[old_tv_nut]

As I understand it, you did not remove R243 and R239? This means that you have reduced the resistance in this part of the circuit, and it will no longer bias the transistor or the CRT correctly. You must disconnect R243 and R239 and replace them with the pot (from H to L). 5k may or may not be close enough in this circuit - it is certainly outside the 5% tolerance of the original resistors. To get back to 4k ohms, put a 20k (18k or 22k is close enough) resistor across the pot (from H to L).