Pot Problem Question

[Ed_Chambley]

I have never ran into this problem, but I have a 1 meg alpha pot on the treble of a 5f4.  It works fine until it is turned up almost all the way, then it creates a static sound and no other sound will come from the amp.  Is this simply a bad pot or could anything else cause this?

[sluckey]

The pot is definitely a suspect. Change it and see.

[OldHouseScott]

Might also be a bad treble cap leaking DC onto the grid of the next stage, but the pot is the prime suspect.

[frankeg]

I have the same issue! I've restored a 1970 SF Twin and the vibrato channel volume pot is doing the same thing that your treble pot is doing. The pot and all the coupling caps (orange drops & silver mica) are new. I did not change the bright switches or their caps which seemed to be working fine. The vol pot ground is in parallel with the mid pot and tied to the tubes cathode ground. I am slowly working my way through this and will let you know what I find. Please post your results as well...frankeg

[Geezer]

Parasitic oscillation? .......as the gain of the higher frequencies is increased by rotating the pot?

Maybe get it into the bad condition, then try moving some wires around to see if anything changes.

G

[frankeg]

Parasitic oscillation? It's a possibility. I checked the vol pot with an ohmmeter and in the last 1/3 to 1/4 turn to full volume actually dropped the ohms from the mid wiper to ground. I know the pots are interactive and this may not be an issue but at power it is the same area that begins to make the scratchy sound. Tonight I'll compare the wiring and ohm readings with the normal chan which is identical and I'll check for dc leakage. I will also chopstick the wires and may try to move the ground lead around. I'll post the results. I do have a scope and a signal gen so I should be able to find it. It's just a matter of time...

[frankeg]

Maybe get it into the bad condition, then try moving some wires around to see if anything changes. I like this suggestion. Thanks!

[Ed_Chambley]

Mine was just a bad pot.  I like thee feel of alpha pots, but you cannot take them apart without destroying them. 

[frankeg]

Found It! The fiber eyelet board on this 1970 SF twin is conductive. I have anywhere from .4 to 1.5 vdc from the board material to chassis ground. My particular problem was a bare wire jumper connected from the 250pf treble pot to the next eyelet. The wire was pressed down on the board. When I gently pried it up off the board the noise stopped. I will replace the jumper. I cleaned the entire board with alcohol last night and put a heat lamp on it which I'll do again tonight. BTW, this twin spent the last 15 yrs in a unheated garage in south central Pennsylvania...