[KCMike]

Really hoping for some advice on how to procede with this. Thanks

[jerryhawthorne]

Mike, with one leg of that 12K resistor lifted from the pot that controls the RF screen voltage you should be able to measure end to end on both pots to assure proper value for max. Then sweep the center tap to one end for measuring each pot and hopefully both move smoothly from zero (roughly) to max value of the pot. Easiest to do this on an analog meter to watch the needle move smoothly. Digital is OK but drives my mind crazy. I wouldn't "open" them up at this point, let's see if they respond to resistance as they should.
Best, Jerry

[KCMike]

Just got in and seen this thanks Jerry. I will try that and post tomorrow what I get.

[KCMike]

Tested the vol. control. with resistor lifted.
Using analog meter. set on 1k scale. The antenna side of pot reads 90 ohms to 50,000ohms as it should.
The grid control side of pot confusing at best for me.
Starts out at 90 ohms as pot is rotated up when at 2k it quickly moves to 6k. as you then goes smoothly til you get to 50.000. but it keeps going to end up pegging needle.
went to 10k scale and got up to 2 meg. ohm. which was right before full volume. When it hit full vol. it started dropping down.

[KCMike]

Well I went ahead and dis- assembled the volume pot. Interesting design.
I cleaned the metal contacts slide in each pot with alcohol. I'm not sure what the contact ball that slides on metal is. It looks like cork or wood under magnifying glass.
I pretty sure it is not that. Anyway after cleaning it still reads the same resistances.
Here is pic. of it.
[Image: http://i1336.photobucket.com/albums/o654...9ad277.jpg][Image: http://i1336.photobucket.com/albums/o654...748018.jpg]

[jerryhawthorne]

Oh my Mike, certainly looks like a single one I had on my Fada that was opening up. I ended up replacing it with a "modern" control. The first picture shows well the workings, not at all like a newer pot. The resistance aspect is layered on the bottom and the wiper with the nub on it moves around over the springy semicircular piece of metal pushing it down to make contact with the resistance substrate under it if that makes sense. I'm not certain what the taper of the pots are but knowing that I'm certain Mark Opat could make you up something that would work. My problem was that the layered resistance material under the springy piece was compromised. No fixing that that I know of. If the second pot that your having trouble with measures good, end to end but jumps around from wiper to end you may have some crud ( technical word) in that area of jumping keeping the pressed down spring from contacting that area. Good end to end measurement seem to indicate that the resistance "plating" is in tact. Perhaps someone can suggest something that won't hurt the resistance coating but may clean that area and the area under the springy material.
Perhaps a tiny adjustment on the wiper with the tip to apply a little more pressure on the flaky one? Could be fun, or not.
Good luck, Jerry

[KCMike]

Thanks Jerry for helping to make that clear. I was not aware of the material under the springy surface was where it got the variable resistance from. I will check with Mark Opat. Sounds like this would be un -rebuildable. At least I now know where the problem is.

[jerryhawthorne]

Mike, nothing to loose trying to add a little tension to the "nub" section. It appears the base layer is fine. Think of the springy section as the wiper, touching the resistance section when the nub goes over it and presses down. The springy section is directly attached to the center tap of pot . As it is pressed down with tension from the nub part, it contacts the base resistance material and depending on the rotation of the shaft, it will change the resistance. Perhaps the nub on the bad section has worn down a little.
Jerry

[KCMike]

I will give that a try and post if it worked or not.
I did see that the model 42 uses the same type of pot. but differnt values that can be used in the 48. Information on nastalgia air.

[Arran]

I think that some soaking the resistive elements in varsol/mineral spirits, and then alcohol should clean the crap buildup out of it. I found an old tone control of a similar design in a Canadian Westinghouse 801/Columaire 8 chassis, the wooden peg or dowel works exactly how Jerry described. In fact it helped things by adding a small amount of lithium grease on the dowel side of that spring metal disk.
Regards
Arran

[gregb]

Be very careful with the resistive element as it is just a layer of carbon on a piece of cardboard and the carbon is very easily removed. Any scrubbing with a tool or brush could remove it. I know this as I tried cleaning one very gently with a Q-tip and some contact cleaner and that took the carbon off.

Gregb