On/off volume pot replacement part 500c

fahfisher500

Active Member
My on/off volume pot is going bad (weak and intermittent left channel, better with high volume), I have read the forum and understand that this is a largely unobtainable part, is there a suitable replacement part that is readily available? I know almost nothing about electronics but would be game to try to replace this part myself if I had clear instructions on what to do. I own a soldering iron and a voltmeter but that is it when it comes to electronic repair equiptment. Thanks for all advice.
 
I think I'll try the deoxit first. I had it worked on by Terry DeWick when I put it back into service several years ago and it has worked great until this week. I really don't know what I am doing with electronics, but if it is just a matter of unsoldering and resoldering a few wires I think I could handle that. My concern is that the replacement will not look like the original and I will not know which wire to solder to which pin.
 
Take pictures before you unsolder anything, then you'll know where the wires go....good luck with it!
 
fahfisher500; No need to unsolder anything to use De-Oxit. There will be two slots where the wires are all connected on the Volume pot. Spurt the DeOxit in there and work it in and around with the knob (make sure you are unplugged 1st) Work the knob back and forth about 50 times, vigourously. Then give it a shot of Faderlube and work it in the same. Wait an hour before plugging in and starting up.
 
Or you could do what I did, purchased a Alps 500K blue velvet volume pot, and put a switch on the back of
the Fisher. Unless you want to keep it original.
 
Voluminous pictures will address your re-construction concerns. Daunting at first but good pictures will really put you at ease.

Also, I believe there is nothing like a complete teardown and cleaning out of the pot. Years of wear on the traces together with all sorts of liquids shot in there inevitably leaves a slurry of goo that hardens and negatively impacts the functioning of the control. I've attached a couple of shots of the the pot of my 500-c that I took apart when I got up enough guts after tiring of the scratchiness and intermittent sound. Notice the speckles of dirt that fell out and remain. Once I cleaned the entire control and lubed it, no more noise!

DSCN2027.JPG DSCN2028.JPG
 
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I think I will try to clean it first with the deoxit/faderlube and see how that does. Then I may try to repair it as shown above. I will put my full ignorance on display and ask about how to safely attempt this. I have read in many posts about the danger of working on tube equipment with very high voltage components which may be hazardous even when unplugged. Of course I will unplug it, but what else do I need to do to do this safely. Thanks for the help.
 
If you're going to remove the pot, then working with it unplugged will be 99.9999999999999999 % safe. The high voltage caps actually drain after a while if they were recently energized. You can put a (DC) meter on them and watch them drain. You can do it quickly by shorting them with, say, a screw driver but only if you're into drama. Otherwise, leaving it unplugged for a half hour should be enough.

Since the pot is mounted on the front, just have that side of the unit closest to you when you work. That way they'll be little reason to reach over the chassis.

Hopefully the sprays will correct your issue. If not, and you decide to take it apart, don't forget the pictures.
 
It can also melt the tip right off the screwdriver, leaving you with a reminder of why you aren't supposed to do that.
 
I've done this for one of my Fisher amps and the treble control of the Eico Hf81, once you
clean and lube a little it should be good unless the carbon is so worn it not any use. just watch
those metal tabs when your putting it back together.
 
Follow up with resolution of the problem. I tried cleaning the pot with deoxit and fader lube and it actually seemed a little worse. Tried again and it was definitely worse. I called Mark Oppat and he helped me troubleshoot it to be sure it was the pot and it was. He built me a new pot with the on/off switch which I finally got around to installing yesterday and the problem is solved!. I'm thankful for all the advice given here and will post again if there are any changes. I highly recommend Mark, he was very helpful and patient with my very elementary questions and made the new part within 1 week.
 
Do you have pictures of the new pot? I just got a power switch from Mark for my original pot, but it is a bit scratchy and I am wondering if this is a good route to go.
 
Do you have pictures of the new pot? I just got a power switch from Mark for my original pot, but it is a bit scratchy and I am wondering if this is a good route to go.

Are you saying the new pot from Mark is scratchy? Did you try spraying some DeOxit in the new pot?
 
Sorry if that was confusing. My 500C came with the original volume pot but no power switch assembly. I got the power switch assembly from Mark to put on the back of my original volume pot, but the original volume pot is scratchy so I am considering getting a whole new power switch/volume pot assembly instead. A friend sprayed some Deoxit in my old pot and I disassembled/cleaned/reassembled and it is still scratchy.
 
Make sure you don't have a bad coupling cap leaking some DC into the pot. That makes them very noisy. The pot itself might be OK.
 
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