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Last updated 13th March 2003

Troubleshooting The Converter Stage

The converter stage may sometimes be referred to as the mixer, or first detector stage, the latter being an early term.

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:

The converter stage discussed here uses a 6A8 pentagrid tube. One portion of the tube functions as a tuned amplifier and the other portion as the local oscillator. Grids G3, G4, and G5 along with the cathode and plate form a tetrode amplifier. Grids G1 and G2 along with the cathode form a triode oscillator. Some receiver designs use a separate tube for the oscillator.
The theory of operation of the converter stage can be explained by expanding on each of the four functions above. Refer to the diagram below.

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: The input frequency being received, the oscillator frequency, the sum of the input and oscillator frequencies, and 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:

Failure of either the oscillator or mixer section of the converter tube will result in no signal getting through to the I-F amplifier stage. A quick check can be made of each stage using the following procedures. Assume all other stages are functioning properly.

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 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 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 another receiver next to the receiver being tested. A transistor set will do just fine. Tune the second 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 frequecy of the second receiver and heterodynes with the weak station being receive.

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

Service Date Chart For An Inoperative Oscillator Stage
Procedure Reading Possible Cause
Voltage check Oscillator grid reads zero or positive Confirms oscillator is not working
Oscillator anode reads zero Feedback coil L7 open.
Oscillator anode dropping resistor R-20 open.
Oscillator anode by-pass capacitor C-20 shorted.
Oscillator anode reads low Oscillator anode by-pass capacitor C-20 open.
Voltages normal except for oscillator grid Defective converter tube. Substitue a known good one. If trouble persists, problem may be in the oscillator grid circuit. Make ohmmeter checks as shown below.

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.
  • Service Data Chart For An Inoperative Or Abnormal Functioning Mixer Stage
    Symptom Abnormal Reading Check For
    Inoperative Zero volts on mixer plate (pin 3) Open primary L8 of I-F transformer T-4.
    Zero volts on mixer screen grid (pin 4) Shorted screen by-pass capacitor C-21.
    Missing +100V source.
    Zero volts on cathode (pin 8) Open cathode resistor R-18.
    Shorted cathode by-pass capacitor C-18.
    All voltages OK Defective tube
    Hum Open mixer grid coil L-5
    Modulation Hum Defective converter tube (cathode to filament short or leakage)
    No reception on low end of band Weak converter tube
    Check oscillator for critical oscillator conditions
    Distortion Short-circuited AVC capacitor C-29
    Weak reception - high noise level Open AVC capacitor C-26
    Squeal Open screen by-pass capacitor C-21
    Open plate by-pass capacitor C-22
    Tube shield improperly grounded
    Incorrect routing of wiring
    Squeals or birdies when tuning certain stations Image frequency interference (refer to troubleshooting the RF amplifier)
    Noisy, intermittent operation Defective converted tube
    Corrision in input transformer T-3
    Check tuning capacitor for shorts or poor wiper contact
    Check wiring for loose connections, bad solder joints
    One station all over tuning range Tuning capacitor not tuning
    I-F tuned to wrong frequency
    Receiver will not track on alignment Tuning capacitor stator plates incorrectly spaced
    Typical Voltage Readings
    Circuit 6A8 Pin No. Volts
    Plate 3 245
    Screen 4 100
    Oscillator grid 5 -15
    Oscillator anode 6 200
    Cathode 8 3

    There are other variations of the oscillator circuit. As mentioned earlier, some designs use a seperate tube for the oscillator.


    A Word About "Tracking Of The Oscillator"

    Since the intermediate frequency stages are fixed tuned, the oscillator and tuning to the input of the mixer stage must track properly so that the two circuits are always 455 KHz apart over the entire tuning range. The input circuit to the mixer must tune a range of 550 to 1,600 KHz, while the oscillator must tune 1,005 to 2,055 KHz. If the oscillator does not track with the tuning of the input so that there is always a 455 KHz difference between the two, the intermediate frequency produced will no longer be 455 KHz and degraded performance will be notice.
    Receiver alignment instructions usually include a tracking adjustment for the upper and lower ends of the band. The capacitors used to do this are the trimmer C-6A, across the main tuning capacitor C-6, and the padder C-7, which is in series with oscillator coil L-6 and tuning capacitor C-6. The trimmer is used to set the high end of the band, while the padder sets the low end.
    In some receivers, the oscillator section of the tuning capacitor has been designed to maintain the intermediate frequency difference without the use of the padder capacitor. In this design, the rotor plates of the tuning capacitor are smaller and differently shaped than the rotor plates of the other sections of the tuning capacitor. This type of capacitor is one having a cut-plate oscillator section. The shape of the plates automatically maintain the tracking of the oscillator. In this case, the only adjustment necessary will be the trimmer adjustment at the upper end of the band.

    ęBill Harris 1997