Repair or Replace Volume Control? Airline 62-181 (1931).

[Winky Dink]

I finally did a temporary hookup of the replacements for eight condenser block capacitors so I could power up the set. My first issue is a nasty volume control. After power-on, the first half of the sweep yields nothing, then loud crackling increasing with the audio volume.

The control was not made to be disassembled, and even if I managed to open the case I don't know if it would be repairable. I sprayed some contact cleaner on it, hoping it would magically seep through the seams. It didn't.

This is the part in question:
https://picasaweb.google.com/lh/phot...eat=directlink

Is it possible to find a modern replacement (on/off and volume, 2.5Kohms, D-shaped shaft...), or should I look for used vintage components? I looked at Mouser and Allied but didn't find anything that I thought would work.

Advice?

- Winky

[sean]

I bet it would come apart. It looks like if you loosen the set screw on the power switch actuator you could pop the metal back off of the control. It is probably had a piece of cardboard with carbon on it as the resistive element inside. Some of the carbon has probably worn off. I have had some success in repairing these type of pots by using a graphite pencil to apply new carbon. You can rub some on then check the resistance of the pot with a meter. More carbon gives less resistance and less carbon gives more resistance.

[Reece]

As noted, it is probably repairable, and worth a try. Remove the switch and the actuator arm that operates the switch. See those little dimples around the perimeter of the back cover? slip a thin blade in there and begin to loosen those dimples and work the cover off. Hook the pot (actually it's a rheostat per the schematic on NostalgiaAir since it only has connections at one end and to the rotor) to an ohmmeter and determine where the jump in resistance occurs. Examine and you will probably see a crack at that point if it's a carbon control. You can get rear window defroster repair paint at an auto parts store and fix the crack. The repair has to be smooth obviously to allow the slider to pass over. Let that dry and then use pencil lead as Sean suggested to scribble over the carbon surface.

It might be a wirewound control; some early ones were in the screen circuit like this. Then look for a break in the wire at the suspect point. This sort of wire isn't easy to solder. You'll have to figure on a mechanical repair of the break. Then lightly sand the slider and the track where it slides.

Let us know how it goes! BTW if you end up having to replace the control it would need to be a linear control, not audio taper, in this circuit.

[Winky Dink]

Ohmeter shows 8K decreasing to 0K at end, roughly linear--not what I expected. I will try disassembly today. Thanks.
-Winky.

[Reece]

That's odd if it's supposed to be 2.5K...quite a drift upward. Does it have the same part number on it as listed in the NostalgiaAir schematic? There were two part numbers depending on the serial number of the chassis.

[Winky Dink]

Spent the day playing with the volume control/switch. Cracked it open, cleaned it, tested it. The arm makes contact at the outer edge of the graphite race, and the sharp resistance drop happens at the chipped area.

https://picasaweb.google.com/lh/phot...eat=directlink

Added graphite to the chipped area and retested it. Result is similar to that in the first picture except it starts at 20K ohms instead of 10K. Interesting that you asked about the part number. Much earlier I collected this information:

https://picasaweb.google.com/lh/phot...eat=directlink

I also gleaned all the part numbers I could from Montgomery Ward and Wells-Gardner in Rider's. This is a list of the volume control part numbers. I also found a note that their part numbering system was changing in the early 1930s.

https://picasaweb.google.com/lh/phot...eat=directlink

All I have to go on is that my schematic says R1 2,500 ohms.
I have a spray can of Slip Plate (dry lubricant--graphite and nickel in water) which I used to coat a CRT. I may experiment with that on strips of paper or felt to see if I can create a good resistance curve.

Thanks again. I'll put this away until tomorrow night. Have a good President's Day.

[old_coot88]

I've used india ink to make custom resistors in the past. Never used it in a pot, but might consider it in a situation such as yours.
Bill(oc)

[Reece]

Could you bend the rotor a little so that it takes a new path, just inside of the old one, where it doesn't hit any chipped area? Maybe put a little kink in the brass strip, or carefully surgically remove it, tin both cut parts, and overlap it back on, clamp with alligator clips, sweat solder it.

