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Check the resistance on the HV adjust pot. My 25CC50 daily watcher developed intermittently unstable HV a couple years ago, different HV reg circuit with 2 VDRs and 6JH5 pulse regulator, but same 3 Meg HV pot feeding the regulator grid. It turned out that the HV-adj pot had drifted up to about 6 Megs. Probably couldn't source enough grid current to the regulator. Also, if you find you can't adjust the HV to spec, check the 5% resistors in the regulator, they need to be close to correct value.
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