[droptop]

Hey everyone,
What would cause an arc inside a 21CYP22A? My beloved CTC7 Grenoble developed this out of the blue after near flawless operation over the last 3 years with no warning. After exhaustive testing and troubleshooting I’ve located the arcing sound by removing the metal crt cover and running the set face down with the crt and HV compartment open for visual inspection. It is definitely coming from inside the crt which tests full scale 100% good on all three guns with no internal shorts indicated. The trouble started when I happened to notice an occasional vertical roll as a video disturbance flashed across the center of the screen. Upon investigation I found that turning the brightness up caused an arc that was audible but not visible and the picture would show visible noise that we’ve all seen as an indicator of arcing somewhere. Turning the brightness higher causes it to get worse along with lowering the HV and focus voltages. At a slightly less than normal brightness level it makes a beautiful picture.

Every voltage and current reading is correct throughout the B+, audio, vertical and horizontal circuits. Having said all that, does anyone know the cause? I’ve been told that connecting all elements together at the base and impressing a high voltage (2K) between this and the second anode might burn out whatever is causing the arc. The internal dag coating is in place showing no signs of “letting go” around the ultor connectors. If I did this procedure which anode button would be used?

I’m really hoping someone here knows about this. The CRT in the set is in absolutely pristine condition from a visual as well as emissions standpoint and makes a great picture. I hate the thought of losing a tube this good.

Thanks for any input the community might have.
Rick