KK4DAS MB 20 Transceiver Complete

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.”

73 from Great Falls,

Dean

KK4DAS

KK4DAS MB 20 Transceiver - now Receiving on 20 meters

 

This week's homebrew adventure was to complete the receiver side of what I am now calling the KK4DAS MB 20 transceiver - MB is a call-out to Bill, N2CQR's Mythbuster rigs which this build is both inspried by and patterned after.  

First order of business was choosig the IF freqency which was easy in this case.  The VFO is at 9MHz so if I put the IF at 5MHz, well my 2nd grade math teacher taught me that 9+5=14 - which puts me right in the 20 meter band.   

I ordered a batch of 5MHz crystals from Mouser for the IF but they had not come in yet - and I was impatient to get started so I built the BFO and balanced modulator using a crystal from the junkbox figuring I would swap in a 5MHz crystal when it arrived.  The BFO and two diode balanced modulator is copied directly from Farhan, VU2ESE's original BITX20 schematic. It went together easily.  Despite conventional wisdom I did not attempt to mach the diodes - I just took the first two off the tape.

BFO and balanced modulator

Nulling out the carrier

To null out the carrier I injected a 2MHz signal into the AF port of the modulator and put the scope on the IF port.  Using the FFT mode of the scope I was able to clearly see the carrier and mixing products.  I was able to effectively null out the carrier using the trimmer cap and trimmer pot that are part of the design. The two peaks either side of the middle are the first sum and difference mixing products.  The display is centered on the carrier frequency and you can see that it has been nulled out.

The crystals arrived and the next job was to build the crystal filter.  I used the Dishal crystal ladder filter design software to design a 6 pole Cohn/QER filter.  The advantage of the Cohn filter is that it uses a single value of capacitor between each crystal.  The QER (Quasi Quasi Equi Ripple) modification to the Cohn adds an addtional crystal in parallel at each end of the filter and reduces the amount of ripple.  When building a filter the recommendation is to pick crystals fom your batch that are the closest in frequency to each other and the Dishal software requires you also to know the motional parameters for the crystal. I used what is called the G3UUR method to evaluate the crystals. I had previously built the simple crystal evaluation circuit and quickly sorted the crystals. I then used the Dishal software to determine first the motional parameters and then to calculate the capacitor values and input/output impedance of the filter.  The calculated input and output impedance were spot on0and I calculate the LC values needed for matching to 50 ohms.

   

Sorting the crystals using the G3UUR circuit

The completed 5MHz, 2.5KHz filter

The last board I needed for the receiver was an audio amplifier.  I opened up by box of "boards that could be rigs someday" and found an audio amplifier stage I had built for Pete, N6QW's SimpleSSB a few years ago.  Its a single 2N3904 pre-amp followed by an LM-386.   With all the boards needed for the receiver at hand it was time to connect everything up.  

The completed KK4DAS MB 20 Receiver

I placed the crystal filter at the rear center with the MostlyDIYRF TIA IF amplifier boards on either side. I left space adjacent to the TIAs for the transmit IF amplifiers. The audio amplifier is on the lower right and just above that is the BFO / balanced modulator board.  The receive band pass filter is on the upper left below the first IF amp.  At the far left is the future Tx RF amplifier stage - not yet wired into the circuit.  I mounted the antenna connector on a piece of single sided PCB.  To keep the DC wiring straight I decided to use blue wire for DC on Rx and yellow wire for the DC on Tx.  Red is used for always on DC.   

A big advantage of modular construction is that since all of the modules have been tested individually it increases the liklihood that the build will work well when everything is connected.  To my great pleasure that turned out to be the case and the receiver woked perfectly on first power up.  There was one glitch - the tuner maxed out at 14.260 MHz about 100KHz short of covering the entire 20 meter band.  I thought I was going to have to rebuild the IF with 5.2MHz crystals but Bill pointed out that Yaesu and built in a variable cap to move the VFO passband up or down. I zeroed out the capacitor and that gave me full coverage across 20 meters.

Adjusting the Yaesu VFO for full coverage on 20 meters

Now that the receiver is built I will spend a few nights following Farhan's advice and take pleasure in listening to the receive I have just built.  

All I need to build for the transmitter is a microphone amplifier and the final low  pass filter.  Then add some relays for Tx/Rx switching and then I can work on fit and finish of the cabinet.  

73 from Great Falls,
Dean
KK4DAS