Why not video AGC in the TRK12?
 

I've got a question about the TRK-12!

Why did they apparently remove the video AGC system from the sets
in later production? My chassis obviously had at one time had AGC but it
was chopped out. (This chassis was used as some sort of engineering test bed.)

Was it impulse noise? I note that VRAT in England has been having
a discussion about that for the British 405 line (white is higher modulation,
reverse of ours) system. I understand the difficulties of AGC in the 405 line
system, but not in ours ... the original TRK12 schematic is exactly as I would expect
a non-gated AGC to be, that is, first the video detector which is an averaging
detector (for the RF) followed by a second diode used as a capacitive input
rectifier with long time constant, acting as a peak detector on the video.

Why drop AGC?

This is just a guess: It could have been there was some worry that it would be miss-set in a shop near a transmitter then deployed to a fringe location, and not worked right there. Manual IF gain control would allow the user to get it right.

The post-war 630, and a few of it's later RCA made variants (but not all) lacked AGC, so clearly RCA had some reason to have an aversion to it for a while.

Hazeltine patents, perhaps? Hazeltine might have pulled the licensing on AGC....