Unknown potentiometer

[Mjaue]

Hi,
I am restoring an old Proton P1100 preamplifier. It has a lot of issues but I am slowly getting it back to working condition.

There seems to be an issue with the potmeter controlling the base, so I removed it and tried to clean it. While it got better it's still not working as intended.

It's a dual 10k pot. I measured the resistance A+B in a clockwise direction: it went from 0 Ohm to about 4.3 kOhm before dropping to 0 Ohm in the middle position. Turning further clockwise from the middle position gives 4,4 kOhm to 8,6 kOhm at the end.
Measuring B+C gives the inverse result with infinite resistance in the middle position.
From what I can measure it is a linear pot.

What type of pot have these characteristics in the middle position? The service manual calls it a center open pot, but this yields no results when I search for it.

Markings on pot are 601F CS.T10KBX2

Any help would be highly appreciated.

 

[gadget73]

No idea what you'd call that, quite likely its a custom job and you may not be able to find a sub unless its an original part.

What exactly is the problem with it? Sounds like it measures OK.

 

[Semanon]

No idea what you'd call that, quite likely its a custom job and you may not be able to find a sub unless its an original part.

What exactly is the problem with it? Sounds like it measures OK.

I agree that it seems to measure OK. Center position should be defeat, so it should take the pot out of the circuit A to C should be infinite because it’s not a continuous track.

Have you looked at the schematic? Maybe post a clip of the tone section.

 

[Mjaue]

Yes one channel is measuring fine, the other one not so much. Will try to get in with some cotton and electro cleaner to clean it up.
I have attached picture of the circuit board and the circuit diagram.
Marked the pot in yellow.

 

[raycarr00]

Numbers read a 10k to me.

 

[Mjaue]

It is indeed a 10k pot. It's the centre position that baffles me. I will try to reassemble and measure again after work today to see if it can be salvaged.

 

[raycarr00]

some here on ak have used tarn-x and then rinsed them. i haven't tried it.

 

[Mjaue]

Thank you for for the tips.
I have just put the pot back together again and although not perfect, it's much better than it was before.
I will leave it in place for now so I can get all the other stuff sorted out in this amplifier.

I will update this post if I do find a replacement for this kind of pot for future reference.

I'm much obliged to you all for replying and highly appreciate the knowledge and common interest found on this forum.

 

[Mjaue]

The preamp is now fully rebuilt (I will link to the rebuild thread when that is up).
While the scratching sound is gone from the potentiometer, the center defeat function is quite finicky, probably due to the wear of the self centering function.
If not fully centered one of the channels connects to the tone amp circuit causing a phase shift of the signal at the output stage.

There is a also a slew rate issue that I have not found the reason for.


I will be putting in a normal pot in this position and possibly a tone control bypass switch instead.
While the original potentiometer is a novel idea of a tone bypass system it cause a lot of distortion to the sound when not working as intended.

 

[Bob the Tech]

Old post, but I'm facing a similar issue w/ distortion in one channel sensitive to the bass pot. That nomenclature "CS." and for treble "CO." stands for "Center Short" and "Center Open" respectively. The bass pot is apparently designed to short end to end when centered. Measuring the bass pot (CS, in circuit) I get short A to B, B to C, A to C when centered. The treble pot is open end to center when centered.

Interesting and I'm guessing replacements are never going to be found.

 

[Mjaue]

Hi Bob,
I have yet to find a suitable replacement, but I've not been actively looking for one either.
What amp is your pot in?

 

[Bob the Tech]

Hi Bob,
I have yet to find a suitable replacement, but I've not been actively looking for one either.
What amp is your pot in?

Proton 1100 pre

 

[Barmaleykin]

Old post - but for someone, my solution will work in the future.
To restore BASS pot, I have used Conductive Paint for Circuit Keyboard High Conductivity:

Amazon.com: WANJAO Conductive Paint for Circuit Keyboard Repair High Conductivity | Copper Conductive Low Resistance, Strong Adhesion，Smooth and Delicate for Membrane Keyboard Car Defrost Repair, 20ML : Tools & Home Improvement

Amazon.com: WANJAO Conductive Paint for Circuit Keyboard Repair High Conductivity | Copper Conductive Low Resistance, Strong Adhesion，Smooth and Delicate for Membrane Keyboard Car Defrost Repair, 20ML : Tools & Home Improvement

www.amazon.com

Here is the result - picture attached.
After assembling all parts, the potentiometer works with no issues.