[R-390] Local & Line potentiometers [NOW] AGC troubleshooting

[Charles Steinmetz]

Craig wrote:

>A little time was spent on the beast this evening.
>1: No ground problems at the end of the pots, even added a jumper once
>again. No change
>2: Checking the audio at the CCW ends, less than 5 millivolts with the
>O'scope.

OK, then your problem is way too much audio, and the most likely 
suspect is the AGC.  Before you swap modules, you will be well served 
by collecting some data.

First, make sure there is a jumper installed between TB102, Terminals 
3 and 4.  If not, install one and see how it works now.  Assuming 
there is a jumper:

Set the AGC to "MED" and tune the radio to a good, strong, local 
signal (like a strong AM broadcast station).  Measure the DC voltage 
at TB102, Terminals 3 and 4 with a high impedance meter (VTVM, DVM, 
or scope, with an input resistance of >= 1M ohm; not a VOM).  It 
should be significantly negative, -10v or more.  If it is, you have 
no AGC problem and the fault lies elsewhere.  But if the voltage is 
only weakly negative, or zero, you have an AGC problem.  If so:

Turn off the radio, and pull the plug.  Set the AGC time constant to 
"MED."  Remove the jumper between TB102, Terminals 3 and 4.  Measure 
the resistance to ground from each of these Terminals.  Terminal 3 
looks back into the AGC detector, and should read in the neighborhood 
of 500k ohm due to R545, R546, and R547.  Terminal 4 is the AGC line 
feeding the RF and IF circuits and should read essentially infinite 
(>> 1M ohm).  If you have gotten to this point, one or the other of 
these Terminals will probably show a much lower resistance to ground 
than this.  Trace the circuit to find the leaky component(s).  If 
Terminal 3 reads less than ~500k ohm, the usual suspects are C551, 
C548, C547, C545, and C544.  If Terminal 4 reads less than 1M ohm, 
the usual suspects are any of the several dozen bypass caps on the 
AGC line in the IF and RF sections.

If, on the other hand, the resistance readings are OK, suspect V508, 
V509A, and associated circuitry (Z503 and C546, especially).

Best regards,

Charles