a real challange?

[Yamamaya42]

snip/ paste
Try this:

1. Place a 47k resistor between pin 11(cathode) of the CRT and ground.

2. Connect a .05uf, 630v cap from pin 3 and 4(plate) of the 6K6 vertical output
to pin 11(cathode) of the CRT.

Basically you need a positive going vert deflection pulse at the CRT cathode in order to blank the picture during vertical retrace.
----
Pure speculation at this point, not really sure, the RCA 630ts service manual shows the wave at this point to be 700v ptp, perhaps that was too high for the cathode of the 12lp4 , and some resistor in the line is needed?