20m SSB Transceiver
One of the great things about the BitX20 is that there is a discussion group for BitX20 builders located at http://groups.yahoo.com/group/BITX20. This group not only provides help for those building BitX20's, but also provides discussion about modifications.
The BitX20a kit only comes with the board, components, a tuning cap, and two potentiometers. You have to supply the enclosure, microphone, connectors, and anything else the radio will need. This suited me fine since I have a well-stocked junk box. I opted to use a CB enclosure to house the radio since I had plenty of them sitting around waiting to be used, and they're about the right size I need. I also had a few CB microphones which could be used for this transceiver.
Once the BitX20a board was completed and the receiver tested, I tried several microphones to use with the radio. The one I settled on was from an old Sharp 23 channel CB. I had to cut it from the PC board as it was hardwired in. Since the CB was in working condition I felt a slight pang of guilt doing this, but the chances of me needing to use an old 23 channel CB are nil. Especially since I do have some working 40 channel CB's around which I don't use. But since the mike works great with the BitX20a, I didn't feel too bad about it.
On the BitX20 group there is also a schematic for a microphone amplifier which I thought about putting into the radio. However, once it was tuned up properly, there was no need for it since I was getting 10 Watts out on voice peaks. I also felt that if I really needed an amplfied mike, there are plenty of amplified CB mikes available. As it turned out, I wouldn't have room for a mike amp board.
My idea was to fit a piece of blank PC board inside the chassis to mount all the other boards to. I found the size by putting the original CB board on top of the blank PC board and marking it, being sure to mark the screw holes to mount it to the chassis. I had to make the new PC board larger than the CB's board to accommodate the BitX, meter, and freq display boards. I also wanted to mount the main tuning cap to it, so it had to reach the front panel.
The BitX20a board was a tight fit how I originally wanted to install it, and I had to turn it 90 degrees. I constructed the S/power meter circuitry on a small piece of scrap PC board by cutting grooves into the copper in sort of a grid pattern to make soldering pads for the components. I then soldered the components directly on top of the PC board, sort of Manhattan style. I wired the meter board to the BitX board and was pleased to see that it worked OK. On transmit, though, the meter seemed to want to dip down instead of go up, and I realized that I had wired in a diode backwards. Turning the diode around solved the problem. I installed the meter board in front of the BitX board behind the front panel. The meter makes a nice addition to this project.
I hadn't originally planned to add a frequency display to this radio, but after seeing some other BitX20 projects on the BitX20 group I decided to put one in. I had built some CW transceivers without frequency displays, but was only tuning about 50 KHZ with them. The BitX20a would tune some 150 KHZ or so, and it would be nice to know what frequency I was on in case I wanted to meet someone for a sked, or to at least be on an even frequency.
I decided on the Digital Dial kit also available from Hendricks QRP kits. It is a nice small LED display which would fit nicely into the CB case. The kit consists mainly of SMT parts, which after about 30 nerve-wracking minutes I managed to solder to the board. I turned it on to test it and was pleased to see that I had a working display. However, it wouldn't read any frequency from the BitX20a. I traced the signal input using my benchtop frequency counter, and found that I had a bad solder connection on Q1. After resoldering it, it worked fine. I had a working frequency display, but it took me a while to figure out how to program it to read the VFO + IF frequency to get the exact frequency readout.
Since space was tight behind the front panel, I decided to mount the DIgital Dial board to the piece of PC board I mounted the other boards to and connected it to the numeric LED display using ribbon cable. Once the Digital Dial board & LED display were mounted, I cut a piece of red clear plastic from the display from an old cable box. This I attached to the chassis in front of the LED display which fit just behind the front panel. According to the manual for the Digital Dial kit, a red lens over the LED display makes it more readable.
Since the boards would be mounted upside down in the case, attached to the piece of scrap PC board mounted to the chassis, I was concerned with heat dissipation from the heat sinks attached to the IRF510 finals. I noticed that the heat sinks got very hot when tuning up the radio, so I added a couple of extra TO-220 heat sinks to the original heat sinks. I had also previously drilled a grid pattern of holes in the blank PC board above where the IRF510's would reside. This would help keep the heat from building up and heating the BitX20a PC board.
I have since modified the radio by adding a very small cooling fan, such as one that would cool a CPU, to the back of the radio. I cut a roughly 1 1/4" hole and covered it with some old window screen material. I added the small fan and connected it to +12V to turn on when the main power switch was turned on. Since the fan is only 5V and supply to the radio is 12V, I had to use a 30 Ohm resister in series with the fan. Since smaller 1/4W resistors seemed to heat up quite a bit, I found an old 5W resistor which works well. Unfortunately, the small fan caused a loud whine in the raido's audio, so I put a 1000uF cap across the + voltage and ground as close to the fan as I could. This solved the problem. I then decided to add a small toggle switch in the rear of the radio to be able to turn the fan on & off manually in order to save battery power. If I'm in the middle of a QSO, I can turn the fan on to keep the finals cool, but if I'm just listening or tuning around, I can turn it off.
Once all the boards were installed & everything was working properly, I finished the front panel by drilling & cutting holes for the pots, mic connector, freq display & meter. I then painted the inserts white, adding rub-on lettering, and added a label I printed from my computer, then oversprayed them with clear spray paint to preserve them. Once they dried, they were epoxied onto the CB's front panel.
The LM386 on the BitX20a board provides enough audio to power the original speaker which came in the Royce CB. I decided to incorporate it into the design, so I wired up the stereo headphone jack with the NC switch going to the external speaker jack with a NC switch going to the speaker. This has the result of routing audio to the internal speaker if nothing is plugged into the external speaker jack, and both the external & internal speakers are muted if headphones are plugged in.
I love incorporating LED's into my homebrew projects, so I wanted to put red & green LED's on the front panel to show when the radio is in transmit or receive mode. Luckily, the BitX20a board has a connection which provides a ground and +12V for transmit or receive to drive an external function. This worked out great for this purpose so that I didn't even have to try to find a spot on the bottom of the PC board to connect to. I simply wired the anode leads to the TX/RX connections and the cathodes were wired together to a 1K resistor which was connected to the ground connection. Since only one LED would be on at a time, only one resistor was needed. When the radio is in receive, the green LED is on, and when the PTT on the mike is pressed, the green LED goes out and the red one comes on. I mounted the LEDs below the meter, just like the original CB had. This was done primarily to save space on the front panel so that I could put the BitX20a label next to the frequency display.
Also to save space I placed the pushbutton switch for the frequency display on the rear of the case. I figured that I wouldn't use it all that often, but it is there if I need it. This has the result of a clean front panel layout that's not cluttered.
Once the outer case was painted, I installed a nice tilt stand that I picked up at a hamfest for $2. It really came in handy in this instance and tilts the radio an a nice viewing angle. I also added a microphone bracket from a CB radio which gives me a spot to attach the mike when I'm not using it.
I'm really happy with how this project turned out. The BitX20 is an excellent design and this radio turned out very nice. It makes a nice radio to take camping or otherwise operate portable.