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Philco model 14 Late. Rewinding coils - Printable Version

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Philco model 14 Late. Rewinding coils - Mikeym53 - 12-15-2021

I finished the restoration on this model 14 console. After the complete recap. I rebuilt all the Bakelite blocks and changed the filters. I brought it up on the variac and noticed that volume was low due to low gain. I touched the grid of the 6A7 mixer and it came to life. Sure enough, the antenna coil primary was open. I removed it and there it was the dreaded cotton covered wire that had green spots in it and was rotten. I used a dental tool to count the tuns before I removed the winding. It came apart in pieces. I rewound it using #34 enameled wire, it was only 6 turns. I reinstalled the antenna coil and slowly put power to the set. No change. Next I did a voltage check and found no plate voltage on the 78 RF amp. tube. Now the RF coil primary was open. I removed it and it was same thing, more cotton covered wire. This coil was 18 turns. After rewinding it and reinstalling the coil I put power to the radio, and it played well for a few seconds, then nothing. Dead radio. I suspected the oscillator coil, and I was right. The winding coupled to the 6A7 second grid was open. It was 18 turns as well. After rewinding and reinstalling the coil, the radio plays great. After doing a full alignment, its ready to go. In retrospect it was quite a bit of work and diagnostics, but in the long run I'm glad that all three coils failed on the bench rather than later on after the chassis was installed.
Mike McCrow

RE: Philco model 14 Late. Rewinding coils - Chas - 12-15-2021

I can explain corrosion in these early Philco coils but it does not work for any coil that has not got a celluloid liner. What happens is the celluloid breaks down over time because of moisture penetration, some of it becomes nitric acid, thus attacks the wire.

The assumption I have is that there no reason not to believe that the cotton covered wire was coated with colloidion a cellulose sealant, that too broke down and ate the wire.

What I want to make clear is it is not the cotton covering in of itself but a treatment given that has failed.

Chas

RE: Philco model 14 Late. Rewinding coils - Mikeym53 - 12-15-2021

Thanks, Chas for the explanation. I had read in the Philco repair bench about the celluloid liner, and how it can break down from moisture and over time attack the wire. Your theory about the colloidion sealant makes sense. This is great tek. Thanks. When you come across this wire in a Philco coil, the main thing is identifying it by the cotton covering that has corroded and looks green. Since the wire has lost it's integrity to hold together. It is important to count the turns by using a fine point
tool that you can drag across the winding and count each turn. If you try to unwind it to count the turns, it will fall apart and you will be lost.
Mike

RE: Philco model 14 Late. Rewinding coils - Kestas - 01-17-2022

Before digging into a restoration, my survey includes checking all coils and transformer windings, to get a good prognosis of what needs to be done.