Ham Radio Station AC2C

Building the Elecraft KX1 QRP Transceiver

This page contains some details of the construction of my Elecraft KX1 (S/N 02185) kit.

Please note that each image on this page is linked to a larger, high-resolution, image. Smaller images are displayed here to enable faster loading. Clicking on any image will result in a high-resolution image opening in a separate browser window.

I recently got the bug to try what Hams call QRP mode - seeing what you can do with low power (usually considered to be 5 Watts of power or less). Up until this time, I have occassionally turned the power output control on my IC-756ProIII transceiver down to very low levels to see what I could do and was intrigued enough to invest in a QRP-only radio.

Click to enlarge

After looking at a couple options, I decided on the Elecraft Model KX1 portable, battery-operated, transceiver that comes as a built-it-yourself kit. The image on the right is from the Elecraft web site. I hope that my completed unit will look somewhat like it.

The KX1 comes in kit form and, with all available options, is provided as four separate kits:

  • The KX1 Transceiver covering 20M and 40M
  • A plug-in set of iambic paddles
  • An Internal Antenna Tuner (ATU)
  • An expansion board for 30M and 80M

I purchased all four kits at once and this page documents my experience in building the kits.

Click to enlarge

The Iambic Paddles:

I started with the iambic paddles just to see how well the old eyes could handle the small details and also to get some practice with soldering. The paddles are a very simple kit, as kits go, and can easily be completed in just a few hours.

The only difficulty I had was with the final steps of connecting the small wires from each paddle board to the center and ring terminals sunk inside a very small hole in the aluminum block.

With the paddles completed the first evening, I then decided to start on the internal antenna tuner (ATU). I knew that the final assembly and adjustments would have to wait until the KX1 was done, but wanted to get some more practice with the soldering before tackling the more complex KX1.

Click to enlarge

The Internal Antenna Tuner Unit:

The KX1 ATU Kit comes packaged in a variety of bags containing all the components and wire necessary. The first step (very important) is to do a complete inventory checking the kit contents against the parts list contained in the user manual.

After the parts inventory is complete, the construction starts with mounting and soldering the 7 relays used to select the L/C combinations required to match whatever impedance is present at the antenna terminals. The instructions are EXCELLENT and compare very favorably from what I recall of the very good HeathKit instructions from years gone by.

Click to enlarge Click to enlarge

After all the relays are installed and soldered in place, the remaining discrete components - consisting of resistors, capacitors, diodes, and an RF choke - are installed. Components are mounted on both sides of the board and space gets a bit cramped around the relays.You need a well-regulated soldering iron and very fine solder to keep it from flowing too much. You also need to be careful not to accidently touch other components when soldering new ones.

The pictures on the left and right here show the board after all discrete components have been installed on the board. The next steps are to wind and install three Inductors and one transformer on small toroidal cores.

Click to enlarge Click to enlarge

The antenna tuner unit contains three inductors that must be wound onto toroidal cores. L1, on the left, consists of 12 turns of #26 enamel wire on a T37-2 toroid core. L2 on the right, consists of 17 turns of the same wire on the same core.

This project was the first time in my many years of kit building that I had wound an inductor on a toroidal core. I must admit that I was a bit concerned of the small details, but the job was much easier than I had expected. I found that after you get the first 2-3 windings around the core, you can use a pair of needle-nose pliers to tug on the threaded wire to pull it taut.

As for counting the number of turns, just count each time the wire passes through the center of the core.

Click to enlarge Click to enlarge

Inductor, L3, on the left, is exactly like L1 and L2 except that it consists of 25 turns of wire and takes a bit more time to ensure that you get the exact number of turns.

Transformer T1, on the right, was the last toroidal core to be wound. To make it, you start with two 10" lengths of enamel wire - one wire is red and the other is green so that you can separate the wires at the ends. The instructions tell you to twist the wires together so that they cross each other 3 to 4 times per inch.

To twist the wires together, I held one end of the two wires in a PanaVise and used a pair of hemostats to hold the other end. Then I turned the hemostats 40 times to give 40 turns in the 10 inches of wire. Wrapping the wires around the toroid cores was fairly easy.

Click to enlarge Click to enlarge

With inductors L1, L2, L3 and transformer T1 mounted on the board, the final step in the construction is to adhere the rubber foot to the bottom of the board and to insert the master control unit (U1) into it's socket. This final step is the only part of the ATU kit that requires close attention to avoid electrostatic discharge (ESD). I have a wrist strap that provides 1 megohm between the strap and my station ground. The pictures on the left and right here show the final ATU board ready for installation into the KX1.

