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It would be the video out a 12GN7 or 12BY7 something like that. Its the last on before the cathodes of the CRT. Its plate is connected directly to the cathodes of the CRT (thru the service switch and the drive pots) is sets the voltage of the cathode so if the that tube is not right it will effect the plate voltage, which then effects the CRT cathode voltages (and therefore bias), ultimately can cause the crt to draw too much current, which loads down the HV, which then blooms, and if only slight bloom can look like focus problems.
did you double check that focus voltage? I would use a HV prob so as not to load it down too much. if it really is that low (like the other poster said should be 4-5 kv) then it has to be a problem in the focus circuit, and those filter caps are not going to help anything. I have a simpson 260 analog that goes up to 5kv and use that one as well for checking focus voltage.
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