Issue with a Fisher Volume Pot

[eduarsan1]

Guys I need your wisdom

6 Months ago I got a 500K control volume from Mark Oppat and last week the left channel starts to be noisy like scratching between min. and 9 o´clock and then I asked to Mark about the pot and here is their answer:

"Hi Luis,
Remember that sometimes the control is not the actual problem, and I doubt it is here either. These do not “age” that fast so for this item to change in that time is nearly impossible. Have you recapped that unit? Also tubes will cause this issue too, even though you think the control is at fault.

I know you think it can not be, but believe me, capacitors and tubes will exhibit this condition.

Here is a simple test:
Reverse the leads on the center terminal of the control. That is, take each center lug wire and move it to other section of the control. If the problem moves over to the other channel, the issue is from the control back to the input of the unit. If it STAYS on the same side, the problem is AFTER the control thru to the output stage.

I think you will find your problem this way."

The amp is a Fisher 400 fully restored with IBAM, EFB, coupling caps and tubes as well.

Thoughts??

Honestly IMHO I think is a quality issue on the pot

Regards

Luis

 

[sony6060]

Valid start of diagnostics IMO. I do find it odd not to be the potentiometer when the static issues stop above the 9 0'clock position. But.....

 

[dcgillespie]

Luis -- Check to see if there is any DC present between the top and wiper terminals of the questionable pot, and ground, at various control settings. If so, then DC is leaking through a coupling cap -- either from the plate voltage of the previous stage, or from the bias voltage for the op-amp buffer stage -- either of which could cause the problem as well.

Let us know!

Dave

 

[RS Steve]

hmmmm, wonder if my TA 600 could have that issue. :scratch2:

 

[BrokenHill]

I got two of those Oppat 500K control volume pots, they now sit in a box and you don’t want to know what I think of them.

Installed two high quality mono pots and will never look back, yeah I know it’s not stock and it altered the Fisher 400 appearance
but I don’t care, I’d rather have performance then the stock look.

Just my thoughts.

 

[fred soop]

I got two of those Oppat 500K control volume pots, they now sit in a box and you don’t want to know what I think of them.

Installed two high quality mono pots and will never look back, yeah I know it’s not stock and it altered the Fisher 400 appearance
but I don’t care, I’d rather have performance then the stock look.

Just my thoughts.

You could get a dual pot from Digi-Key, though it would be linear rather than audio taper. Then the linear response can be modified to more closely approximate log taper response with the proper external resistors. That would get you back to a single control for both channels.

 

[eduarsan1]

Many thanks for the suggestions!"

 

[sony6060]

hmmmm, wonder if my TA 600 could have that issue. :scratch2:

No. When I rebuilt your TA-600 it has new coupling caps and all were tested for leakage at rated voltage on a Heathkit capacitor tester. The pot is simply worn and unfortunately the TA-600 has a long shaft. If it was short shaft like the 400/500/800 I would have replaced it from NOS on hand.

The static quits once the volume control is set when on the bench. Did that change? When I applied DeOxit, it improved greatly. But, still not perfect.

 

[RS Steve]

No. When I rebuilt your TA-600 it has new coupling caps and all were tested for leakage at rated voltage on a Heathkit capacitor tester. The pot is simply worn and unfortunately the TA-600 has a long shaft. If it was short shaft like the 400/500/800 I would have replaced it from NOS on hand.

The static quits once the volume control is set when on the bench. Did that change? When I applied DeOxit, it improved greatly. But, still not perfect.

It acts up sometimes, mostly when at 9 to 10 oclock setting, above that volume it is quiet.

 

[sony6060]

It acts up sometimes, mostly when at 9 to 10 oclock setting, above that volume it is quiet.

If I can find one, I'll send it. That long shaft volume control with bass tap and switch is one rare bird.