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Last updated 13th March 2003

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

ęBill Harris 1997