CTC 10 HV affected by brightness

[Turle]

Been stuck on this for weeks. Initially I thought it was a focus problem, but The HV on my 10 drops to around 14kv whenever the brightness is up, and it jumps around depending on what’s on screen, I have no idea what could be causing this. I’ve pulled the cap off the regulator tube and it made no difference, the rest of the hv tubes are fine. the power supply seems to be working fine. I even tried a different CRT but it made no difference, does anyone know how I could fix this??

All help is appreciated. Thanks again