[R-390] R390A AGC troubles

[Bill Coleman N2BC]

Maybe my earlier posts were lost in the fan / audiophile / Depot Dawg 
discussions.

Observations:

1) Low AGC voltage (zero signal = -0.5V)
2) Suppressor grid bus (V504 & V508) is +3.4V, should be -2.5V)
3) Z503 will not peak before it's slug "tops out" in the core

I have isolated the decks, measured everything that is measurable, replaced 
caps.  I have isolated individual IF stages and swapped tubes.  All without 
making progress.

The more I poke, measure and isolate, the more I am convinced that the 
problem is in the AGC Amp stage(s) rather than out on the AGC bus.

All resistance measurements along the AGC bus are OK, voltage measurements 
when fed with a separate source (9V battery) are OK.

Along the way I managed to destroy Z503 so I pulled one out of a 'cadaver' 
IF strip.  I noted that it's slug is fairly well centered in the core.  Upon 
replacing Z503, the 'new' one behaved the same way.... increasing stage 
output as the slug was cranked toward the top but it runs out of adjustment 
before peaking.

I fed the AGC Amp stage directly with my siggen at 450kc and Z503 will peak 
with the slug slightly further down in the core.

I have snugged all the ground points in the entire IF chassis & replaced 
everything in the V508 stage except the tube socket.

I think I'll disassemble the gear train, polish the parts and reassemble 
blindfolded, that will be less frustrating!

Ideas greatly appreciated

73, Bill   N2BC

PS:  I have no fan installed, use a plain highly oxygenated line cord, and 
the receiver's mixed Stewart Warner / Collins / EAC heritage is usually just 
a lovable as my Lab/?/Shepard best friend.