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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:
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.
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.
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.
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.
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.
Check for the following conditions:
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.
ęBill Harris 1997