Do you reach for the RF gain control when you tune your local AM broadcast station?  One of the most commonly heard complaints from owners of  "boatanchor" communications receivers and other vintage high-performance radios is:  "The audio is distorted on strong  signals."  While audio distortion may be the result of an inoperative or poorly tuned stage or stages in the receiver, once the first-order bugs are exorcised and full sensitivity is realized, this distortion is almost always an AGC problem.

THE NEED FOR GAIN CONTROL
 
    The overall combined maximum voltage gain through the RF and IF stages of common receivers is on the order of a million.  For AM and SSB reception, all of these amplifiers must operate in a linear mode, i.e. they must not saturate or clip, if the signal’s modulation envelope is to be accurately reproduced.  Obviously, the full gain of the receiver is needed only for the weakest signals.
    In tube type receivers the RF and IF gain control is almost universally accomplished through the magic of  the remote-cutoff pentode.  The gain of amplifier stages using these "variable mu" tubes, such as the 6K7, 6SK7,  and 6BA6,  can be varied electrically by changing the bias on the signal grids.  Because the gain of a single stage can not be reduced to zero, gain control signals are generally applied to all the RF and IF amplifier stages and sometimes to the mixer(s).

AUTOMATIC GAIN CONTROL
 
    In almost all receivers since the early 1930’s, especially for AM reception, the RF and IF gain is controlled automatically.   This not only accommodates fading, but avoids "blasting" on strong signals or missing weak signals all together when tuning across the band. Throughout the 1930’s and  early ‘40’s this function was referred to as automatic volume control (AVC).  In the WWII era, with the introduction of systems, such as TV and radar, where the parameter being controlled is not audio volume, the term automatic gain control (AGC) begins to creep in.  For our purposes, it means the same thing.
 
    In broadcast receivers, the AGC function is universally present, but is generally hidden from the user.  The "volume" control knob effects only the gain of the audio amplifier.
    Communication receivers usually have both automatic and manual RF gain controls.  The manual control is intended for use in situations, such as CW reception, where the AGC circuit may not function properly.  Any quality communication receiver, in proper operating condition, will receive all but the most grotesquely strong AM signals with minimal distortion in the AGC mode with the manual gain control wide open
 
IMPLEMENTATION

    AGC is accomplished by recovering a DC signal from the detector stage that is proportional to the signal strength and applying it to the gain-controlled stages in such a way as to reduce the gain as signal strength increases.  In tube-type receivers, the AGC voltage will always become more negative with increasing signal strength.  The AGC signal must be low-pass filtered to remove the audio component of the signal, lest the AGC system suppress the modulation of the incoming signal.
    In most common applications, the AGC signal is picked off from the junction of the last IF transformer secondary and the detector-diode load resistor.  (Which may be masquerading as the audio gain control potentiometer.)  The AGC signal will pass through a series of high value resistors on it’s way to the control-grid return connections of each of the gain-controlled stages.  This AGC "bus" will also have a number of capacitors from the various resistor junctions to ground.  The larger caps, at the detector end of the bus, establish the basic AGC time constant,  while the others further filter the AGC signal and
bypass stray RF signals to ground.

THE PROBLEM

     The receiver is in ostensibly good operating condition:  All stages have known good tubes with the appropriate voltages on their elements.  The set is aligned properly and all the adjustments could be peaked.  And, you can hear weak signals at least down to about 10 microvolts.  When tuned to a distant AM station the audio sounds good.  When tuned to a local; distortion is obvious, and goes away when you switch to MGC and reduce the gain.  The condition can also be observed by connecting a modulated signal generator to the antenna terminal.  You should be able to bring the signal level up to the better part of a volt before serious distortion can be heard or the audio output "folds back" to a lower level.
    The AGC bus is generally a very high impedance circuit so as not to unduly load the detector circuit.  Furthermore, because the gain of the controlled stages increases EXPONENTIALLY with a decrease in AGC voltage, even small amounts of leakage will cause the AGC system to malfunction.  The source of this leakage is almost always the capacitors on the AGC bus.  How much leakage is acceptable?  Al’s law:  10 megs is a significant problem, 1 meg is a disaster.

THE FIX

    In receivers with paper caps, especially the dreaded "Micamolds" or anything coated with wax:  REPLACE ALL THE PAPER CAPS.  In fact, do this before you even turn the set on and save yourself a lot of trouble.  A quality vintage receiver can often be brought back to life and play quite nicely by just shotgunning the caps and making no adjustments at all.  Then a "light" alignment puts it in primo condition.  I keep coming back to an observation I heard from an old time race car mechanic:  "Don’t even try to tune junk."
    So you do the above, and things get better, but there’s still a problem with strong signals.  What now?  Make a Xerox copy of your schematic, and highlight the AGC bus.  Disconnect the bus from the diode-load resistor and measure the resistance from the bus to ground.  It should be essentially infinite. This will enable you to find any caps you missed (or decided to ignore like those inside IF cans), other leaky components (I’m starting to find leaky postage stamp mica caps.) or physical shorts.  Most digital meters measure to 20 megs and are usually adequate.  The Hewlet Packard HP 410 series of meters  will read 100’s of megs and are recommended for the truly paranoid.
    Some receivers, the R-390 family in particular, have a high value resistor to ground at the far end of the AGC bus.  You need to disconnect it to perform the above test.
    When you look at a schematic, the AGC circuit is all but invisible.  So it’s not surprising that it’s often overlooked as a source of trouble.  Hopefully, this article will help get the old sets playing properly again.
 
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