the pot by shorting wiper to ground (at the pot, nowhere else) and there should be zero audio coming through.

 

[Enzo]

And consider the practical aspect. How often do you listen to it at zero volume? In other words, does this condition prevent you from use of the system in some circumstance? or is it just a "matter of principal"?

 

[Karl vd Berg]

And consider the practical aspect. How often do you listen to it at zero volume? In other words, does this condition prevent you from use of the system in some circumstance? or is it just a "matter of principal"?

That's true.

But something really weird has just happened: output drivers are fried. The amplifier is dead. And I'm just desperate. 🙁

 

[Karl vd Berg]

Discovered the reason (I hope it's the only one!).

Bias values were too high, around 330mV when it should be 6mV!

Strange, because I took measurements for few days after recapping, monitoring the thing as I always do. Seemed pretty stable around 6,03mV

It became pretty hot with distorted sound and then... poff! Smoke out of it! 😱

I'm still in shock.

I took this morning a couple of pictures that show these two resistors, and God knows whatelse went melted/destroyed...

An externally hosted image should be here but it was not working when we last tested it.