Pre-show testing found everything initially working OK but after an hour the monitor focus had softened. The focus adjustment pot was hard over but still short of being able to pull in the focus.
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I setup the BK467 to verify the CRT. No issues were reported - no shorts, no problem with the focus and the gun drives all looked good. As far as I could tell the CRT had no issues and looked good apart from focus.
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Next I verified all the resistors in the focus chain and the focus adjustment pot. Again everything measured correctly - nothing open or bad value. The only anomaly I came across was that the screen pot measured 4M3 rather than the schematic value of 4M7, not significant enough to be the cause of the focus problem.
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I connected up the HV probe to measure the bleeder & focus voltages. On cold power on the bleeder voltage into the focus chain was 7,790V and the focus voltage tapped from the focus pot was 3,870V. Adjusting the pot from it's hard over position caused the focus voltage increase further, indicating that the focus voltage was too high. The problem manifested as a sharp picture on cold power on that gradually lost focus over time. With the HV probe the problem was clear - the bleeder voltage gradually increased, reaching ~8,100V over a 20 minute period (resulting in a corresponding focus voltage of ~4,200V.)
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One additional data point was that the LOPT on this chassis was hissing. I'd seen this before when the separate HT lead had come loose causing poor contact between the lead cap and LOPT socket. I removed the lead, cleaned it and reflowed the end cap but this made no difference to either the hissing or the bleeder voltage. With no time to further investigate, I reduced the B+ voltage to reduce the HV overall to mitigate the focus overvoltage, improving the focus enough for the game to limp along to the show.