[R-390] R390/URR Restoration

[2002tii]

Steve wrote:

>The AGC circuit is not working at all.  In going through the IF deck 
>and testing individual components I found that R554 is a funny 
>shaped 470 ohm molded resistor with round ends and a rough body. All 
>the schematics I have show this to be a 2.2K resistor.  It is in the 
>cathode circuit of the AGC amplifier, V509.

???  R554 is in the plate circuit of V509 -- it is the power supply 
decoupling resistor.  Nominal 180v B+ comes from switch S107, through 
a pi network filter (C531, L503, C530), through R554 to the 
decoupling cap (C542), then through Z503, which is a low-Q tuned 
circuit that is the plate load for V509.  V509 is a 6BJ6 Class A 
amplifier, which should draw 5-10 mA of plate current.  This will 
drop 10 or 20 volts across R554 if it is 2.2k, or 2.5 to 5 volts if 
it is 470 ohms.  The difference between 160v and 177v on the plate of 
V509 won't make any practical difference.  Theoretically, 2.2k will 
give somewhat better decoupling, but again, there would almost 
certainly be no practical difference.

The 390A has a nearly identical AGC amplifier circuit, using V508 
(6BA6), R542, C545, and Z503.

Whether R554 is 2.2k or 470 does not have anything to do with your AGC problem.

You may want to tune in a nice, strong signal and look at the voltage 
at the plate of AGC rectifier, V510.  You can measure this at TB102 
Terminals 3 and 4, which are normally jumpered, if you are using a 
meter with 10M or 11M ohm input resistance (e.g., a VTVM).  If you 
are using a meter with lower input resistance, measure at pin 6 of 
V510.  This voltage should be strongly negative, at the very least 
-10 Vdc, with S107 in the "AGC" position.  If it isn't, turn the 
radio off and measure the resistance of the AGC line to ground.  If 
you disconnect the link from TB102 terminal 3 to terminal 4, with 
S107 in the "AGC" position Terminal 3 should show ~500k ohms to 
ground, and Terminal 4 should show essentially infinite resistence to 
ground.  Very likely, one terminal will have less resistance to 
ground than this because of a leaky capacitor.  If the AGC line 
resistances are OK, Z503 could be open -- check for 150 V or more on 
the plate of V509 (pin 5) with S107 in the "AGC" position.

Happy troubleshooting,

Don


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