[R-390] R390A AGC Troubleshooting technique.

[Charles Steinmetz]

Alan wrote:

>my question is about a troubleshooting technique
>documented in the Army TM. There they suggest opening the
>AGC loop and provide a negative supply to the AGC line,
>terminal 4, from -10 to 0 V.
>    *   *   *
>I see in the Y2K book, their was a request
>for evaluation of a good AGC line current.

I'm not sure what you mean by "a good AGC line current."  The AGC 
line resistance to ground is determined by several 
resistors.  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.

So, if there is -10v on the AGC line feeding the RF and IF circuits 
(i.e., at Terminal 4 with Terminals 3 and 4 disconnected), the 
current will be 10v/1.8M, or 5.6 uA.  Any other current is the result 
of leakage to ground from any of a large number of possible paths, or 
grid current from the controlled tubes.  But you don't need to 
measure currents (or even know what they are) -- just measure the 
resistance of Terminal 4 with the radio turned off.  If it is less 
than 1.8M ohm, use the ohmmeter to find the leakage, and fix it.  If 
it is ~1.8M, then you probably do not have any problem with the AGC 
line feeding the RF and IF circuits.

Note that if you are getting -5v on the AGC line, the leakage is 
relatively minor -- you don't have a 10k ohm or less leakage path (if 
you have an AGC leakage problem at all).

What is the measured resistance from Terminal 4 to ground (with 
Terminal 3 disconnected)?  What is the measured resistance from 
Terminal 3 to ground (with Terminal 4 disconnected)?

Best regards,

Charles