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A 3-pin LEMO is the input connector on Sennheiser SK-series bodypacks, and sooner or later every sound kit needs one worked on — whether that's wiring a new lavalier to it, converting a mic from another transmitter brand, or repairing a cable that's failed at the connector. It's one of the easiest jobs to get subtly wrong: the joints are tiny, the bias voltage has to reach the right pin, and there's no room for a cold solder joint inside a LEMO housing. The steps below cover a fresh termination; a repair is the same process once the old connector is removed and the cable end is cleaned up.
Pinout, key steps and the manufacturer sequence. richardhubbard.au
The layout below is the standard arrangement for a Sennheiser SK 3-pin input. Confirm it against your specific microphone and transmitter before soldering — bias arrangements vary between mic models, and the wrong pin means no audio or no power.
| Pin | Signal | Typically connects to |
|---|---|---|
| 1 | Ground / screen | Cable shield / mic ground |
| 2 | Positive bias / power | Mic power feed |
| 3 | Audio signal | Mic capsule output |
Wisycom's LEMO-fitted transmitters — the MTP40S and its relatives — have no single fixed pinout. What each pin carries depends on the MIC Mode selected in the transmitter menu. Two things follow from that, and both cost time on set.
The pin order itself is the familiar one — ground on 1, power on 2, audio on 3, the same as the table above. What changes is whether pin 2 is doing anything at all, and whether the audio line is carrying a bias voltage on top of the signal. A perfectly soldered lead can still be silent because the transmitter is in the wrong mode, so check the menu before reaching for the iron, and again before deciding the joint is at fault.
| MIC Mode | Pin 1 | Pin 2 | Pin 3 |
|---|---|---|---|
| 2 wires | Ground | — | Audio |
| 2 wires + bias | Ground | — | Audio + 5.5 V |
| 3 wires | Ground | 5.5 V | Audio |
| 2 wires & phantom | Ground | 3.1 V (powers a PHA48) | Audio |
| 2 wires + bias & phantom | Ground | 3.1 V (powers a PHA48) | Audio + 5.5 V |
Pin 1 is ground in every mode, and the gain range is −60/40 dB throughout, so the choice is really about how the capsule gets its power: down the audio line as a bias, on its own conductor, or not at all.
If you need PTT, choose a two-wire mode. The full push-to-talk options — normal and muting — and the PTT LED indication are only offered in 2 wires and 2 wires + bias. The three modes that use pin 2 for power drop back to disable or no-data, and lose the PTT LED with them. That is a wiring decision as much as a menu one, so settle it before you cut the cable.
Values here follow Wisycom's published MIC Mode options. Firmware and models vary, so confirm against the manual for the transmitter in your hand before committing a joint.
Slide the boot and strain-relief collet onto the cable first — the number one rookie mistake is soldering a perfect joint with the housing stranded on the bench. Strip back just enough jacket to reach the solder cups without leaving exposed conductor once assembled.
Tin each conductor and each solder cup separately before joining them. A thin, even coat on both sides makes the final joint fast, so the tip spends the least possible time near the fragile connector insulation.
Work one pin at a time in a fixed order so nothing is missed. Ground first gives you a stable reference, then bias on pin 2, then audio on pin 3 — straight along the numbers, with no jumping back. Each joint should be quick, shiny and concave — a dull or blobby joint is a cold joint and will fail on set, not on the bench.
Heat-shrink any adjacent joints that could touch, then — before you close the housing — put the multimeter across the pins. Confirm continuity where you expect it and, just as importantly, no continuity between pins that should be isolated.
Close the strain relief so the cable, not the joints, takes any pull. Fit it to the bodypack, dial up the gain and confirm clean audio at a sensible level. A working bench test under real power is the only proof the job is done.
The steps above are how I work through it. What follows is the manufacturer's specification — the exact order and dimensions LEMO give for assembling this connector, summarised from the mounting instruction they supplied in July 2026. Where my habits and LEMO's figures differ, follow LEMO's.
A word on the threadlocker. Steps 9 and 10 are LEMO's instruction and I've reported them as written — but they are worth a hard think before you reach for the bottle. I've had these connectors come across the bench with Loctite on the screw ring, and they are effectively unrepairable: the ring won't come off without destroying the connector, so a job that should have been a ten-minute re-termination becomes a new plug.
That trade-off is the whole question. Threadlocker is there so the ring can't vibrate loose in the field, which matters on a connector that lives on a moving performer. But if you expect to re-terminate this cable — and on a lav rig you almost certainly will — a ring you can actually undo later is worth more than one that never loosens. My own habit is to leave it dry, tighten it properly, and check it as part of routine maintenance.
Prefer to have it done right the first time? Re-terminating and maintaining lav rigs is part of running a location sound kit. For any other cable, the pinout diagram generator covers the wiring. It's all part of location sound work across WA — get in touch if you'd like a hand.