MKARS80
The MKARS80 is a relatively simple but ingenious mono-band 80m QRP transceiver kit. It was developed by Steve Drury, GL6ALU, at the Milton Keynes Amateur Radio Society, hence the name MKARS80.
Detailed information on the MKARS80 can be found on Steve’s site: http://www.radio-kits.co.uk/.
I bought three MKARS80 from Steve as I intended to experiment with them and gain more experience building transceiver kits.
MKARS80 Mk1
The first MKARS I made was assembled strictly by the book. I used the excellent illustrated step by step assembly instructions that can be downloaded from Steve’s site. I used the supplied metal box to accomodate the PCB and made a slightly different design for the front panel inscriptions. Also, I made an aluminum handle to carry the rig, a handle that did double duty as a stand to tilt the rig to a more comfortable operating position.
From kit .....
.... to transceiver !
The little rig worked fine first time after switch on and I was surprised by its sensitivity. However, I found that there were some things that needed improvement. First, there was the tuning. The main tuning is performed by a polyvaricon. Since the local oscillator frequency is higher than the signal frequency, the polyvaricon has to be turned the wrong way; turning it clockwise decreases the frequency on the LCD and vice versa. Most of us are accustomed to the opposite. I assume this has to be a typically British custom, comparable to driving on the wrong side of the road and I thought about changing it. Not the driving, just the tuning...
Besides being the wrong way in my view, I find the tuning procedure time-consuming since one has to use two knobs; the polivaricon for coarse tuning and pot for fine tuning. I decided to improve that at the same time. Third, LCD readability is insufficient in sub-optimal lighting conditions, so I felt I should change the LCD for a back-lit version. Also, I thought about another and more traditional housing with the controls at the front and connectors at the back.
VARACTOR TUNING
For the tuning, I started to experiment with a varactor I had handy, the Philips BB112. Soon I found out that it could easily tune the entire band and thus replace the polyvaricon. So I removed the polyvaricon and replaced it by a simple circuit with the BB212, a few C’s and R’s and a 10-turn pot. With it I could now tune the entire band the right way and with even twenty times more precision than with the polyvaricon, since the polyvaricon tunes the band in half a turn whereas the pot does it in 10 turns. The simple schematic can be found here: Varactor_tuning_MKARS80.GIF.
Although there is nothing new about varactor tuning, I was the first to apply this type of tuning to the MKARS80.
BACKLIT LCD
The next step was the LCD. I did not find a back-lit LCD that was pin compatible with the original one but I did not bother. I was now at a point where the rig would not fit the original housing any longer because of the 10-turn pot and since I was not too fond of the original housing anyway I decided to find an entirely new one. Fortunately, googling my way through the various offers of metal box suppliers on the internet, I found a suitable aluminum box at a wholesale company at only 20 kilometers away from my home. It so appeared that this box was going out of production and they had only five left. They sold the lot to me for 2,50 Euro each. Was I happy. Now I was free in the layout of the front panel.
My first MKARS80 with carrying handle ....
.... also serving as a stand .....
.... to improve the operating position.
I designed a front panel for the MKARS80 on my computer using Corel Draw and printed it on matte photo paper. I cut out the holes for the controls, the LCD and the LED. Once that was finished I sprayed the now ready bezel with quick drying clear matte PU varnish to make the front more scratch proof. The bezel was now ready be glued to the metal front of the box once the rig was ready. So far so good. Now, I had to make multi-wire cables to connect the LCD on the front to the PCB on the bottom of the box and also cables for the individual controls. I decide to use connectors on the PCB side, so that at least one side of a cable could be removed easily for experiments or repair.
... and now with the LCD and the controls in place , waiting for the new bezel.
The extension cables with connectors, I later provided the two LCD cables with an earthed shielding in order to prevent “birdies” in the receiver.
The simple varactor tuning circuit takes the place of the polyvaricon.
A second connector for the microphone was installed underneath the MKARS80
A few days later, the MKARS 80 Mk 2 was ready to go on the air. I used a large tuning knob that I provided with a glue-on finger dimple to make tuning across a band segment even easier.
