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Last updated 13th March
Troubleshooting the RF Amplifier Stage
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 action
- Elimination of image-frequency
response - peculiar to superhetrodyne receivers
Theory of Operation:
Refer to the schematic diagram
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+
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
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 receiver to remain fairly constant over a wide
range of signal strengths for a given setting of the
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 circut, 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.
Data Chart For Troubleshooting the RF Amplifier Stage
= 0. Other voltages normal
(L4) of interstage transformer T-2.
Open plate resistor R-4.
Plate de-coupling capacitor C-4 shorted
voltage = 0. Other voltages normal
capacitor C-14 shorted.
Screen resistor R-14 open
short in gang tuning capacitor C-2.
voltage = 0
cathode by-pass capacitor C-1
Check for open winding (L1-L2) on antenna
Open plate by-pass capacitor C-4.
Open AVC by-pass capacitor C-30.
RF stage out of alignment
by-pass capacitor C-14.
Tube shield not making good ground connection.
corroded antenna transformer T-1.
Open AVC by-pass capacitor C-30.
Corrosion in the interstage transformer T-2.
Defective gang tuning capacitor C-2 (check for
grounding wipers making poor contact).
Dirty trimmer capacitor C-2A
by-pass capacitor C-30.