[R-390] R390A AGC troubleshooting procedure

[Charles Steinmetz]

I wrote:

>[TB102] Terminal 4 is the AGC line feeding the RF and IF circuits 
>and should read essentially infinite (>> 1M ohm).

Dave wrote:

>No, there's a voltage divider in the RF deck.  R201 and R234, on 
>Figure 5-18, page 39 in Y2K-R3.
>Resistance to ground will be 1.8M .  When you unplug P108 off the RF 
>deck, terminal 4 should go open.

Right you are, Dave -- thanks!  Here's a further corrected version of 
the procedure:

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 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 approximately 1.8M ohm 
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 ohm, the usual suspects are C551, 
C548, C547, C545, and C544.

If Terminal 4 reads significantly less than ~1.8M ohm, 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.  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 dirty, but this 
is 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