The KX1 - Part I:

I started by doing a complete and thorough inventory of the parts in the KX1 kit. Most of the parts are fairly simple to match against the parts list in the manual. The capacitor listing gave me the biggest challenge as some capacitors are supposed to have wider lead spacing that others.

The only definite error I found in the parts list was for J3 - the 1/8" jack used to connect the key/paddle on the front of the transceiver. I was careful not to jump to a conclusion and meticuously completed the parts inventory before concludng the error. At the completion of the inventory, it was clear that the part that had been provided would not fit to the PC board.

I sent an email to the parts request email address provided on the Elecraft website and got an immediate confirmation. I called Elecraft later in the day and confirmed that they had mailed the replacement part that day.

It took about 4 hours to complete Part I of the assembly. The assembly steps consisted of mounting and soldering some resistors, resistor packs, a few diodes, some capacitors, a crystal, an LED, a few transistors, and some voltage regulators. At the end of Part I, you are asked to make some simple resistance measurements before performing the first "smoke test".

Click to enlarge Click to enlarge

The pictures on the left and right show the KX1 PC Board at this point. Luckily, a quick study of the schematic shows that the missing J3 plug has no effect on the tests conducted in the step.

After the initial resistance tests are done, there is an inital "Alignment and Test" phase that involves connecting to a power supply and testing the control and display circuitry of the CPU. It may be one of those "you gotta be there" moments, but it is hard to express the satisfaction in seeing the display come alive and respond to button presses.

So far, everything seems to test out perfectly, and I will proceed with the next steps in the assembly manual - watching closely for anything that may be affected by the missing J3 jack.

The KX1 - Part II:

Working off and on throughout the day and evening, I finally finished Part II - assembly and test of the receive circuits. As in Part I, the instructions were excellent with detailed diagrams where needed.

Throughout the entire assembly, there was only one place where it seemed like I had the wrong size part. L9 is supposed to be a micro sized 100 microHenry inductor, but the one I had was mini sized and did not fit well within the outline provided on the PC board. I bent one end lead under the component to make it fit.

Probably the two trickiest steps involved installing two components to some previously installed components by soldering the component leads directly to the PC board pads. No holes are provided for the components (I assume holes will be provided on the next revision of the PC board. It is a bit difficult to hold the components steady with one hand, hold the board with the other hand, and then try to solder the whole thing together.

Click to enlarge

The very last steps in Part II involve winding two inductors on toroidal cores. At this point in the project, I starting to feel very comfortable with winding toroids and think I'm actually fairly good at it. The photo on the right here is of the two inductors L1 and L2 - L1 (top in the photo) has 13 windings and L2 has 14 windings.

Throughout the assembly steps, you are constantly reminded to make sure that everything is properly aligned on the board, fully seated, and that no component can exceed 5/16" height above the board. There are some areas where components have to be installed first, then bent over against the board to make them no higher than 5/32" above the board.

Once construction is complete, there are some quick resistance checks to test for continuity or possible shorts. Then the real excitement starts - Power up and align the receiver.

Click to enlarge Click to enlarge

In these steps you apply power, plug in a set of stereo headphones, and use the paddles to test the sidetome generator. You may recall that I am still without the J3 paddle/key jack, but I was able to use a little jumper between the PC board pads where it mounts to test the functionality. The photos on the left and right show the KX1 PC Board as it exists at the completion of Part II of the construction.

It is a real thrill to hear the sidetone come to life and adjust the sidetone volume and pitch. Next comes a quick check of the S-Meter and Voltmeter displays along with a test of adjusting the keyer speed between 8 and 50 WPM.

In the final steps of the testing, you connect an antenna to the receiver and align the two band-pass filters. What a thrill to hear signals coming through the headphones !!!

The alignment procedures amount to adjusting trimmer capacitors as you listen for a "peak" in the signal strength.

The KX1 - Part III:

Click to enlarge

It took 3-4 hours to complete the assembly portion of Part III - Transmitter and Low Pass Filter components.

After installing the remaining resistors, capacitors, and diodes, it is time to wind the last toroidal inductor and a toroidal transformer. At this point in the construction, I'm getting to be an old hat at this process.

Inductor L3 consists of 18 turns of enamel wire on a FT37-43 ferrite core. Transformer T1 has a primary winding consisting of 8 turns and a secondary consisting of 2 turns and is also wound on a FT37-43 ferrite core.

After L3 and T1 are wound and installed, the power output transistor is mounted on the PC Board using a small stand-off. The stand-off serves to hold the back of the transitior against the metal front panel which serves as a heat sink.

