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Last updated 13th March 2003
Troubleshooting The Detector/AVC/1st Audio Stage
The detector stage is often referred to as the demodulator stage, or sometimes as the second detector to distinguish it from the mixer which is sometimes referred to as the first detector, a name often used in early receivers.
Theory of Operation:
Demodulate, or remove the audio component of the received signal - The input signal fed to the detector comes from the preceding I-F stage. This is an alternating signal composed of the carrier signal which is modulated by the audio component. The detector or demodulator section consists of the cathode (pin 3) and the two diode sections (pins 4 & 5) of the tube, which form a half-wave rectifier. The incoming signal is rectified (the lower half of the wave form is chopped off) by this circuit. Resistor R-26 and capacitor C-26 act as a filter to remove the RF component of the signal, leaving only the audio portion
Amplify the audio signal and pass it on to the audio output stage - This audio signal appears across the volume control R-27 where it is coupled to the grid of the triode section of the tube through coupling capacitor C-31. The amplified signal appears on the plate circuit where it is passed on to the audio output stage through coupling capacitor C-32.
Develop the AVC (automatic volume control) voltage - A negative voltage is developed across the volume control R-27 as a result of the rectification of the signal. ResistorR-28 and capacitor C-28 filter this negative voltage and smooth it out to a DC voltage. The amount of AVC voltage developed depends on the strength of the received signal. The stronger the signal, the more voltage developed. This negative bias voltage is applied to the grids of the preceding R-F and I-F stages. When the incoming signal is strong, a high AVC voltage is developed which reduces the gain of these preceding stages. A weaker signal, develops less AVC voltage which increases the gain of these preceding stages. Thus, the AVC voltage acts to hold the gain fairly constant over a wide range of signal strengths. This keeps the audio from becoming overly loud when tuning from a weak station to a stronger one. The wire that feeds the AVC signal to the preceding stages is called the AVC bus.
A quick check for operation of the
detector stage can be made by applying a modulated signal
from the signal generator to the grid of the preceding I-F
stage. If the modulated tone from the signal generator is
heard in the speaker, the detector and audio amplifier
portions of the stage are working.
The operation of the AVC portion of the circuit can be checked by measuring the AVC voltage across the volume control potentiometer R-27. If the AVC circuit is functioning, a negative voltage will be read. By adjusting the signal strength of the signal from the generator, this voltage should vary accordingly. A stronger signal should produce more AVC voltage, and a weaker signal, less AVC voltage. Note: by design characteristics the AVC circuit can handle signal strengths only up to a certain point. If the output from the signal generator is increased beyond that point, the AVC voltage will no longer increase with increase in signal strength.
The charts below give symptoms and possible caused for the detector/AVC/audio amp circuits. Refer to the schematic diagram above.
ęBill Harris 1997