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R417 (CR409 prevents current from flowing into the U402B negative node) so R418 must have
one volt across it and therefore the cathode of CR408 must be at plus two volts. R421 has
plus one volt across it and therefore is conducting 0.5 mA into the negative node of U402A.
R420 is drawing 0.25 mA from this same node so the difference, 0.25 mA, must be flowing out
of this node through R423. The output of U402A is at -4.4 V. Current is drawn from the
amplifier summing node through CR406 from approximately -0.6 V at the cathode of CR406.
This puts 3.8 V across R424 (maximum with R426 in a full clockwise position, zero with R426
in a full counter clockwise position). This current is summed into the focus amplifier input
and produces the correction for beam position.
The right/left focus correction is identical to the top/bottom. Both circuits sum into the
Focus Gate Amplifier input. The result is an asymmetrical, upside-down, truncated pyramid
approximation to an asymmetrical parabolic bowl. By adjusting the heights and offsets of the
pyramid for the best fit to the bowl one can achieve a focus uniformity over the whole CRT
screen.
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