K2 Fixed Level Audio Output

I started by making a very simple modification for a low level audio output with level that was independent of the AF pot setting.

The next step was to modify my front panel board by adding an audio output (mic plug) via a 25 x audio buffer and a zero-beat indicator which uses the same signal. The added circuit is built Manhattan-style on the upper right on the back of the front panel board, see image.

In this way three things are achieved:

1. The amplified audio output is independent of the AF pot setting.
2. The zero-beat indicator is also independent of the audio pot setting.
3. The ‘no-wire’ philosophy of K2 is maintained, as there are no wires that need to be run to any of the other boards.

The schematic of the audio buffer and the updated zero-beat indicator is here. Note that the zero-beat indicator’s component values have some minor changes from the original design, that gave a nicer behavior of the display.

I also have added an optional signal from the sidetone circuit on the K2's control board (requires a plugable strap between the boards). This gives a visual check of the zero-beat indicator when you transmit, and it is also nice for demo purposes as both the received and the transmitted CW are output and can be decoded using e.g. a PC.

My Elecraft K2 Modifications

When I got my license in 2001, I discovered the Elecraft K2 transceiver on the internet and noticed the excellent reviews that it had received. Its receiver outperforms most, if not all, of the commercial rigs on the market. It combines state-of-the art performance with the thrill of building something yourself. It is also important for me that the kit designers encourage experimentation and modification.

My basic K2 with no options, serial #2198, was assembled in August 2001. I have later added the options that are needed to make all the front panel labels meaningful: the KSB2 single sideband option, KAT2 automatic antenna tuner, KAF2 audio filter and real-time clock, KNB2 noise blanker, and K160 160m/2nd RX antenna.

Morse lives on

Nice drawing of Bencher keyer by Dutch radio amateur
Dick Kraayveld, PA3ALM
(http://www.morsecode.nl/pa3alm.html)

Although morse has been more or less discontinued in a professional context, radio amateurs are active users of morse code. It gives me a kick every time I am able to receive and send morse code, something which a digital 'black box' never can give. The fact that morse has been replaced by satellites, internet and digital communications is as relevant as the replacement of horse transport by cars, sail boats by motor boats or bicycles by motor cycles. People still ride, sail and bike ... and use morse!

AT Sprint (the original)

Photo: LA8EKA

The AT Sprint is a very nice limited edition surface mount CW transceiver designed by KD1JV Steve Weber. The printed circuit board measures 85 by 54 mm. It was quite a challenge, but a fun one, to build! 

It was designed to be a rig of a size fit for camping. It covers the 40, 30, and 20 meter bands with plug-in modules. The whole transceiver fits in an Altoids tin and outputs about 4 W. The rig in the tin weighs about 85 grams. It uses a DDS - Direct Digital Synthesis - chip for the VFO, it has a superheterodyne receiver with a four-crystal filter, audio AGC, audio filter, and a very efficient class E output stage. It also has a built-in memory keyer and a morse code frequency annunciator. It can take any battery voltage between 5.5 and 13.8 Volts.

Mobile Phone Conversion

Several fellow hams in the Asker and Bærum district have converted Mobira (Nokia) MD50 or MD59 Talkman NMT-450 phones from 1985-1988 to the 70 cm band. LA7TM, Nicolai, has played a central role in the conversion, based on the work of Finnish hams who pioneered the conversion and made their experiences and their software available. We then made some changes to the software. These changes are now included in the official Finnish version.

Remapping of Keyer Speed Pot Range for the Elecraft K2

I am beginning to realize that I will never be able to exceed 20-25 wpm CW speed. The speed pot of the K2 however covers from 9 - 50 wpm with 30 wpm in the middle position. Thus the range which is useful to me is just a small fraction of this range.

In order to map 9 - 25 wpm to a larger range, one can add a series resistance to the pot, but this is cumbersome and requires cutting traces on the front panel bord. One could also change the speed pot to a logarithmic one, but this is also a rather involved operation. My solution was to emulate a logarithmic pot by soldering a 2.2 k resistor between the wiper and the CCW end of R1 on the Front Panel Board, the 5K Keyer Speed pot. In this way the pot stills covers the full range from 9-50 wpm, but 22 wpm is now in the middle position. See image for where the resistor is located. The left-hand wire can be found right under the G in "MIC CONFIG." (Note that the Manhattan-style circuit board to the upper right is a fixed level audio output and a zero-beat indicator).

If one had used an even smaller resistor than 2.2 k one could make the low speeds cover an even larger range of the pot, but I don't recommend this. In contrast to substituting a log pot, the added resistor makes the pot load the circuit more as the pot is turned in the clockwise direction, so that instead of 5 k load, it now presents 5 k in parallel with 2.2 k or about 1.4 k. I would not recommend loading the control circuit even more.

Elecraft K2: Using the KPA100 loudspeaker when the KPA100 is remoted

When the KPA100 power amplifier is external to the K2, the KPA100 loudspeaker is unused. I wanted to use it for other rigs that need a loudspeaker. The minijack connection on the back of the KPA100 can be used if its internal switch is shorted.

This can be done in a simple way by mounting a male header and plug in the unterminated "AF In" connector (J5). The lower pin on the header should be soldered to the warm speaker cable. See middle right in the image (press image to zoom in).

If the KPA100 is mounted in the K2, J5 is plugged into the K2 main board, and when it is used stand alone, J5 may be plugged into the new header. In this way the KPA100 loudspeaker connection is compatible with the original design when mounted in the K2, and its loudspeaker can be used for other rigs when the KPA100 is separate from the K2.

Be sure to measure the resistance at the end of a mini-jack cable plugged into the external speaker connector before trying the loudspeaker. It should measure about 4 ohms, and if it is less than 1 ohm you have made a short to ground, so check the header once more.

25 part transmitter for the QRP Minimal Art Session

My Minimal-Art-Session rig shown here was made for class B of the QRP-MAS competition which is in May every year. In class B the transmitter should have a maximum of 50 components, and the fewer the number of components, the more points one will get per contact. With such simple rigs, only CW is viable.

This rig was made by combining ideas from several designs that I had seen and it was finished the same day as the competition. I ended up with 22 components in the transmitter except for the output filter. The rules say that no matter what, the output filter counts as three components. I guess this is in order that no one is tempted to simplify the filter too much and start emitting harmonics.