Ken Harker WM5R
Using the Elecraft K2/100 with both the Elecraft MH2 and the Heil Pro-Set

When I purchased the Elecraft K2/100, I got an Elecraft MH2 hand microphone to go with the transceiver. Even though I plan to use the transceiver primarily with headset microphones, having a hand mic around is useful, especially for such a portable radio.

 KenwoodElecraft MH2
AF 1 1
PTT 2 2
DN 3 3
UP 4 4
5V NC6
GND 7 7
GND 8 8
The headset microphones I plan to use with the radio are a Heil Pro-Set and a Heil Pro-Set Plus. Both models of the Pro-Set terminate in a 1/4" stereo jack for the headphones and an 1/8" mono jack for the microphone, and require an adaptor for use with the 8-pin microphone connectors on most radios. The three common standards are Kenwood (red,) Yaesu (yellow,) and Icom (blue.) The easiest way to wire the Elecraft K2/100 microphone configuration block (P1 on the Front Panel Board) for use with a Heil Pro-Set is to set it up for a Kenwood microphone, and use a Kenwood (red) adaptor for the Heil Pro-Set. Wiring the microphone configuration block for Kenwood microphones is easy - you connect six of the pins straight across, which you can do with standard PC board jumper blocks (such as those supplied with the Elecraft MH2 hand mic,) and avoid soldering altogether.

The Elecraft MH2 hand mic comes wired such that all you need to do on the microphone configuration block is put eight jumper blocks straight across. You actually don't use DN, UP, or FUNC, but there's no harm in connecting those anyway. The only difference between this microphone configuration block setting and that for a Kenwood mic, is the use of pin 5 for FUNC and pin 6 for 5VDC, neither of which are used by Kenwood microphones. Using the Kenwood (red) adaptor for the Heil Pro-Set, only pins 1, 2, 7, and 8 are actually used, so connecting FUNC to pin 5 and 5V to pin 6 is entirely compatible with the Heil Pro-Set/Kenwood adaptor combination.

Unlike the Heil Pro-Set or Heil Pro-Set Plus, the Elecraft MH2 is an electret microphone, and requires a small DC voltage applied to the microphone audio line to work. The Elecraft MH2 comes with a 1/4W 5.6K resistor that the documentation instructs you to solder between pins 1 and 6 on the back side of the microphone jack on the Front Panel Board of the Elecraft K2/100. If you do this, however, every microphone you plug into the radio will see a small DC voltage on the mic audio line. This is a problem for microphones that are not electrets (such as the Heil Pro-Set) because the voltage will bias those mic elements and make them almost completely insensitive. The level of audio that will reach the Elecraft K2/100 will be far too low.

Photo of the inside of the microphone connector
1/8 watt 5.6 Kohm resistor soldered
inside microphone connector.

The solution is to solder a 5.6K resistor between pins 1 and 6 inside the connector shell of the Elecraft MH2 hand mic, instead of inside the Elecraft K2/100 itself. This way, the Elecraft MH2 will still get a small DC voltage on the microphone audio line for its electret element, but a Heil Pro-Set plugged into the radio will not. In order to fit the 5.6K resistor inside the connector shell, though, you will probably need to use an 1/8W 5.6K resistor instead of the supplied 1/4W 5.6K resistor. You won't be able to find this resistor at your local Radio Shack or Fry's, but Mouser and Digikey both have them. This will require good soldering skills, as it is a delicate, tight fit. Another option might be to open up the microphone shell and find the appropriate wires inside to solder a 5.6K resistor across. I have not looked inside, myself.

It may still be necessary to adjust the mic gain when switching back and forth between the Elecraft MH2 and a Heil Proset or Heil Proset Plus, but at least you will not have to open up the rig to adjust the microphone wiring header block.

Last Updated 14 April 2016