[Central-Electronics] re shorted focus scope pot on 200V

[Fern]

    Finally got to work on my central electronics 200V exciters. I have one 
where I could not focus the display. Trouble shooting it proved that the 
focus control R305 (500K potentiometer) has a short of approximately 24 ohms 
to ground on the wipper arm. Has anyone had this problem before and what is 
the best place to procure a correct replacement?
    Thanks from Fern   VE7GZ