75 meter AM Transceiver
The 'Wheat Box'
Take a look inside!  January
- Warning!!! - The linked page displays naked components! Viewer discretion is advised.
- 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 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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