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I may have solved the mystery. Today I turned on the set and forgot about it. When I returned 3 hours later the focus had deteriorated. I let it cool down, turned it back on, and the focus was fine, though the control was full up.
When I first started restoring the set, I noticed that it had a solid state rectifier in place of the 3A3 focus rectifier. Because I wanted the set to be as original as possible, I took it out and installed a 3A3. The high voltage died. I assumed I had a bad 3A3, replaced the solid state rectifier, and planned to get back to that issue later.
There are 4 parts in the focus circuit; the rectifier, a 1500 pf doorknob on the cathode of the rectifier, the focus pot, and a 10000 pf doorknob on the wiper of the pot. First I replaced the 10000 pf doorknob. Still had poor focus after a couple of hours. Then I put a 3A3 in place of the solid state rectifier. High voltage died, plus the 3A3 got so hot that the glass cracked.
I had to quit for the day. I think that the 1500 pf doorknob is leaky, more so when it gets hot. The excess current drain caused by the leaky cap destroyed the 3A3, and, when the solid state rectifier is in place, it loads the flyback enough to reduce the width.
Tomorrow I'll test the theory by replacing the doorknob. Stay tuned.
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