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Time for an update!
Ed Milbourne indeed did supply the unobtainable pots. Progress then got interrupted by travel in May. The bad news is that two of three existing pots were bad, and of the three removed from a junker by Ed, tow were bad. Guess this was a weak point in this chassis.
Attached scan and previous attachments show original DC convergence circuit with 100 ohm pots tapped at 25 ohms (CTC-5 DC conv003), and an attempt I made to get results using a separate fixed resistor divider for the tap point CTC-5 DC conv004). this didn't work becuase the dynamic convergence drives are in series with this resistance and wete thrown out of whack. CTC-5 DC conv005 shows the final thing I cam up with, replacing each tapped pot with two pots and a SPDT switch to select either and upper or lower wiper position.
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