Click to enlarge Click to enlarge

I had difficulty getting the stand-off connected to the PC Board exactly as instructed in the manual. The provided screw was just too short. In the end, I left off the split washer on the bottom side of the PC Board.

The replacement jack for J3 arrived and assembly and alignment could be performed. The completed PC Board, ready to be mounted to the inside of the front panel, is shown in these photos.

Before mounting the completed PC Board to the front panel, you need to do some visual inspections, make some resistance checks, and put the button caps on the push button switches.

KX1 and ATU Assembly and Test:

Time to put the KX1 Transciever and the internal antenna Tuner unit (ATU) together and start testing and alignment.

Click to enlarge Click to enlarge

Shown here are how the two units look when the ATU is plugged into the KX1.

It is incredible to see how the two units fit together inside the container - it really made me appreciate all those continual reminders to make sure everything fits snug against the PC board, make sure the components are vertical, make sure the height does not exceed 5/16", etc.

I had a minor "operator error" problem in following the instructions for calibration and alignment of the ATU but after getting some advice from Don Wilhelm, W3FPR, the alignment, calibration, and testing went very smoothly.

Click to enlarge Click to enlarge

With the basic KX1 and ATU completed, the last thing I did was to mount the battery holders in the base of the cabinet. For this step, I deviated slightly from Elecraft's recommendations. I went to a local hardware store and got some #2-56 x 1/4" screws and hex nuts and used the hex nuts on the bottom of the cabinet to provide added physical strength.

The photo on the left is the KX1 image from the Elecraft web site - also shown at the top of this page. The photo on the right is my completed KX1.

I am very pleased with the finished radio and want to get a few QSOs logged with it before going ahead with building and installing the 30M and 80M option. Running only 4 Watts or less of output power will require a bit more patience and some new skills.

The following table lists some of my first KX1 QRP QSOs - ALL with 4 Watts or less:

Date Band Call Name QTH Miles RST
28-Mar-09 20M N5KIP Dick Baton Rouge, LA 1,012 559
28-Mar-09 20M KE5YKH Don Ocean Springs, MS 910 569
28-Mar-09 40M N9FC John Indianapolis, IN 503 559
29-Mar-09 40M KU8T Tom Fort Wayne, IN 457 529
29-Mar-09 20M WD5CCW Mike Columbia, MS 914 539
29-Mar-09 20M KK7YJ Jim Missoula, MT 1,931 539
29-Mar-09 20M W6UT Dennis Stockton, CA 2,378 529
30-Mar-09 20M PA2PCH Ad Tynaarlo, Netherlands 3,877 539
30-Mar-09 20M F5IN Michel (Mike) Pithiviers, France 3,829 579
30-Mar-09 20M SM5NZG Heide nr Stockholm, Sweden 4,072 559
30-Mar-09 20M DL1CC Wolf nr Berlin, Germany 4,209 579
03-Apr-09 20M OK1FPS Pavel Praha, Czech Republic 4,271 579
03-Apr-09 20M 7X3WJA Hamid Laghouat, Algeria 4,300 129
03-Apr-09 20M EI0CZ Bren Clonlara, Ireland 3,036 559
03-Apr-09 20M RX6AM Mike Novorossiysk, Russia 5,370 569

Okay... I think that's enough for the basic 2-band transceiver. Time to start working on the 30M and 80M option.

Adding the KXB3080 30M & 80M Option:

Click to enlarge

As with all the kits, the place to start with the 30M and 80M option is with a complete inventory of the parts packed in the package.

Shown on the right are all the parts included in the option package. There are several surface mount devices (SMDs) pre-mounted on the board. The provided parts matched one-to-one with the included parts list.

Based on recommendations from a couple others, I included a dime in the photo to help in judging the size of the parts.

KXB3080 Part I - Modify the KX1 PCB:

The first step in adding the 30M & 80M option is to modify some components on the main KX1 PC Board. The instructions are fairly good, but clearly assume that you are quite experienced with kit building.

Click to enlarge

Capacitors C1 and C26 are replaced on the KX1 board and inductor L7 and capacitor C27 are removed. Then, a PC Board trace is cut and modified to provide for new circuitry. Inductor L6 is then moved from the component side of the KX1 PC Board to the non-component side of the board.

Finally, a new Transformer (T2) is wound using 33 turns of #28 toroid wire and 2 turns of #26 toroid wire. I found that the smaller (#28) toroid wire was a bit more brittle and I had to be a careful not to break the wire. I was glad that I had plenty of experience in winding toroids by the time I got to this one.

When completed, the new T1 Transformer is placed on the KX1 board using the holes left from removing capacitor C27 and inductor L7. The photo on the left shows the details of the portion of the KX1 PC Board where the modifications for Phase 1 are performed.

