When I was a youngster, I had a friend who lived down the street who, like me, was fascinated by shortwave radio. His Dad had a National NC-173 receiver and we would explore the airwaves with that big metal box for hours. Of all the different stations we listened to, our favorite by far, was the AM gang on 3885 KHz. That was the hook that eventually drove us to get our novice tickets and start a life long interest in ham radio. Years later I bought and restored my own NC-173 and it's still fun to listen to the 75 meter phone band on that radio.
I had the opportunity to buy a large quantity of TA7642 single chip radio ICs and began designing a simple 75 meter receiver loosely based on the features of the NC-173. The primary goal was to design a wide band AM receiver that sounded good and was sensitive and selective. I settled on a hybrid design using a NE602 mixer/oscillator to down convert 3800 KHz to a 455 KHz IF. This is followed by a 455 KHz ceramic filter which couples into a TA7642 which provides IF gain, AGC, and envelope detection. The resulting base band audio is amplified by a TA7052 IC with BTL output and produces 1 watt of HiFi audio. The VFO is varactor tuned and covers about 100 KHz of the 75 meter phone band. Connect the AMR75 to an 80 meter dipole and an 8 ohm communications speaker and you have a great setup. To increase the utility of the receiver I added a 455 KHz BFO (National called it a CWO) which provides the ability to tune in SSB and CW stations. It's old school, just the like the NC-173, so you have to be careful to back off on the RF gain and tune in the correct sideband. AMR75 controls includes RF gain, AF gain, Main Tuning, and Bandspread Tuning. The BFO is turned on and off by toggle switch. I put my receiver into a metal enclosure but it could just as easily be mounted on a piece of wood with front and back panels made of PCB material.
The kit is easy to build with all through hole components and 1/4 watt resistors. Only one toroid to wind, the VFO inductor. Alignment can be done with a general coverage receiver. -Steve K1EL
AMR75 Features
- Varactor Tuned 100 KHz Coverage
- Hi Fi Audio Output
- 455 KHz IF
- RF and AF Gain controls
- Main and Bandspread Tuning
- 455 KHz BFO for SSB or CW tuning
- TA7642 Single Chip radio IC
- 9 to 13.5 VDC supply required
- NE602 front end mixer
- 455KHz Ceramic IF filter
- Board Size: 3.9 in. by 2.1 in.
AMR75 AM Phone Receiver for the 75 Meter Band
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