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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
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