Volume/potentiometer troubleshooting.

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Volume/potentiometer troubleshooting.

Post by NashvilleRad on Sat Mar 02, 2013 6:51 pm

Kinda' sorta' related to my other post (http://www.tuberadioforum.com/t1941-dial-lamp-holder-repair), I'm working on a Sonora RCU-208 and just finished recapping.

I haven't had a chance to go back through the set to check things out - no hot components or tubes, but one very noticeable problem: the volume is present but barely audible while cycling the volume pot -- but shifts to full volume when the volume pot is turned all the way to the full volume stop - the breakpoint is very definite.

I'm sure I'm spacing something out, so I'm wondering is if there's anything I've forgotten to check that would cause this, and preclude (please, please preclude) cracking open the pot and cleaning it, or having to replace it entirely.

You know me, I'll be happy to post pictures/schematic if requested. The set does have AVC, by the way.

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Re: Volume/potentiometer troubleshooting.

Post by Motorola man on Sat Mar 02, 2013 10:29 pm

Connect an ohm meter across the leads of the volume pot and watch for a smooth change in resistance while turning the knob. If it's not smooth, try cleaning it first. If there's an opening in the volume pot housing, spray some de-oxit or other contact cleaner directly into the opening and turn the pot back and forth a few times. Repeat test after the pot has dried out. Repeat the test. If it's still not changing smoothly, take the pot apart and do a more thorough cleaning. Check for open circuit if it's a wire wound pot.
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Re: Volume/potentiometer troubleshooting.

Post by Ben Delk on Sun Mar 03, 2013 9:53 am

I had one like that on a Silvertone. When I took the pot apart the 1st 3rd of the grahite was missing. Very little volume until you hit the remaining piece. Then what a shock, almost full volume all at once.
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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Sun Mar 03, 2013 10:22 am

Yeah ... I'm going to have to force myself to check the pot.

The thought of desoldering and taking the pot apart is daunting.

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Re: Volume/potentiometer troubleshooting.

Post by Ben Delk on Sun Mar 03, 2013 1:34 pm

NashvilleRad said:
Yeah ... I'm going to have to force myself to check the pot.
The thought of desoldering and taking the pot apart is daunting


Jump on it man....if I can do it, anyone can............

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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Mon Mar 04, 2013 7:39 pm

Hey Ben,

It's just carving out the time that's the hard part.

I'm on the road and still have to get by work to scan the dial cover - I will get it done though.

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Re: Volume/potentiometer troubleshooting.

Post by Motorola man on Mon Mar 04, 2013 9:21 pm

Looks like you're not the only one with a pot problem. I just finished recapping a Howard 418 and found that it has no bass response. It has a secondary wiper in the volume pot which feeds the bass control pot, then on to a 6F5G for a "bass boost". The tube is good, so that narrows it down to.... the two pots.
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Re: Volume/potentiometer troubleshooting.

Post by Bill Cahill on Tue Mar 05, 2013 2:30 am

You want daunting? Try replacing three bad tube sockets in an antique tv! Now, that's daunting!!!!!!!
TV repairs, anyone????????????
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What a Face Rolling Eyes Embarassed Shocked Shocked Shocked Shocked Shocked

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Re: Volume/potentiometer troubleshooting.

Post by Ben Delk on Tue Mar 05, 2013 7:54 am

NashvilleRad said:
It's just carving out the time that's the hard part.
I'm on the road and still have to get by work to scan the dial cover -I will get it done though


NashvilleRad....I understand.....did the road tour for several years. Can't say I miss it. Get the scan done when you have time. No rush.
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Re: Volume/potentiometer troubleshooting.

Post by Dave Allen on Wed Mar 06, 2013 8:40 pm

Pictures would really help if you need a replacement. If it's a spline shaft it could be tough to find. A flatted shaft is much more common and not hard or expensive to replace.

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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Thu Mar 07, 2013 9:48 pm

Dave, I'll check. I was actually zooming in on a few pics I had to try and figure out what I needed to do when I roll back into the house on Saturday.

