My first thought was to wire up the power and antenna connections to a DP3T switch and just switch between the boards. I would have wired up the AF Gain and tune pots in parallel to each radio. However, I did not know if there would be any interaction between the two "dead" boards and the selected "live" board. After studying the schematics and getting advice from others, it looked like my best bet would be to get a rotary multiconnection switch.
I did get such a switch from Dave Bixler at Ozarkcon. Dave gave me a couple of old I/O computer switchboxes which had DB-25 jacks on the back. One was just an A/B switch, but the other had four positions. I figured it would be my best bet.
I opened up the switchbox and removed the switch. It indeed had multiple connections, 29 to be exact. It is basically a 29 pole, 4 throw switch. I could use three of the positions to switch between the three Small Wonder boards, and the other to switch the radio off.
I disassembled my three QRP rigs and set the boards aside. All the hardware from these three radios went into my junk box, except for the parts I'd need. I dug out a CB enclosure from an old Royce 1-660 which was nice & large. It would be big enough to house all three boards, plus a small Norcal keyer that I pulled from my old 10 meter beacon. The CB enclosure also had a window for the channel display. I decided to use a two digit LED display to light up and show which band is selected. I had a nice, bright green LED display that I salvaged from an old cable box.
I started working on the enclosure to get it ready to house the Small Wonder boards and the keyer. I figured out what connections I'd need for the LED display, then wired it up. I finished out the front panel and selected a couple of nice knobs, one for the selector switch and one for the AF gain pot. I first chose a nice, large knob from an old junk SW radio as the tuning knob, but later replaced that silver knob with a large black knob which looks better with the other black knobs. I used a flat square button for the keyer's program button which I mounted on a small piece of PC board and mounted under the band display.. Next to that I drilled a hole to pass audio from the keyer's piezo speaker. I cut a couple of pieces of PC board which I mounted on the CB's original front panel to make a new front panel. The one on the left contains the band switch, AF pot, TX LED, and headphone jack. The one on the right contains the tuning pot.
I cut a large piece of PC board to mount the Small Wonder and keyer boards onto, which I inserted into the CB's chassis frame. I used computer standoffs to mount the various boards to the large piece of PC board.
I started by making the antenna connections. I used two poles for each connection to handle the current across the switch, and made the connections so that the antenna goes to ground when the radio is switched off. This was done by using the otherwise unused "OFF" position connections on the switch. I also used two poles for each of the power connections, as well as separate poles for the LED display and keyer power. The AF and tune pots needed three poles each, and the keyer and audio out needed one each. I also wired up the TX LED to the switch.
The keyer has a paddle jack (3.5" stereo) on the rear of the radio, as well as a larger 1/4" jack for a straight key. I wired the output of the keyer to the straight key jack and then to the switch, so I can use either paddles, straight key, or if I need to, an external keyer. I also installed a mono 3.5" jack in the rear panel to pass audio for an external speaker.
Once everything was wired up, I tested it out and found that the keyer's program switch was shorted. On closer inspection, I found that I had wired up the two wires correctly, but that I had labeled them (using a marker) incorrectly. I simply switched the two wires around and soldered them in place. I hooked it up again and found that just about everything worked OK, except that I had very low audio on 20 meters. I couldn't figure it out until I turned the AF gain pot down and the volume went up. I switched the two connections from the 20 meter board and it worked OK.
Overall, I'm happy with the way this project turned out. Having three bands in one radio which can be changed with a turn of a switch is much easier than having to unplug antenna, key, headphones, and power from one radio to another. I'm also glad I decided to install a keyer directly into the radio so that I don't have to worry about taking along a keyer, or making sure that keyer has a good battery. The MultiWonder connected to my tri-band portable dipole makes an excellent setup for camping.