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The Hallicrafters have a well known history of having this coil issue. I would be very surprised if the 4.7pf cap was the cause, but a quick squirt of freeze spray would answer that question. The Philco 7" sets have a similar fading symptom, though the coil in that set operates at around 17Khz, and is layer wound, not a universal wound multi-pi construction like the RF HV coils, which operate at 100s of Khz.
I had an exact duplicate coil would for the Philco set, and much to my disappointment, it exhibited the same fading problem as the original coil. My mistake may have been in having the coil potted in varnish (enamel?), though I will never know for certain.
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John Folsom
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