KXB3080 Part II - Assemble and Install the KXB3080 Option PCB:

Click to enlarge Click to enlarge

The next step in adding the 30M & 80M option is to complete and install the KXB3080 option board. The KXB3080 PC Board mounts directly atop some components on the KX1 PC Board and is hard soldered in place.

There are three bare wires that are "straight-through" connections using existing holes in the KX1 PC Board. There are six additional wires that are prepared and cut to length for connection to various existing holes on the KX1 PC Board.

One of the wires is routed through a hole in the KX1 PC Board and is later connected to the Low Pass Filter that is installed in Part III.

The photos on the left and right here, show some details of the portion of the KX1 PC Board affected by the installation of the KXB3080 Option PC Board.

KXB3080 Part III - Assemble and Install Low-Pass Filter LPF1:

Click to enlarge

The last assembly step is to assemble and install the low pass filter that replace the L1 and L2 inductors on the existing KX1 PC Board.

I would invite comment from others that have completed this project, but feel that this little assembly was the most difficult step in the entire process. The photo on the left shows the two inductors, L1 and L2, that are wound and mounted to the small PC Board. Each inductor has two sets of windings - one winding with 9 turns of #26 toroid wire and one winding with 15 turns of #26 toroid wire.

Click to enlarge

At this point in the construction project, winding the inductors is fairly easy. However, mounting them on the small PC Board and getting the wires to line up well while having the toroids line up with the outline on the PC Board was a bit of a challenge. The instructions were okay, but could have included a few more close-up details for my satisfaction.

Finally, the existing L1 nd L2 are removed from the KX1 PC Board and the new Low-Pass Filter PC Board is installed directly on the KX1 Board. The photo on the right shows the details of the Low-Pass Filter Board as mounted on the KX1 PC Board. You must use extreme care in placing the Low-Pass Filter Board to ensure that it will mate well with the Antenna Tunner option board that fits directly above it.

KXB3080 Part IV - Reassemble, Test and Align the KX1 with KX3080 Module:

The first time I reassembled, and started testing the completed unit I immediately encountered two problems.

The first thing I noticed was that the trimmer capacitor labeled "30B" had no effect on the tuning of the receiver sensitivity. The second symptom was that the transmitter output power on all bands was lower than I expected. I did not know whether the two problems were related or might be completely independent.

I requested some advice from Don Wilhelm, W3FPR, to see if he had any suggestions for me and we exchanged a few emails.

The first symptom, that trimmer capacitor labeled "30B" had no effect on the tuning of the receiver sensitivity, was a bit tricky to find but was easy to fix. By studying the schematics closely, I determined that trimmer capacitor "30B" was actually C4 on the KXB3080 PC Board and that the wires labeled "A" and "B" in the assembly instructions placed C4 in parallel with KX1 trimmer "20A" when 30M was selected.

Click to enlarge

Armed with this information, I decided to see if trimmer "20A" had any effect on 30M receiver sensitivity. A quick test confirmed that trimmer "20A" did, indeed, affect 30M receiver sensitivity. Thus, either trimmer "30B" must be defective or there must be a wiring error.

A very close inspection of the completed circuit board showed that I had a wiring error. The photo on the left is the same one shown above under KXB3080 Part II. The red arrow shows wire "B" as initially installed and the green arrow shows where wire "B" should actually be terminated.

After removing wire "B" from the incorrect terminal and terminating it correctly, the 30M tuning problem was solved and the receiver was working very well on all 4 bands - 80M, 40M, 30M, and 20M.

Working to solve a "Low Power Output" Problem:

With the receiver working quite well, I started concentrating on the low power output symptoms. I was getting only 1-2 Watts output on 40M and 20M and a bit over 3 Watts output on 30M and 80M. Some advice from Don Wilhelm, W3FPR, fairly quickly had me concentrating on the Low Pass Filter (LPF) PC assembly that had been the very final steps of the KXB3080 option modification.

After a few visual checks, I decided to remove the LPF Board from the KX1 PCB and then removed the two dual-wound toroidal inductors from the LPF PC Board. I carefully stripped and tinned the leads from each winding and then carefully placed the toroidal inductors L1 and L2 back on the LPF assembly. After reassembling the LPF assembly on the KX1 PC Board, I noticed some increase in power, but was still not completely satisfied with the results.

As of the current time, I am getting the following power output levels with a 13.8 VDC Power Supply:
          80M : 4.09 Watts
          40M : 3.50 Watts
          30M : 4.29 Watts
          20M : 3.50 Watts
Some say that I should be satisfied with these results, but I am trying for better.

