tHotD monitor no blue or green

Got this Pedestal style House of the Dead project a little while back, the only issue is the monitor shows completely red with no trace of blue or green! Adjusting either sets of potentiometers for the blue and green does nothing, adjusting the red makes the amount of red more or less saturated as it should. I replaced the transistors on the neckboard, and had a friend of mine recap the chassis board, but the result is still the same after all of that. The picture is still very sharp, although there is minor burn in from usage, is my tube done for? Or is there anything else I should check before looking for a tube replacement?

Update: after a little bit of research, I believe my problem is likely to be the zener diodes, component number ZD311 and ZD321 respectively. I have ordered the parts and will write another update when I receive and install them!

Update: replaced the two Zener diodes and the game is operational now! Hope this helps someone in the future lol