Pilot 602M Volume Pot

[Krasch]

I just powered up my Pilot 602M after recap. I checked and changed all resistors that were out of spec.
One channel was weak. Found volume pot way off. Spec calls for 1 meg 300k tap. Mine is 622k and 633k on the other channel total. Control at 9:00, from wiper to audio input is 108k and the other channel is 72k. How can this be repaired? I sprayed it with plastic safe contact cleaner and Deoxit D5.
Thanks

 

[Shawnnnn]

I'm dealing with the same problem on a Bogen amp that I just recapped and replaced a lot of resistors on.

It also has a 1 meg volume pot that is not equal on both channels. It's very noticeable when playing music.

I switched all the tubes around just to be sure it wasn't a tube issue, but the volume was still way off. Voltages were also pretty even on both sides. So I took the pot apart but it really didn't look that bad, and after a more thorough cleaning the channels were still off. When I put it back together a few tabs broke, even though I only bent them back once so that sucked too. Maybe you will have better luck taking yours apart than I did.

I could only find one 2 gang 1 meg audio pot on all the usual websites. It was only a few dollars so I'm going to try it, but it's not even close having the right length stem. If it works I'm going to try and use a shaft coupler and the old stem.

Other than finding a donor unit I'm not sure what else there is to try. Maybe others will have some more ideas.

 

[Krasch]

I tested mine using a 1000hz input with a voltage high enough to trigger my scope on the wiper of the pot. The one sine was about half of the other channel. The difference was thru out the range. What I need to test is to remove the wires going from the wiper to the balance to make sure there is no load somewhere else. I tested with loudness off. The tap seemed to cause a dip in the curve between 11:00 and 1:00 on the pot. I think I have a weak channel also. I swapped the wires from the balance pot to the first af amp tube and it got worse. Hard to test this till I get clean inputs to it.

 

[gadget73]

rare that things drift low, but any other component across it will cause it to read low. Make sure nothing else is there to throw your reading before drawing too many conclusions. Might want to unhook it and check the thing out of circuit to be absolutely certain.

If it truly is the pot and not anything external, the fix is you replace the pot.
https://www.oldradioparts.net/index.html

is the usual source for these type of things.

 

[Krasch]

The pot is a Clarostat 36-76A 140-6141
Resistances are as follows Total left 1.06 meg and .94 meg right


Tap to ground and to Input didn’t show much difference in knob position.
Knob positions were guesses for each column.
I couldn’t figure out the 300k tap for spec.

 

[gadget73]

ok so its reading 1M or near enough across the whole track. Looks like the two wafers drifted differently. Only real fix there is replacement wafers unfortunately. A hack fix would involve bridging some resistors from either end to the wiper of one side to make it match up in one spot but its still not going to track exactly right anywhere other than the spot you fixed it at.

for example if you wanted to fix it at 12:00, a 1M resistor from ground to the left wiper would get it pretty close, 328k vs 338k.
at 9:00 that would give you 117k on the left vs 78k on the right. Less wrong, but still not right.
full open you'd be at 500k vs 1M (approx)

I have in the past done this hack at whatever volume control position I used the most and just compensated with the balance control elsewhere.

something I did once consider but have never actually tried is baking the control to see if that would drive moisture out of it and shift things back closer to right. I suspect it won't do anything but if its a case of a control thats going to the trash can anyway you don't have much to lose.

300k tap should be 300k from the ground side, or thereabouts.

 

[Shawnnnn]

Can you get replacement wafers somewhere gadget?

 

[gadget73]

I've bought NOS pots of the same brand and style and robbed them for parts before. Problem with buying old pots is you're trading one old part for another old part, so its not always a guarantee the "new" parts are perfect either. I don't know of a source of just those wafers, and often you're out of luck when it comes to loudness taps. I think Mark Oppat builds pots in a similar way, using donor parts to assemble something that works but I've never actually asked him.

 

[Shawnnnn]

I sent an email off to Mark.

Mine has a 10k balance pot going down the middle so I'd really like to find a nos part, but it seems pretty impossible. Hopefully Mark can help me, and possibly Krasch

 

[Krasch]

I ordered one from old radio parts. He's going to build one. Price was right and it looks like I'll get it right away. He knew just what I needed.
Great to know it should be one I need. I didn't want to cobble something and hope the loudness sounded correct. I'll post when I get it and how it works.