I’ve enjoyed 2 years trouble-free service with the Elecraft KX3 transceiver using the original microphone. However, after about 50 SOTA activations I found the MH3 microphone randomly keying itself. Luckily, the PTT on the microphone can be disabled from one of the menus and then the XMIT button on the bottom left of the radio’s front panel can be used to key the transmitter on and off. Several more SOTA activations followed using this method. Problem is that it occupies two hands making logging and answering calls slower than it should be. Yes you can use VOX, but in the variable weather environment on a summit, this is not an attractive solution. Something had to be done about it. A new microphone shipped from the USA costs north of A$100 so that was ruled out. Repairing it myself seemed the only option.
The mic cable was exercised along its length to find the position of the problem. It turned out that PTT was being keyed by flexing the cable near the microphone itself. Plugging the mic cable into the KX3’s CW key jack and pressing the PTT I found that operation was normal and unaffected by cable flexing.
The full schematic of the MH3 is available from Elecraft here. There are four terminals on the 3.5mm plug. The tip and sleeve are for the electret microphone and the 1st and 2nd rings are for the PTT and up/down buttons. The two sets of terminals are electrically isolated from each other.
With the intermittent nature of the fault, it seemed as though there was a short between two conductors when the cable was flexed. A multimeter was placed across each combination of the terminals of the plug in turn and conductivity measured. It revealed a short between the 1st ring and the sleeve. These are on separate circuits.
The microphone was opened up by removing three screws from the back. Inside there is a circuit board mounted on the back half containing a switch and some resistors. Two wires run to the electret element on the front half of the mic through a ferrite core. There are 4 wires from the cable soldered directly into the board. Connections are:
|Ring 2||PTT common||yellow||4|
Looking at the board, there did not appear to be any shorts and the wiring looked normal. I removed the switch from the board by undoing the screw and then undoing 3 more screws allowed the board itself to be freed. It was held in tight by friction with the plastic standoffs so it needed to be assisted by levering with a screwdriver. With the board free, the cable can be easily removed from the back of the mic housing.
The rubber strain relief can then be slid down the cable after separating it from the metal clip. The metal clip is clamped around the cable and is loosened by prizing the two ends apart with a screwdriver. It can then be slid down the cable as well. The cable sheath can then be opened up starting at the end with the bare wires by carefully slicing along it with a sharp knife. Be careful not to strip the insulation off any of the wires within. I did this in sections of about 3cm at a time and then pulled the wires through the opening.
Inspecting the wires after extracting from the outer sheath there was a narrow section on the green wire where the insulation had been sliced through. The red and black wires are unused. In the red wire there was also a gash so it seems there was a fault with the original wires. The exposed copper of the green wire was shorting to the uninsulated copper shield wire when the cord was flexed. The copper shield wire is wrapped around the white wire and after reaching the end of the sheath the shield wire is insulated with a black covering.
With the wires extracted, I tested the mic with the radio. Flexing the cable caused no assertion of PTT any more. Looked like the problem had been licked!
The copper of the green wire was in good shape and not nicked so the opening was wrapped in green insulating tape. The stripped cable sheath was rewrapped around the white/shield and unused wires and insulation tape wrapped around that. There was no need to shorten and resolder wires as there’s plenty of room inside the housing for extra cabling.
The metal clip is slid back up the cable and clamped down 1 cm from where the slicing stopped. The strain relief is repositioned over it and the board screwed back into the housing along with the PTT switch. The end of the cable can then be bunched up inside the back of the mic and the strain relief anchored back into the mic housing taking care to align the positioning pin to prevent rotation of the cable. Reattach the front housing of the mic and you’re done!
My MH3 has a slightly shortened cable, but is now fully functional again. The repair took less than an hour and saved me 100 bucks. If your mic plays up I’d highly recommend having a go at fixing it yourself.