HVSD I remember on these caused vert blinking / collapse.
That is above the CRT on a terminal strip w/ an SCR.
Other common thing IIRC was plate resistors in the horz osc.
Pix was OK but the 36MC6 had short life.
Also move one of the 5GH8's around & see if the symptoms change.
Dont trust a checker.
73 Zeno
LFOD !