[R-390] R390A AGC Troubleshooting Procedure [revised 2/17/20]

[Charles Steinmetz]

R390A -- AGC troubleshooting procedure (revised 2/17/2020):

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 time constant 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.  In that case:

Turn off the radio, and disconnect the power plug from the AC mains. 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 resistance 
(>10M ohm).  [*NOTE* that some digital ohmmeters/multimeters do not work 
for this test.  The best instrument to use is the ohmmeter function of a 
real VTVM (vacuum tube voltmeter).]

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.

Best regards,

Charles