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The reason you are losing sync is because when the brightness is turned up high or the contrast advanced, you are clipping the sync with the diode! I suspect the sync take off is at the plate of the video amplifier.
Compare your circuit with the RCA 630ts and note R148 which will alleviate the sync clip.
It is a mystery how it works all unless the diode was installed opposite to how you depict it. The CRT electron current flow in the original design is from is ground through the brightness control up thru R32 the 150K resistor to the cathode of the CRT. The diode depicted in your drawing would obstruct the beam current and there is no other path unless C19 the 0.05ufd coupling capacitor is very leaky!
Last edited by Penthode; 03-30-2020 at 12:38 AM.
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