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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Sat Mar 09, 2013 10:02 am

Well ... here it is. I'm going to *guess* it needs replacing. There's more of the resistive graphite (?) pieces that fell out in the chassis I'll have to clean out.

So, I'm looking for a 500K Clarostat (or analogue) with on/off switch, about 2" long from the inside face of the pot, and about 1-11/16" from the top of the threads the shaft is 1-3/4" long, not including the threaded bushing section for the chassis mount, and 2-1/8" including the threaded section - of course I can find a longer one and saw it off.



Last edited by NashvilleRad on Sun Mar 10, 2013 3:13 pm; edited 1 time in total

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Re: Volume/potentiometer troubleshooting.

Post by Ben Delk on Sat Mar 09, 2013 10:08 am

Ouch, that is messed up indeed. You might check with Mark Oppat at OldRadioParts.
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Re: Volume/potentiometer troubleshooting.

Post by sprman on Sun Mar 10, 2013 11:43 am

Good find. Should be able to find a replacement for it.Stay away from pots made in Mexico, have had nothing but trouble from them at work...Sprman

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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Sun Mar 10, 2013 3:09 pm

I've corrected my earlier measurements - the shaft is 1-3/4" long, not including the threaded bushing section for the chassis mount, and 2-1/8" including the threaded section.

I've seen others looking for this same length, and the closest I've seen is an Alpha pot that is 1-1/4" .... I did send an email to Old Radio Parts to track one down.

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Re: Volume/potentiometer troubleshooting.

Post by Dave Allen on Mon Mar 11, 2013 2:09 pm

NashvilleRad wrote:.... I did send an email to Old Radio Parts.....
I'm sure Mark will have something. Beware, the Alpha pots are nice and new but metric. The shafts are 6mm (.236"), not 1/4" and the bushings are 9mm (.354") vs. 3/8".

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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Mon Mar 11, 2013 6:17 pm

I spoke with Mark today and have a replacement on the way Very Happy

Seems like some of the major manufacturers would have these, or at least a one-size-fits-all and you trim the shaft lenth as needed.

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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Sun Mar 17, 2013 8:41 am

Video coming up of the set - the pot from mark Oppat works nicely bounce

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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Mon Mar 18, 2013 7:47 am

Although, there is a very slight, continuous hum that is really only noticeable when tuning between stations.

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Re: Volume/potentiometer troubleshooting.

Post by Ben Delk on Mon Mar 18, 2013 7:55 am

Could it be near a fluorescent light, on a curcuit with ccf lamp, frig, etc.?
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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Mon Mar 18, 2013 9:01 pm

Hey Ben, actually, all of those could be possible.

There is a fluorescent light over the work bench and I turned it off to check. And, it is plugged in to the single (new) GFI outlet in the basement that may share the line with something else.

The one cap I didn't replace, that did test out of spec, was the one mica cap that goes to ground off of the pot/on-off switch.

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Re: Volume/potentiometer troubleshooting.

Post by fixinmyphono on Mon Mar 18, 2013 10:45 pm

it's always that last cap trust me Laughing
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Re: Volume/potentiometer troubleshooting.

Post by NashvilleRad on Tue Mar 19, 2013 9:50 am

I do have a silver mica replacement for it ... it is termed the "IF Bypass Diode" and is a 100 MMF brown mica cap.

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Re: Volume/potentiometer troubleshooting.

Post by GaryRabbitt on Mon Mar 25, 2013 2:22 am


IF the hum increases when you are tuning in the station, it is probably "modulation hum" . Symptoms, hum when the station is tuned in, but not in between stations.
Line filters/ caps at the switch or the AC incoming line usually cure that. The values are usually a .01 thru a .05 or so. 100pf doesn't should like much of a line filter, plus a cap that small and being mica usually wouldn't go bad.
I would still replace it and see if the hum goes away.
I don't know what the person who drew up the diagram was sipping, but a cap is not a diode, especially not a. IF bypass diode.
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Re: Volume/potentiometer troubleshooting.

Post by analogdino on Mon Jun 17, 2013 11:44 pm

I've often wondered if you can "fix" worn carbon pots with a soft lead pencil marking on the track... never done it.
Cheers,
Roger

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