I have a heap of notes from working on sys 7 that i shall dig out.

IF it is the blanking signal then a test to confirm this is to jump the 5v to pin 37 ( from memory), if game boots then, yep blanking signal is at fault.

Blanking HAS to be a nice smooth 5v - any ripple will throw it out.

ill get back later with what ive made note of from previous workings.

The 5V jump test is on Marvins site - check there first as im going from memory here from 6 years ago