Intriguing CRT issue with a Tektronix 2225 oscilloscope

[oldwhitelodge]

I have days on looking at the the horizontal output amplifier of this scope due to an issue. The x scan works absolutely fine at slow scan rates, but as the picture shows. RHS X output becomes highly distorted at higher scan rate as if there was low-pass filter operating, hence me susprecting that it was a capacitor problem. I disconnected the inputs and found that the drive from the attenuator board was perfect.

I could not find a problem even after changing capacitors. I never thought to disconnect the CRT itself at the output until the end. I used a tube from a working 2225 and got a perfect X scan! The amount of time I've wasted on this!

Anyway, I have no idea why the CRT would cause this type of distortion - has it gassed. I cannot work it out as I would have though the X-plates would have a very high resistance! I inspected the tube quite closely but could not see anything wrong.

The picture attached shows the distortion on the x-plates of the 2225.

Any ideas?

Chris