75 meter AM Transceiver

The “Wheat Box”

Take a look inside!  September 4, 1999

• Frequency coverage: 3.75 MHz to 4.00 MHz, with about 20 KHz of “overcoverage” at band ends.
• Front panel layout:
• Backlit S/RF meter
• Backlit Frequency Readout
• Controls (L-R):
• IF Bandwith Selector
• RIT Selector
• RIT Tune
• PWR/AF Gain
• Main Tuning
• LED Lamps above Main Tuning (L-R):
• Transmit indicator
• RIT enabled indicator
• 4 KHZ filter selection
• 8 KHz filter selection
• Rear panel connections
• Antenna
• PTT microphone
• Speaker
• T/R control line for mating linear amplifier
• Case interior is fan cooled

• Basic scheme: single conversion to the 455KHz IF frequency with high side injection.
• Selectivity:  provided by either a 4KHz or 8KHz Collins Mechanical filter, front panel selectable.  (These are the same devices used in the high performance R390A receiver.)
• S-meter response from an unmodulated carrier input:
• S0 - No input signal
• S¼ - 0.3µV (just above AGC threshold)
• S1 - 1µV
• Mid-scale - 30µV
• S9 - 100µV
• Full scale - 30mV
• Off scale, almost pegged - 100mV
• Image rejection:  >75db from two fixed tuned bandpass filters.
• Digital frequency display resolution:  100 Hz
• RIT tuning range:  ±4 KHz
• Tuning rate:  approximately 30 KHz per turn
• AF output:  3.5 watts RMS into 8 ohms.

• Basic scheme - Heterodyne transmitter.
• The output amplifier stage uses a Motorola MRF148 power FET biased for class AB linear operation.
• Method of modulation: Linear Series Modulation applied to the driver stage.
• Power output: 1 watt carrier (4W PEP @100% modulation)
• Spectral purity: Mixed products and harmonic energy output less than –50dbc, except second harmonic which is less than –45dbc.

Simplified Block diagram - K9GDT 75m AM Transceiver

Why not?  It’s no secret that SSB and CW are more effective when the going gets tough.  I operate those modes, too.  I enjoy AM operation because it sounds so good!Also, the folks who operate AM are typically a friendly and courteous bunch, willing to share their considerable technical expertise with fellow hams.

Some links to great AM web sites:

• W5AMI- Brian’s “AM North America” - AM operator’s page.
• WB3HUZ- Steve’s fine site: “The AM Window”

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