[R-390] AGC troubleshooting

[Charles Steinmetz]

Here is a guide to AGC troubleshooting (I've posted this before, each 
time with some minor revisions):

Best regards,

Charles


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).

The R390 is very similar, although the part numbers are different.