[R-390] R390A AGC

[Bill Coleman N2BC]

My R390A has an AGC problem.  With no signal the AGC voltage is -0.5V
(measured with an HP410C VTVM).  The AGC responds properly to signals (goes
more negative to a max of appx. -15V).

Resistance measurements on the AGC line look OK:

IF strip @ J512-6 = Infinity
AGC line @ J512-4 = 482K
RF deck @ J108 = 1.82M
With P512 and P108 in place, the AGC jumper on the rear = 378K

The only other voltage that is wrong is the IF strip suppressor grid bus,
measures +3.4V should be -2.5V

I suspected Z503,  I couldn't get it to peak - Output increased as I cranked
out on the slug but it ran out of adjustment range before peaking.   Just
replaced Z503 (nasty) and have the same result.

I've gone thru 3 wholesale changes of tubes in the IF deck.

I've replaced C545, C546, C548.   (I keep coming back to the AGC IF Amp
area)

Ideas?

73, Bill  N2BC