[bandersen]

The blanking circuit picks a signal off the vertical output, runs it through the K1 couplate applies it to the CRT grid. So it could be the couplate is bad or the CRT is so far gone, the blanking signal isn't effective.

Yes, you hook up the sig gen that way. Only the TV get's plugged into the isolation transformer. Nothing else!

Wire the new CRT filament exactly as the old one was. You don't need to change anything. The resistance in the tube filaments vary with the current running through them. Sort of self regulating. Just try it and you'll see what I mean. The new CRT will have about 6.3 VAC across it without any changes needed.