Elecraft K3, rear mic jack not working

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Elecraft K3 – Rear mic jack not working





I recently decided to upgrade the headset in my “modern” station. Although my Heil Pro headset has been working fine, it becomes uncomfortable during longer operating sessions.

At Mike (SJ2W), Bose QC25 headphones combined with Antlion ModMics have been used for many years. After working a few contests there, I can confidently say they are much more comfortable than the Heil headset, and way cheaper, since these headphones and mics are widely used by gamers.

I picked up a used Antlion ModMic and a pair of Bose QC25s, and with that I wanted to clean up the wiring a bit—specifically getting rid of the bulky Heil “K” adapter with its large 1/4″ ptt jack on the front panel.  Since I only use a footswitch for ptt I felt it was annoying to have it cluttering the desk. My plan was to use the rear connectors for audio on the Elecraft K3, since there are plenty of them. Line in, Line out, phones, mic, and speaker output. 

Unfortunately, I could not get the rear mic input to work, no matter what I tried.


ModMic Requirements and K3 Setup

The ModMic V5 requires bias voltage. The K3 can supply mic bias on the tip of the 3.5 mm rear-panel mic connector.

The rear mic jack on the K3 is a TRS connector (tip, ring, sleeve), although the ring is unused. To enable bias:


  • Enter MAIN MENU
  • Select MIC SEL
  • Select RP (rear panel)
  • Press 2 , bIAS is shown, which means that bias voltage is applied.
  • Pressing 1 swithes between .H and . L (high and low mic gain)

Despite this, I could not get any modulation at all.

Tests performed:

  • ModMic on rear input with bias enabled → no audio
  • Heil HC-6 on rear input with bias disabled → no audio
  • ModMic on front mic input using the Heil adapter with FP.L BIAS selected → works perfectly

This confirmed that the microphone itself and the radio’s audio chain were fine.


Investigation

Since no audio passed through the rear mic input regardless of bias setting, I examined the KIO3 schematic, focusing on the rear mic input circuitry.

One component stood out as a likely failure candidate: L6.

  • L6 carries both DC bias and audio
  • It is connected to cabling potentially exposed to ground differentials (PCs, accessories, etc.) and also if bias is activated, it could short to ground, causing L6 to go open circuit.
  • A failure here would completely kill the rear mic input

Consulting the assembly manual showed that removing the audio jack board is straightforward, so I opened the radio for inspection.

L6 highlighted, Note the unused ring connector. It´s perfectly safe to use stereo cables.


KIO3 back panel. Remove top cover, remove the screws on the DB9 connectors, and remove the two phillips screws. The DB9 board is removed by wiggling it from the main board, pulling it out through the hole in the back. The main KIO3 board is removed by wiggling and pulling it up, while gently bending it towards the front of the radio, to clear the standoffs.

Fault Found

Measuring across the inductors on the board quickly revealed the problem:

  • L6 was completely open
  • All other similar inductors measured approximately 4.2 Ω

I did not have a suitable SMD replacement on hand. After scavenging several donor boards with no luck, I found a small axial-leaded 680 µH inductor (original is 100 µH).

Rather than shorting the inductor outright, I installed the 680 µH part as a "temporary" substitute. (Well, we all know that´s going to be a very permanent solution).


Audio board. Note the 3.5mm connector inserted to help measure the signal path.


L6 removed.



Axial leaded inductor converted to SMD.


Reassembly and Result

After reassembling the K3, the rear mic jack was tested again.

Rear mic input fully operational

The ModMic now works exactly as expected, with proper bias and clean audio.



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