The MB 20 transceiver is now complete. The rig is a homebrew 10-watt 20-meter SSB transceiver. The VFO module was an ebay purchase of a salvaged Yeasu FT-401B module. The FT-401B was manufactured around 1970. I bought the module after a recommendation from Bill Meara, N2CQR and Pete Juliano, N6QW. The rig was loosely inspired by Bill’s Mythbuster rigs.
This rig is really a stone-soup build – with bits and pieces of modules lifted from or inspired by many of the homebrew illuminati!
The Yaesu VFO runs at 9MHz so I set the IF at 5MHz and built a 5MHz crystal ladder filter. The rest of the IF module consists of 4 Termination Insensitive Amplifiers (TIAs from W7ZOI, Wes Hayward), 2 on either side of the filter for each of transmit and receive. I also used a TIA to boost the output of the Yaesu VFO by 7dB to drive the IF mixer. The BFO is a Colpitts crystal oscillator using one of the crystals left over from the ladder filter build. The IF mixer is a homebrew diode ring mixer. The balanced modulator / product detector is a 2-diode circuit that I liberally borrowed from Ashhar Farhan’s Bitx-20 module schematic. The power chain is two 2N3866 pre-amp/driver stages feeding a RD06HHF1 MOSFET final amplifier, very loosely modeled after the Bitx-40 module PA. The 20-meter band pass filter is a standard double tuned circuit, the Tx LPF is a W3NQN LPF design with values taken from the QRP Labs kit instructions. The microphone amp is a one transistor NPN amp configured to support a homebrew electret microphone. The circuit originally came from Farhan’s sBitx that I modified for my electret mic. The audio amp is a single stage driver and an LM-386, that I first built for Pete’s SimpleSSB.
The front panel and base are plywood from my wood shop. I use copper roofer’s foil to create a solderable ground plane. Almost all the circuit boards are homebrew Manhattan style. The IF TIAS are on boards that were sent to me by Todd Carny, K7FTC, of MostlyDIYRF. The Tx/Rx switching is relay based.
Lastly, the digital frequency display is Ardunio Nano and a surplus TV prescaler chip that divides the IF frequency by 64 so that it can be counted by the nano. This is my revision of a circuit described by IMSAI guy on his YouTube channel that has also been build by many others. I rewrote the firmware to provide a rock solid frequency display with 100 Hz precision. The digital display makes fine tuning the VFO much easier – and its very good to be able to tell at a glance where I am tuned to. The red 3D printed bezel for the OLED display was designed by my friend Leon, NT8B
It puts out about 10 watts. I plan to build an external 100 watt amp to accompany it, but I have been making QRP contacts just about every day into an off-center fed dipole up at about 35’.
As Pete always says: “If you know stuff, you can do stuff”
And as Bill says: “There is a lot of soul in this machine.”
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