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the brightness increase is due to the momentary short of the cap on the cathode of the video out. changes the bias of the tube, causes it to conduct hard, drops the plate voltage thru the load resistor, direct coupling to cathodes result in lower cathode (momentary) at the CRT, changes the bias to more conduction (less difference in G1 to K) so more eletron flow in the CRT.
I would start with the assumption that the problem is in the video out, so try a new video out tube. check voltages of that tube as well.
You should also try a new blanker tube since it is part of the crt bias circuit (unlike the CTC-12 this set uses G1 voltage adjustment to set CRT bias not the K's).
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