iN NEED of POT

[Pete Deksnis]

Things they say happen in threes. so it is no surprise that three days ago the last Field Neutralization pot I have went belly up. The same one Nick and RCAfan need. I thought I had a spare in an unrestored CTC2 chassis that had been little used throughout its long-ago active life. But this pot too was open. Like the one in my operational chassis, the open is at one end of the wire-wound resistance element. So, I came up with a kluge circuit shown that uses both bad pots and a switch to get the chassis running again.

How's the field neutralization control work? Operationally, rotating the wiper to either side of center changes the direction (and amount) of current flow through the field neutralization coil (which rings the 15GP22 faceplate). Now, a switch is used to change direction of current flow, and the associated pot is then adjusted for appropriate current through the coil.

Another possibility is repair. The 'fried' pot lost about the frst five turns of wire at one end. Picture attached. Perhaps if I'm lucky, I can lay a slurry of solder across the charred turns. But it's probably not doable. (Has anyone fixed a WW pot?). When I checked the other pot, it was NOT burned out -- it seemed to have failed from a design flaw. Picture attached.

The pots have twin wiper assemblies; you can see the two arcs they made as they scraped the turns of wire. There are two places where the resistance wire is compromised. One is a single break about twenty turns from the end. The other a group of breaks in the first ten or so turns of wire. I believe the stress from the wiper was much greater near the end of its travel and easily broke the wires. Close inspection with a magnifier suggests much more abrasive wear from the wiper very near the end of its rotation. If I'm correct, it might explain why a seemingly low-electrical-stress WW pot appears to have a high failure rate.

Pete

[Phil Nelson]

Originally Posted by Pete Deksnis

Perhaps if I'm lucky, I can lay a slurry of solder across the charred turns. But it's probably not doable. (Has anyone fixed a WW pot?).

How about silver-filled epoxy (the stuff in the top of this photo)?



Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Tim]

When I restored my set, I discovered that the pot had opened and I could actually see where it had burned. I was able to scrape insulation off of the adjacent wires and put a drop of solder on it to repair it. The drop was a little bigger than I would have liked but it has worked well and held up with no further problems.

[ChrisW6ATV]

Originally Posted by Pete Deksnis

(Has anyone fixed a WW pot?)
Pete

It is not necessarily the same thing, but I have repaired wirewound rheostats (specifically, fuel-tank level senders from 1970s GMC Motorhomes) where the wirewound element connection was bad at the terminal end. I did this by scraping a loop/turn of the wirewound element near the end so it would accept solder, hooking a bit of copper wire through that turn, and soldering the wire from the scraped point to the terminal.

[ohohyodafarted]

If your are clever, you can actually make a center tapped pot from one that is not center tapped. I did it for the verticle centering pot on my Midwest 16" set.

First find a wire wound pot of the correct wattage and resistance. Then grind off the rivets that secure the two outside terminal lugs to the resistance element. Remove the resistance element from the bakelite case.

Now the tricky part. Using a dremel tool you will need to make a slit in the bakelite case where the center tap terminal needs to be placed. Then place a piece of brass shim stock,about 3/16" wide through the slit and against the inside back wall of the bakelite case. Now re-install the resistance element so that the back side of the resistance element is wedged against the brass shim stock. Use small 4-40 screws and nuts to replace the rivets that were ground off inthe first step to re-secure the terminal lugs to the resistance element. Make sure that the back side of the resistance element is wedged tightly in a "force fit" against the brass shim stock. The brass shim stock now acts as a center tap to the wire wound resistance element.

This fix has been working flawlessly in the Midwest set for years. Give it a try and good luck!