Scratchy head phones

[TommyK]

I have a headphone with the big muffs, Pioneer SE-405, approx 1970's-ish. Each muff has it's own volume knob. It has the curly cord and 1/4" TRS connector.

 

When rotating lock to lock on the volume knobs, the sound drops out, the in as if it is not making contact through the whole rotation. It has gotten steadily worse to the point that each knob only has 1 or two positions where sound comes through at all. The volume at the positions where there is sound are approximately identical.

 

In order to diagnose the problem I used a volt meter to check resistance through the knobs. To accomplish this I touched the black probe to the plug's sleeve, then the red probe to the ring portion of the plug. I then rotated the volume knob, lock to lock, to check resistance. Since there are only a couple positions where I get any sound at the speakers, I expected to see infinite ohms through most of the rotation of the knob, but that wasn't the case. I repeated the procedure probing the tip and sleeve. Here are the results:

 

S/R Right knob 9.5 - 64.0 Left knob no effect

S/T Left Knob 9.5 - 64.1 Right knob no effect

 

R/T both Left and Right at max: 17 (approximately the combined ohms with right and left at minimum ohms setting)

R/T both Left and Right at min: 126 (approximately the combined ohms with right and left at maximum ohms setting)

 

I am surprised that I had any continuity between the left and right. Is this normal?

 

Also, surprisingly, rotating the volume knobs did not cause anything but a smooth count up and count down of the numbers between max and minimum ohms.

 

What else could this poor performance? It appears to me the pots are working as designed. I'm stumped.