[R-390] AGC troubleshooting procedure [WAS: Why is it so ?]

[Charles Steinmetz]

Larry wrote:

> Sure looks like you have an agc problem.

R390A -- AGC troubleshooting procedure:

Throughout this entire procedure, the "FUNCTION" switch should be set to 
"AGC."

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

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 gross 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 power 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 
ohms due to R545, R546, and R547.  Terminal 4 is the AGC line feeding 
the RF and IF circuits and should read approximately 1.8M ohms due to 
R201 and R234.  If you unplug P108 from the RF deck, Terminal 4 should 
then read essentially infinite (>10M 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 ohms, the usual suspects are C551, 
C548, C547, C545, and C544.

If Terminal 4 reads significantly less than ~1.8M ohms, the usual 
suspects are any of the several dozen bypass caps on the AGC line in the 
IF and RF sections, or leakage to ground in one or more of the 
mechanical filters, or (much less likely) leakage to ground due to 
contamination of the fiber insulating washer on the shaft of the antenna 
trimmer capacitor.  It is also possible that the sector of the 
"FUNCTION" switch that shorts Terminal 4 to ground when the switch is 
set to "MGC" or "STAND BY" is mis-timed, broken, or contaminated/dirty, 
but this is even more unlikely.

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

Best regards,

Charles