BAND SWITCHING
Although tuning was now more precise and the tuning knob turning the right way, I still had to turn two knobs to set the desired frequency. To overcome this I decided to make the main tuning even more precise and do away with separate fine tuning. So I split the 3,5 to 3,8 MHz band into three 0,1 MHz portions, selectable by a three-position switch. The upper and lower limits of the individual bands should be controlled by 12-turn trimmer pots, two for each band. I made a small board with these six trimmer pots and was able to adjust the lower and upper band limits with no problem at all. The fine tuning pot on the front panel could now be replaced by the three position switch. The main tuning pot, when tuning a 100 kHz band segment, makes 10 turns, so 10 KHz per turn. This is more accurate than what the fine tuning knob did before. I was afraid that one would notice frequency jumps when tuning with the wire wound 10-turn pot, making tuning insecure, but this was not the case. You will find the schematic for the three-segment-band here: 3-band_switch_MKARS80.GIF.
The revised version of the MKARS80 Mk2, called Mk3, added are the three band segment mod, a new front bezel design and a push button (A.T.) for tuning the remote automatic antenna tuner. Pushing the button generates a tone and thus RF output that the AT needs for tuning.
Click on the picture to watch a short Quicktime movie showing how easy, quick and precise varactor tuning with a 10-turn pot actually is, compared to the original polyvaricon and fine tuning. Band partitioning makes it even more comfortable.
MKARS80 kits found their way to radio amateurs worldwide, probably also to Russia. I found my MKARS80 versions discussed on a Russian internet forum. Unfortunately I do not read Russian, help wanted!
DISPLAY FORMAT
Another mod I wanted to perform was the frequency readout. In its original form the MKARS80 displays six characters for frequency, thus suggesting a frequency accuracy down to the last Hertz. This is somewhat pretentious for little rig like this with a free running VFO (although it is stabilized by a huff & puff circuit). After all, transceivers using PLL and synthesizer techniques, costing ten to forty times more, usually only display frequency to the last ten Hertz and in many older transceivers only to the last hundred Hertz. For me, a display with a hundred Hertz accuracy would be adequate and if this could be changed I would at the same time put some decimal points at the proper places. With help from Steve, G6ALU, and a few other MKARS80 users I was able to change the code in the MKARS80’s PIC. The result is shown in the pictures above. In addition I made a few other changes, like arrows in the display to indicate which way to turn the tuning knob to stay on frequency should the VFO drift.
AGC
I thought the MKARS80 had now come to a point where there was no more room for improvement. Still however, I hoped that there would be a simple way to add an AGC, maybe even an S-meter, without having to alter the original PCB. The MKARS80 does not have any AGC circuit at all, so the operator has to adjust the RF gain manually. Although this is not a problem for most QSOs, the lack of an AGC becomes annoying when one monitors a QSO between a strong and a weak station. The strong station can easily blow your speaker whereas you may miss on the weak station if you forget to turn up the RF gain in time. So I studied the schematic again in order to see if I could make a simple AGC with a minimum number of components. The result is shown here: AGC_PIN_MKARS80.GIF. It uses a cheap standard diode as a PIN-diode. Bias voltage for the diode is derived from a simple rectifying circuit connected to the speaker output. A strong signal makes the PIN-diode conduct and this brings its impedance down. Thus the signal to the first RF stage is reduced. This mod, I found, increased listening comfort.
The only wish I still have regarding the MKARS80 is some form of an S-meter, calibrated if possible. Ideally, it should be displayed on the LCD as a bar or as a digit and take the place of the battery voltage indicator which is of no use to me as I operate my MKARS on mains. I found out that I would have to drastically alter the PIC code and this, I know, would beyond my capabilities. Any volunteers?
Since I was very pleased with the MKARS80, I investigated if it could be modified to a 40m band version, the MKARS40 so to speak. It took me some time and I was only partially successful. Please check the MKARS40 page.
First design of the MKARS80 Mk2 ....
