The 20X1 chassis doesn't have an AGC control. Here's a link to the schematic:
http://www.earlytelevision.org/pdf/A...Sams_100-1.pdf
I'd start by checking the voltage on the 6AS5 audio output cathode.
This set uses a stacked power supply design with the class A audio output tube doing double duty as a voltage divider for the lower B+ supply rail.