Focus coil resistor

[Electronic M]

Yes it could cause HV issues. That resistor is effectively in series with 2 series windings on the flyback. If the pot end to end resistance increases substantially it will limit the energy through the flyback which could reduce HV....Also the yoke helps tune the resonance of the flyback. Disconnecting the yoke WILL reduce HV, so putting a high resistance in series with the yoke should reduce HV.

A quick way to check is take a fixed resistor the same value/wattage as the pot and temporarily replace the pot with it. If you connect the wiper wire to one end of the resistor it'll be the same as setting the centering to one extreme and the set will work fine that way (you will just be stuck with only 2 centering settings until you can get the right pot).