note

1. AM RECEIVER STAGES
RF STAGE
Function:
Many receivers incorporate a stage of RF amplification ahead of the converter
stage. It is the first stage in the signal path. The RF stage receives signals from the
antenna, tunes the desired signal, amplifies it, and passes it on to the converter.
The RF stage provides several advantages.
 Increased sensitivity
 Increased selectivity
 Improved AVC (Automatic Volume Control) action
 Elimination of image-frequency response - peculiar to superhetrodyne receivers
Theory of Operation:
Refer to the diagram below.
Signals of all different frequencies induce a current in the primary (L1) of the
antenna circuit and are coupled to the secondary winding (L2). The secondary
winding and the tuning capacitor C2 form a tuned circuit to select the frequency of
the desired signal which is applied to the grid of the 6K7. The amplified signal will
appear in the plate circuit where it is coupled by the primary of the interstage
transformer (L4) to the secondary winding (L5) to be passed on to the converter
stage. Capacitor C-4 and resistor R-4 in the plate circuit, and C-14 and R-14 in the
screen circuit, decouple the signal from the B+ supply bus.
AVC (automatic volume control) voltage, which is developed in the detector stage,
is applied to the grid of the RF stage through resistor R-30 and L2. Capacitor C-30
is the AVC bypass and also provides an rf signal path for the lower end of L-2 to
ground. The AVC voltage is a negative bias voltage that is developed in
the detector/AVC/1st audio stage. The AVC voltage is proportional to the strength
of the received signal. This negative voltage is applied to the grid of the RF (and
also the converter and IF stages) and automatically adjusts the gain of these stages.
Stronger signals develop more AVC voltage, reducing the sensitivity of the stages,
while weaker signals cause the AVC circuit to develop less AVC voltage, thus
increasing the sensitivity. This AVC action causes the output volume of the

2. receiver to remain fairly constant over a wide range of signal strengths for a given
setting of the volume control.
Troubleshooting:
Trouble-shooting an RF stage is fairly straight forward. Using a signal generator, a
modulated signal is applied to the antenna terminals and if the stage is working
properly, the amplified signal will appear in the plate circuit, across the interstage
transformer T-2, to be passed on the converter stage. Assuming all other stages are
working properly, the output will be heard in the speaker.
Below is a chart of symptoms and possible causes. Assume all following stages are
working properly. Refer to the schematic diagram above.
Service Data Chart For Troubleshooting the RF Amplifier Stage
Symptom Abnormal Reading Possible Cause
RF Stage
Inoperative
Plate voltage = 0. Other
voltages normal
Open primary (L4) of interstage transformer T-2.
Open plate resistor R-4.
Plate de-coupling capacitor C-4 shorted
Screen voltage = 0. Other
voltages normal
Screen by-pass capacitor C-14 shorted.
Screen resistor R-14 open
All voltages normal Check for short in gang tuning capacitor C-2.
Defective tube

3. Cathode high Open cathode resistor R-1
Cathode voltage = 0 Shorted cathode by-pass capacitor C-1
Dead tube
Weak signal All voltages normal Weak tube.
Check for open winding (L1-L2) on antenna
transformer T-1.
Open plate by-pass capacitor C-4.
Open AVC by-pass capacitor C-30.
RF stage out of alignment
Oscillation All voltages normal Open screen by-pass capacitor C-14.
Tube shield not making good ground connection.
Noisy operation All voltages normal Open or corroded antenna transformer T-1.
Open AVC by-pass capacitor C-30.
Corrosion in the interstage transformer T-2.
Defective tube.
Defective gang tuning capacitor C-2 (check for
grounding wipers making poor contact).
Dirty trimmer capacitor C-2A
Poor tone
quality
All voltages normal Shorted AVC by-pass capacitor C-30.