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Last updated 13th March
2003
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Troubleshooting the RF Amplifier 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 action
- Elimination of image-frequency
response - peculiar to superhetrodyne receivers
Theory of Operation:
Refer to the schematic 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 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 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.
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 |
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. |
Typical
Voltage Readings
Tube element |
Pin No. |
Voltage |
Plate |
3 |
240 |
Screen |
4 |
100 |
Cathode |
8 |
3 |
©Bill
Harris 1997
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