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Why do you say its not regulating, I thought you said the shunt current changed with the brightness and was between .5 and 1.5 with a dark to bright raster and the HV held to 23-24kv all signs that HV regulation is working as it should.
Now you may indeed have a over load that exceeds the ability of the HV to supply current at excessive beam current in the CRT, but this is unrelated to HV regulation. The shunt can only regulate within a beam current operating range, go beyond that and its NOT a regulation problem, rather its a load problem, You keep saying it like those can't be separated. you could confirm this by observing the shunt current when the blooming happens, the current should drop to 0 as the KV drops to 19kv. this would indicate the regulation is working as designed and you are simply overloading.
My guess is you are simply over driving the CRT to make up for a brightness deficiency real or imagined. there could be an issue with the service switch that may be making the setup not work properly, but that can be worked around, not all set had setup switches you just do it by eye with a live signal.
I am sure you will suddenly have the solution and it will all be fine.
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