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ms660 · 6th Feb 2017, 8:17 pm

AGC is an abbreviation for Automatic Gain Control, also sometimes referred to as AVC (Automatic Volume Control)

The AGC voltage is developed from the signal and it's subsequent rectification, in some receivers the voltage is obtained via the normal detector circuit, in other's (including yours) a separate diode is used, it's the right hand one in the EBC33's envelope as viewed in the schematic, the signal to feed that diode is taken from the anode circuit of the IF amplifier via C24 (50pF) the rectified voltage will be -ve and will be proportional to the signal strength, that voltage is used to control the gain of the IF valve and the Mixer valve, when those valves control grids go more -ve the gain of those valves is reduced, that basically equates to less gain with a strong signal and more gain with a weaker signal, the overall effect is to keep the detected volume level reasonably constant when receiving either a weak signal or a strong signal.

Lawrence.

6th Feb 2017, 8:17 pm	#97
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bush A.C.91 AGC is an abbreviation for Automatic Gain Control, also sometimes referred to as AVC (Automatic Volume Control) The AGC voltage is developed from the signal and it's subsequent rectification, in some receivers the voltage is obtained via the normal detector circuit, in other's (including yours) a separate diode is used, it's the right hand one in the EBC33's envelope as viewed in the schematic, the signal to feed that diode is taken from the anode circuit of the IF amplifier via C24 (50pF) the rectified voltage will be -ve and will be proportional to the signal strength, that voltage is used to control the gain of the IF valve and the Mixer valve, when those valves control grids go more -ve the gain of those valves is reduced, that basically equates to less gain with a strong signal and more gain with a weaker signal, the overall effect is to keep the detected volume level reasonably constant when receiving either a weak signal or a strong signal. Lawrence.