[Steve D]

I am working on a model 600 and removed the pot/switch to fix the switch part which was on all the time. I successfully de-greased, cleaned and lubed the switch part which works fine now but discovered that the pot section has a bad carbon section near the 7 o'clock position.

The resistance reads 35K from end to end (should be 15K) and as the wiper is moved from CW to CCW and reading from CW to wiper the resistance goes down monotonically until I reach the 9 o'clock position from where it increases again. Upon close inspection it appears that the carbon is cratered and worn from the 9 to 7 o'clock position probably since the control was operated in that area most of its life.

I know pencil led can be used as a resistive element and I would like to perhaps make a paste from some type of glue or paint and pencil lead which I can fill the pitted areas with and establish continuity. Does anyone have any ideas or experience doing something like this.

BTW this control is connected at the antenna input which is why the value is so low.

Thanks,

Steve D

[gary rabbitt]

Hi Steve.
I have not tried any of the resistive paints, and the mix of graphite and glue didn't work well for me on a different volume control. You can check this page out talking about resistive paint.
http://www.kobakant.at/DIY/?p=634

I have heard of using the paint to repair rear window defroster grids. Again, I have not used it myself.
http://www.amazon.com/Permatex-15067-Win...B000HBI9YQ


I was thinking, if the wiper could be carefully bent, and the contacts touch a section of the element that's not worn, you might be able to get more life from the control. It shouldn't take more than a few mm to bend in te good area.

In bending the wiper, you might want to remove the shaft and wiper from the housing. Remove the little C clip by the shaft, and it should slide out the back.
Bend the contacts ever so gently, then slide the shaft in again to check it's position.

By sliding the wiper and shaft out, you can then add a little pressure to the wiper so it makes good contact. If you bend the wiper without removing it, you may run into the wiper not having good contact pressure on the element... if that makes sense.

Try this, using the Ohmmeter, connect to one lug, then use the other probe tip as a wiper, moving it around the arc. That way, you can see if the 'good' area of the element is capable of givning you the right resistance. Don't scratch the element with the probe.

That's the ideas I have, if I had the same issue.
Good luck!
Gary.

[Steve D]

Thanks Gary, the window defroster grid ink is a great idea. I think I'll try that.

I did try dissolving pencil carbon in epoxy glue and although the carbon powder reads between 100 ohms and 1K, when dissolved in the glue it reads very high, >1M ohm. I guess there is not enough concentration of carbon to become conductive. The powder must have to be much finer grit.

I did remove the wiper part and bend it to get more contact pressure but no luck as the element is actually vaporized near the CCW connection and I noticed that the contact appeared partially melted. I think lightning may have struck this set at some time as the CCW terminal is connected directly to the antenna. I'd better check the antenna coil and capacitor as well.

Steve D

[Dick Finn]

Just a thought, you can buy graphite in small squeeze bottles for use as a lubricant. THis is extremly fine and might work when disloved into a thick paste. Try a hobby shop.

[codefox1]

Not so sure it would be hard to get a 35 0r 50K pot which woud work just as well values are not that critical for such applications.