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I have three older DC H10-13.4 headsets that came my way. Whilst I can hear perfectly well, the mic in each case seems inoperative. I checked electrical continuity and changed one mic for a brand new one but it still does not work. The mic In each case is an M-7A electret type and the radio is a Garmin GNC 255 with a GMA 340 audio panel. It all works fine with my Sennheiser headsets.
Am I looking at incompatibility, a fault or possible finger trouble?

The next step is to get someone who can bench-test them. Any recommendations?

Squelch?

No... at least I don’t think so. I have another set, H10-30 with a different mic that works fine (an M-7/DC).

As an electronics geek it took me a while to understand how GA headsets are wired. I can share how I'd bench test them with the rudimentary equipment I have, if that helps. Out of curiosity I just tried with one of my headsets and this method works with mine. But my understanding of GA headset wiring may well be incomplete. Perhaps different headsets do more wildly varying things than what I know about.

I connected a 330 ohm resistor in series through the headset microphone connector (thinner plug, ground furthest from the tip, and signal in the middle ring) with 10V from my bench power supply. Measuring the potential across the microphone connection with an oscilloscope then gave me around 5V with around a +/- 1V peak amplitude when speaking at the level I would in the aircraft. This is with ANR on - with ANR off I suspect the result will have been similar but different.

As far as I understand GA headset microphones "pretend" to be graphite microphones with some circuitry even though nowadays they are all electret - so should modulate resistance - and historically aircraft would just shove through the regulated 24V-ish supply. One GA headset manual I have (for my headset) specifies a matching impedance of 150-1000 ohms and 8-32V; hence my selection of 330 ohms and 10V.

If you have a multimeter you could try to shove through perhaps 18V by using two 9V batteries, a suitable resistor and the headset microphone connector all connected in series and see what potential you can measure across the connector. That'll at least tell you if you have an open circuit. But since I'm not confident I fully understand GA headset circuitry, please don't blame me if you blow something up

The M 7 Electret Mic is a noise cancelling mic designed to operate in place of carbon and amplified dynamic microphones.

It will not operate in circuits that do not supply a DC bias voltage.

So it either has to have the bias provided by a battery pack in the cable set of the headset or will need a bias supply via the mic cable from the panel.

Thanks, you had me there until you wrote “blow something up”!

Flying_john wrote:The M 7 Electret Mic is a noise cancelling mic designed to operate in place of carbon and amplified dynamic microphones.

It will not operate in circuits that do not supply a DC bias voltage.

So it either has to have the bias provided by a battery pack in the cable set of the headset or will need a bias supply via the mic cable from the panel.

Thanks, something to look into. The M-7/DC works fine; it is the M-7A that isn’t. They are fitted as standard to the H10-13.4 headsets that are not ANR so the plot thickens.

Don't mix up ANR with "noise cancelling" mic. A noise cancelling mic has holes in both sides. Extraneous noise which enters both sides, in theory, doesn't move the diaphragm as it applies the same pressure both sides. In this way it's "noise cancelling". You then have to talk quite close to one of the sides in order to be heard properly.

Paul, I did exactly that... oops. What confuses me is that the H10-13.4 headsets were pretty much a standard item. Maybe there is a friendly flying school owner at Blackbushe who would let me try them in one of his aircraft?

I've got a portable intercom you can try them with. I'm not sure of my movements this week, but I should be in the area at least on Thursday evening/Friday and on Sunday (got a Christmas meal to go to on Sunday).

I can also bring my multimeter and google.

Paul, if rdfb is to be believed, a bomb disposal suit may also be useful! Thanks for the offer. If this week is too busy, there’s no rush.

Both the M-7A and the M-7DC require a bias voltage. From memory the M-7DC is a more up to date mic that has better noise cancelling performance.

NickS wrote:Paul, if rdfb is to be believed, a bomb disposal suit may also be useful!

By "blow up" I meant that you might hear a tiny pop and a hiss and have the magic smoke come out. I didn't mean that it'd blow you to smithereens*

* not that I'm taking any responsibility if that happens to you either

They're expensive microphones to be letting the smoke out of as well, though you can get suitable replacements for about 12 quid...