Rawinsonde , just off the cuff, try to spray some contact cleaner deep down the shafts of the selector or control knob.
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Take the top off and inspect the selector switch. A wire may have come loose.
Agree a spray contact cleaner can do good too.
If the switch is free standing it is an easy to replace part. If it is attached to the cucuit board directly, that is harder to do.. as you need the exact replacement to fit it.
I agree with both Ozzy & Elizabeth - get hold of Caig ProGold contact cleaner OR anything similar from your local Radio Shack (if they still sell this stuff anymore?) & spray onto the contact of the inp selector & turn that selector switch back-forth thru all its options again & again to work that contact cleaner in. If there is any oxidation it should resolve that issue.
Elizabeth also has a good suggestion - check the soldered connections to this switch. you might have a weak solder point. this can be easily fixed by reflowing the solder.
Use the Radio Shack contact cleaner first! The ProGold can cause trouble in some circuits (I've seen it contaminate circuit boards to the extent that they had to be pressure washed to remove the ProGold)!! IMO/IME the Radio Shack contact cleaner is the only thing you want to use as it is safe with just about any circuitry, especially if vacuum tubes are involved.
I bought a can of CRC (QD) Electronic Cleaner. I did this before seeing the last 3 posts as I figured this stuff was all basically the same thing.
After removing the knob with a small allen the shaft seemed to have no wiggle room going into the faceplate... extremely tight clearance. I sprayed liberally around the "opening" and worked the control back and forth hoping the cleaner would penetrate. On the other side of the faceplate (inside the chassis) the switch shaft goes into a housing of some sort where I can't get at it there.
When I hooked the preamp back up and tested it worked fine for a couple of days but now it is occasionaly failing again. Each time it fails I simple jiggle the selector to balance the sound again. But it only stays good for a while.
Since the cleaner did seem to have an effect, I guess I will repeat the same action with the cleaner, only work it more and for a longer time.
Any further ideas or suggestions? (thanks)
If you can't get to the switch itself then the cleaner is likely not going to do the job by trying to get it to go through the shaft bushing for the switch!
Your best option at this point is actually pretty simple- operate the switch vigorously to both limits about 50 times and see if it gives you any trouble after that. The only reason its a problem is corrosion from lack of operations.
I work on vintage electronics a lot. The better stuff frequently have 'J pots' in them- Allen Bradley J-Style potentiometers. They are really well sealed. If they get noisy, the technique above actually works fairly well on them unless they are really far gone. In that case you have to drill a small hole in them which is really dicy, but at least then you can clean them.
Twice I have rotated the bal switch end to end maybe 50 times. It works for quite a while and then you do it again. I guess the friction burnishes the contacts enought to eliminate the oxidation and make a good contact. In due time, the cycle repeats.... thanks to all of you who contributed to the issue!