75 meter AM Transceiver

The 'Wheat Box'

  Inside View

  General Highlights
  • 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
  • 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 AM?

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.

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