If the pot does not adjust the high voltage and the HV IN = HV OUT. The transistors in the HV section are shorted. Probably all three of them. You can be guaranteed the transistors are bad if the 82k or 22k resistor (i forget which one now) is burnt.
I put together HV rebuild kits for Bally Driver games. I made about 25 of them, i need to figure out the price point and put them on my website.
I fix these driver boards too.
Thank you
Andrew