The K1 Transceiver

This is my Elecraft K1 Transciever. I’ve done quite a few modifications to it, and have upgraded it with many of the available add-ons.

The modules / modifications I have for it are: 4-band filter board configured for 80M-40M-20M-15M, 2-band filter board configured for 10M and (empty), backlit display, K6XX CW tuning indicator, noise blanker, internal Antenna Tuner, and a digital interface.

Some notable changes to the face of the unit include a different, aluminum knob set, and (only visible during transmit) a red ‘on air’ LED. The K6XX indicator is a blue LED that glows through the RIT/XIT LED.

The K1 is a great ‘little cousin’ to the K2. The K1 is CW-only (it does receive LSB but the filter bandwidth is only 850 Hz at most). It is a very small, very easy-to-build radio, with an amazing receiver and great-sounding CW. It’s a very fun kit to build, and a very easy radio to understand and work with. It’s also quite suitable to modify in various ways.

Inside the radio, everything but the speaker plugs directly with sockets and headers, so there are no wires. Some of the modifications I’ve done have (unfortunately) added wires to the rig, but I’ve done my best to keep the wires to a minimum.

I’ve added a 6-pin mini-DIN port to my K1 which allows for computer digital interface. It uses a pinout almost identical to the TNC port on many Yaesu mobile radios. There is audio-out, Key (ptt), ground, plus a +6v out pin. Of course, there is no audio-in line in this case since the K1 can’t send modulated audio. The audio-out and key in lines are enough though to allow a computer to do your CW for you.

I’ve also added a small switch to turn the backlight on and off. With the backlight off, my K1 only draws about 60 mA on receive. The backlight only adds about 15 mA but still, if running on batteries, every little bit counts.

The internal wiring for the Digital Interface, connects to +6volts, ground, the audio out (line level), and the DOT and DASH lines.

This image, and all subsequent images, can be enlarged by clicking them.

The K1 noise blanker helps eliminate pulse-type noise such as from power lines and automobile ignitions. I don’t have any of that around here at the moment so I can’t test the noise blanker, but it was a simple little module to add, and it’s nice to have it just in case.

Below is my four-band filter board. I’ve configured it for 80 meters, 40 meters, 20 meters and 15 meters. The 20/15 meter half works just as Elecraft designed. The 80/40 meter half has undergone substantial tinkering. The bandwidth is about 150 kHz, and I get a full 5 watts out on 80 meters throughout that bandwidth.

The only problem is the low-pass filter. It’s good on 40 meters, which means 80 meters has no low-pass filter (or rather, it is possible for harmonics to be emitted on 80 meters). So, when transmitting on 80, an external low-pass filter is required. As-is though, 80 meters works great on receive.

The components I modified on the KFL1-4 board to change Band-1 to 80 meters and Band-2 to 40 meters are as follows:

Premix Band-Pass Area:

X4 = 11.5 MHz; X3 = 15.0 MHz

C5a & C6a added in parallel to the variable caps, both are 68pf.

C11 & C12 were changed to 120pf each.

C10 is 10pf.

RF Band-Pass Area:

CL is changed to 680pf, CM is changed to 710pf (680 + 33)

CEa & CFa were added in parallel to the variable caps, both are 150pf.

CK is 10pf.

No changes were made to the Low-Pass area.

Below is my two-band filter board. It’s configured right now just for 10-meter operation. It also has most of the parts populated for 80 meters, but no 80-meter crystal is installed. So for now it’s a one-band board.

I get between 3.5 watts and 4 watts out on 10-meters, and the bandwidth is over 200 kHz. By the way, all the work and experimenting to get 10-meters to work on the K1 proved somewhat academic, as I can’t use the band anyhow – I run a 10-meter beacon here from home.

The component values I used for 10-meter operation are as follows. Please note that the part numbers given all assume that 10-meters is to be Band-2 of the 2 band board.

C6, C10: 82pf

C7, C9: 47pf

C8: 1pf

C16, C20: 220pf

C17, C19: 33pf

C18: 2pf

C24, C26: 68pf

C25: 150pf

L9, L10: 8 turns #26 wire

X2: 36.0 MHz (fundamental)

The following changes were made to the RF board: T3’s windings were squeezed as tight as possible; R11 was changed to 33-ohms; R31 and R32 were removed.

This is the K1 internal antenna tuner. It does a fine job on every band I’ve tried, 80 through 10 meters, and can tune my 90-foot random-wire to 1.5:1 or better.

One of the features I think is really slick about this ATU is that it plugs directly onto the filter board, so there are no dangling wires whatsoever. It’s an ingenious design. I can only guess that this is one of the improvements that evolved into the K1, after Elecraft first made the K2.

This image shows where I have installed the K6XX visual CW tuning indicator. I had to make some slight modifications to the board, but it’s hot-glued directly to the back of the K1 front panel board. The ground plane is also directly connected in a couple places with soldered wire jumpers.

I used fine enamel wire (aka magnet wire) to make the rest of the connections. A very bright blue LED is on the face of the front panel board, and shines into the side of the RIT/XIT indicator.

I do have a pretty good ear for music, but when it comes to CW, I have a hard time judging the tone of received signals, so I do appreciate the visual indication. I’ll be adding one of these to my K2 as well. The only thing it needs is an ‘Off’ switch; I find the flickering light can be distracting sometimes.