AGC problems are the worst. The AGC line feeds both the IF and RF deck. Turn the AGC on, turn to a local AM flame thrower an meter the rear AGC jumper to chassis ground. Should be something over 7 volts.
No AGC troubleshooting starts with checking the IF levels at the input to the AGC rectifier and going from there.
Chuck WA4HHG
SKCC #967
one of my R-390A's has been driving me nuts with an intermittent AGC problem for a few months. the other day i got bothered enought to troubleshoot it. when the smoke cleared the problem was very easy to repair. this is a nice clean Collins unit that i had recapped