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Uni,
I guess I'd measure the cathode current of the H. output tube to make sure it was within spec (not too high) and if not adjust the H, drive control to set it properly. Too much current here will overheat the flyback. Then I would ry to determine if there are any portions of the flyback that are getting locally over warm after a bit of a run. You will have to be careful to be sure the CRT ultor path is shorted/discharged to chassis ground before placing your hand in there as soon as possible after shutting down and unplugging from the mains.
BTW, leakage corona from the 15 KHz AC high voltage output of the flyback makes a hissing/sizzle sound. Once it is converted to DC (after the 3A3) it will make a soft crackling sound, or a loud bang if it actually jumps to ground.
Just be careful. These circuits bite hard if you don't take measures to discharge the high voltage DC stored in the picture tube. Charge will be present at the socket of then 3A3 rectifier, the plate cap of the 6BK4 shunt regulator and on the red wire running to the ultor connection on the CRT bell. There's 25 kVDC on this path when the set is operating.
Make sure that the spring loaded wire that connects the outer aquadag of the CRT to the chassis ground is present and connected properly. If that ground strap is missing the set will bite you.
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