[old_tv_nut]

Another hint: for pin 7, you should set your scope to DC coupling and establish where 0 volts is on the scope screen with the vertical centering control. Then you can see the actual voltage range on the grid.

Some things I don't understand right now:
Why is the DC on pin 7 zero volts with video? The detector diode is working negative with respect to ground, so any IF signal should pull the grid negative.

Why does pin 7 go negative with no video (I assume you mean no RF signal?). Does the scope show a lot of random noise there with no RF? If so, and you don't see snow on the screen, this would mean the tuner and IF are fine and something is causing the noise to be suppressed before it gets to the output (perhaps the noise inverter).