[R-390] R390A AGC Troubleshooting technique.

[Raymond Cote]

Charles and all, in R390A school all that was specified was to check out the caps on the AGC line. Many are really hard to find. Oh and then they said "by the way ring out the resistors. "
Ray

Raymond Cote
KD9CCZ
Hurrah!  My new general call sign


> On Oct 3, 2014, at 09:46, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> 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
> 
> 
> 
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