Another note: this method of volume control doesn't always work as nicely as the one used later in the audio section. Varying screen operation can sometimes get wonky and squawky. If you can get it to work so that it varies volume from zero to loud enough, great. Sometimes at the extreme it could go into oscillation and the instability may vary depending on frequency. Might not happen with your set and hope it doesn't.

Edit: Looking at your pictures again, it looks like the rotor tip doesn't ride on the carbon itself, but on a thin metal contact which gets pushed against the carbon? What I posted above might not work there. Hmm...

[jeyurkon]

Originally Posted by Reece

...
Edit: Looking at your pictures again, it looks like the rotor tip doesn't ride on the carbon itself, but on a thin metal contact which gets pushed against the carbon? What I posted above might not work there. Hmm...

I think you're right. That's pretty weird. Maybe they were trying to reduce wear on the carbon. It looks like the metal would only touch at the outer perimeter. Maybe if he could bend the rotor tip to provide more force it would touch further in?

[Jeffhs]

I took the volume control out of my Zenith MJ-1035 recently and stored it. Will have to unearth it now and use some of the tips you guys have posted regarding getting it working again. The one weird thing on mine is that the third terminal, which appears to connect to one end of the carbon track, has separated from the pot itself. I can't seem to find where this could have been originally connected; the terminal appears to be mounted on a metal ring which is now separated from the control. Can this pot be saved, or is it too far gone?

Pictures attached. Note: It isn't easy to see the detached terminal in either photo (poor resolution--1.3 megapixels--of my cheap Radio Shack Flatfoto digital camera), but it is there -- it's the rightmost one, looking at the front of the pot. Just thought I'd give a heads-up in case it looks, at first glance, as if the terminal is solidly attached to the control.


Thanks in advance.

[jr_tech]

Originally Posted by Jeffhs

The one weird thing on mine is that the third terminal, which appears to connect to one end of the carbon track, has separated from the pot itself. I can't seem to find where this could have been originally connected; the terminal appears to be mounted on a metal ring which is now separated from the control.

Is it still connected to the carbon track, or has it broken off of the insulator that the track is deposited on?
Do you mean that you can't find where the lug was connected to the pot, or can't find where it was connected to the radio wiring?
Is the "metal ring" a ground lug that is around the pot shaft that connects the lug to ground when the pot is mounted through a hole in the chassis?
Sorry for all the questions, but it is difficult to tell from the pictures/description what is happening here.

Here is a pix of a common pot...in this case the lugs attach to the fan shaped insulator with bent over tabs...sometimes small rivets are used.
I have labeled the lugs "A B & C"... perhaps some ohm meter checks between A & B and then B & C while the shaft is rotated would reveal proper (or improper) operation of the pot.
hope this helps,
jr

[Winky Dink]

Two days experimenting with Slip Plate graphite spray. Another day to apply six coats of graphite to the existing substrate. Voila:

https://picasaweb.google.com/lh/phot...eat=directlink

The schematic specified 2.5K for the volume rheostat. Started with an erratic range 20K-to-0 ohms; finished with a smooth, linear 2.7K-to-0 ohms.

Thanks to my mentors and collaborators--we've saved an original part that would have been quite difficult to replace.

-Winky

[sean]

That looks really nice. I might have to go get me a can of Slip Plate for future pot repairs. I wonder how well it will hold up over the long term?

[Reece]

That's amazing. Does that film harden? The control works properly in the radio now? Wonder if this is a potion we should all have for wonky resistance controls. I looked at their site and am interested in the stuff they have to keep the lawn mower and snow blower from getting clogged.

[PaulOF]

That is way cool. Last time I had that problem I mixed up a concoction of Barge cement, graphite, and acetone that I could paint on. Worked well but this looks a whole lot easier.
Paul