There's no need to experiment with values. Motorola added retrace suppression to later revisions of TS-18 chassis as shown in this schematic. Earlier versions had pin 3 grounded. This version has pin 3 going to the junction of a 250pF (C-70) cap and a 220K resistor (R-90). That couples a pulse during the vertical retrace to the grid and blanks the raster.
I got this from Riders volume 4.