I have decided to completely "do over" the LPF assembly and am currently awaiting the necessary parts from Elecraft. The advice I have from Don is to wind the red turns on L1 and L2 as closely spaced as I can manage and then space the green turns over the remainder of the core.

Building a Replacement LPF1:

Click to enlarge

Shown on the left are all the component parts that are required to build a complete LPF Assembly for the KXB3080 30M & 80M option. Not shown in the photo are a short length of insulated hook-up wire and the enamel-coated wires used to wind L1 and L2.

Click to enlarge

Elecraft's reputation for excellent customer service is quite well-deserved. They responded quickly with a price quote for the required parts for a complete "do over" of the KXB3080 LPF and shipped the day after I made payment. The parts arrived in 3 days.

Shown on the right are the replacement L1 and L2 dual-winding toroidal inductors wound with the red turns spaced as closely as possible and with the green turns spread out over the remainder of the core. Winding L1 and L2 in this way causes the leads to come off the core at slightly different locations than originally intended.

Click to enlarge

It is easy to line up leads 2 & 3 to fit into the holes provided on the LPF PC Board however, leads 1 & 4 exit the core at locations a bit removed from the holes where they terminate. It was a bit of a chore to make everything fit into place properly.

Shown on the left is the completed replacement Low Pass Filter assembly attached to the KX1 Board and with the Antenna Tuner Unit (ATU) in place. The small gap in the connecting jack & plug between the ATU and the KX1 is normal and is the same as previous.

In hindsight, I probably would have been just as well off to leave everything as it was without rebuilding the LPF assembly. When everything was done, the measured output power was very comparable to what I had seen previously. But, being a tinkerer, I had to try to see what I could do.

When I was all done, I performed the power output modification that Elecraft provides as a part of the KXB3080 option kit. With the power output modification completed, my power readings, with the ATU installed, and connected to a 13.8 VDC power supply, are as follows:
          80M : 4.46 Watts
          40M : 3.69 Watts
          30M : 5.49 Watts
          20M : 3.69 Watts
Overall, I'm satisfied with these results.

Overall Impression of the Elecraft KX1:

This project was completed in mid-April, 2009, and I will try to return to this page later to relate some of my experiences using the 4-band KX1 for QRP operating.

I think the overall kit is of excellent quality and the availability of Elecraft support, the Elecraft email reflector, and people like Don Wilhelm, W3FPR, make the do-it-yourself experience quite enjoyable. I am very grateful to Don Wilhelm for his quick responses to emails and for his helpful suggestions.

I still maintain that the KXB3080 30M & 80M option is the most difficult part of the whole project. Unless you are an experienced builder and have good quality tools, I would strongly recommend out-sourcing that modification to someone with more experience.

Please feel free to contact me with any comments, suggestions, or other requests related to this project. My email address is available through QRZ dot com or via my call at ARRL dot net.

Operating the 4-band KX1 QRP Transceiver:

My first QSO with the completed 4-band KX1 QRP transceiver was on the 80M band with K4OSO, Milt, in Rockville, Virginia, on 20 April 2009. Milt was very helpful in assisting with the final test of the radio including a test with battery power.

During the test with Milt, I first started out with the main HF radio, the Icom 756ProIII, and then switched to the Elecraft KX1 for the remainder of the QSO. That test allowed Milt to listen carefully for any minor change in frequency and confirmed for me that the KX1 frequency calibration was quite good.

Although Milt is only 120 miles South of me, the QSO was completed in mid-day and Milt gave me a solid 559-569 signal report.

I will log some of my initial QRP QSOs in the table here:

Date Band Call Name QTH Miles RST
20-Apr-09 80M K4OSO Milt Rockville, VA 120 569
26-Apr-09 30M KJ4AOM Ray Richmond, KY 415 569
26-Apr-09 20M KE4YHY Don Arab, AL 632 599
27-Apr-09 40M W1JSB Hanz Sanbornton, NH 401 579
27-Apr-09 20M K0KEX Rick Edgerton, MO 948 569
3-May-09 80M K1LGQ Dennis Brookline, NH 364 599
11-Jul-09 40M K4ZNC John Tallahassee, FL 734 559
23-Jul-09 30M KU8T Tom Ft. Wayne, IN 457 559
25-Jul-09 20M XE1IM Duarte Uruapan, Mexico 2,035 539
25-Jul-09 20M WO8J Joe Redmond, WA 2,310 339
25-Jul-09 20M ZL2BLQ Stan Greytown, New Zealand 8,786 319

Return to Photo Album Page

Valid XHTML 1.0 Transitional