75 meter AM Transceiver
The 'Wheat Box'

  Inside View  

 Warning!!!   The linked page displays naked components! Viewer discretion is advised.  Warning!!!

  General Highlights
  • Frequency coverage: 3.75MHz to 4.00MHz, 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
      • 4KHz filter selection
      • 8KHz 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: 100Hz
  • RIT tuning range:  ±4KHz
  • Tuning rate:  approximately 30KHz per turn
  • AF output:  3.5 watts RMS into 8Ω
  • Basic scheme - Heterodyne transmitter
  • The output amplifier stage uses a single-ended 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 into 50Ω (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

  Data Sheets

  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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