RF AMPLIFIER STAGE

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 (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 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
	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