Troubleshooting The Converter Stage

Function of the Converter:

• Tunes and amplifies the received signal.
• Generates an unmodulated RF signal at a frequency off-set from the received signal.
• Mixes the locally generated signal with the received signal.
• Maintains a constant frequency difference between the locally generated frequency and any signal to which the receiver is tuned. This is known as the intermediate frequency.

Theory of Operation:

Tunes and amplifies the signal - RF transformer T-2 is the input to the converter stage. Tuning of the input is accomplished by the circuit comprised of L-5 and C-5, which feeds the signal to grid G4. The amplified signal will appear on the plate circuit. Capacitor C-5 is one section of the tuning condenser which is ganged to C-2 of the RF amplifier stage and to C-6 of the oscillator stage.

Generates an unmodulated RF signal at a frequency off-set from the received signal. - The oscillator section is comprised of the cathode, grid G1,and grid G2 of the tube, and oscillator transformer T3. Grid G1 acts as the oscillator grid, while grid G2 acts as the anode or plate of the oscillator. Feedback to sustain oscillation is obtained by the coupling of coils L-6 and L-7. Tuning capacitor C-6 across L-6 tunes the oscillator frequency.

Mixes the locally generated signal with the received signal - Because the anode of the oscillator section is a grid (G2) instead of a solid anode, some of the electrons generated by the oscillations will pass through the grid to the rest of the converter tube. The received signal, which is applied to the signal grid G4, will mix with the signal from the local oscillator section. This mixing action will produce four different frequencies which will appear on the plate output of the converter tube.
These frequencies are:

1. the input frequency being received
2. the oscillator frequency
3. the sum of the input and oscillator frequencies
4. the difference (the one we are interested in) of the input and oscillator frequencies

Maintains a constant frequency difference between the locally generated frequency and any signal to which the receiver is tuned - The local oscillator must always maintain a frequency difference between its own frequency and the incoming signal equal to the I-F (intermediate frequency). You will notice on the diagram that the input to the converter is tuned by C-5, and the oscillator is tuned by C-6. These two tuning capacitors are ganged together as indicated by the dashed line connecting them on the diagram. These two sections are also ganged to capacitor C-2 on the input to the RF amplifier. When the shaft of the tuning capacitor is rotated, all three sections tune together. Trimmer C-6A, and padder C-7 are used for oscillator frequency and tracking adjustments. The circuit values of the oscillator section are chosen to make the oscillator always tune higher in frequency by an amount equal to the intermediate frequency, in this case, 455 kHz. So no matter what frequency the input circuit is tuned to, the oscillator is always tuned 455 kHz higher.
As an example, lets assume the input is tuned to 600 kHz near the lower end of the broadcast band. The local oscillator will be tuned to 1,055 kHz (600 kHz + 455 kHz). The converter plate circuit will contain the four frequency components:

600 kHz........Received Signal
1,055 kHz......Oscillator Signal
1,655 kHz......Sum of the above
455 kHz........Difference between the first two

With the input tuned to a signal at 1,200 kHz. The four frequency components generated would be:

1,200 kHz......Received Signal
1,655 kHz......Oscillator Signal
2,855 kHz......Sum of the above
455 kHz........Difference between the first two

From the above examples, it can been seen that the prime function of the converter is to change any received signal to the intermediate frequency of 455 kHz. The following I-F amplifier stage, which is tuned to this intermediate frequency will only accept the 455 kHz signal from the converter to be passed on to the other receiver stages.

Troubleshooting:

Mixer section - With a signal generator, insert a modulated test signal on the mixer grid G4 at the I-F frequency. If the stage is operating, the signal will be amplified and passed on to the I-F stage and the tone heard in the output of the receiver.

Oscillator section - Measure the grid G1 of the oscillator section using a high-impedance voltmeter such as a Vacuum Tube Voltmeter (VTVM). If the oscillator is operating, the grid should read a negative voltage, otherwise the voltage will be zero or slightly positive. Note: on a marginal oscillator section just touching the grid with the meter probe may cause the oscillator to stop functioning. If this is suspected, try this next test. Place working receiver next to the receiver being tested, a transistor set will do just fine. Tune this receiver to a relatively weak station on the broadcast band. Now rotate the tuning capacitor of the receiver under test through the broadcast band. If the oscillator is working it will be picked up in the second receiver and a whistle will be heard as the oscillator sweeps past the frequency of the second receiver and heterodynes with the weak station being received.

For an inoperative mixer or oscillator section, use the troubleshooting charts below for symptoms and possible causes. Assume all other stages are functioning properly.

Check for the following conditions:

Open oscillator coil winding L6.

Shorted oscillator section of the tuning capacitor C-6.

Open padder capacitor C-7.

Open or leaking oscillator grid capacitor C-19.

Open or incorrect resistance for oscillator grid leak resistor R-19.

Leakage across tuning capacitor C-6 or trimmer C-6A.

Tracking Of The Oscillator