Aircraft handlers communication system

I am trying to put together a simple amplifier box which will take two aircraft handlers headsets for training purposes. Finding a schematic of the system used on an aircraft has proved fruitless so I tried to draw a schematic of the headset used and its adaptor box. The headset starts out by having a large 4 pole jack plug, this is inserted into an adaptor box to convert it down to a 3 pole jack. The adaptor box just hard wires from the 4 pole socket to a cable leading to the 3 pole jack plug except that there is a Push To Talk (PTT) button which appears to connect the microphone when it is pressed. The schematic I have drawn so far does appear strange especially with regard to the cable screens but that is how the cables are wired. The problem I have is oscillation which is not surprising the way the wiring is arranged. Does anyone have a schematic of how these aircraft 'Push Back' handlers headsets are wired to make them immune to feedback via the wiring?
This box will not be used anywhere near an aircraft, it is purely as an aid in the classroom.
 

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avtech

Member
2017-12-12 4:04 am
South Oz
What headsets are you using? This is the wiring I usually see:

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Try a google image search for "aircraft intercom schematic" and you'll find plenty of schematics such as:

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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
That wiring is truly strange to my eye. Maybe it reduces glitching when a plug is part-inserted.

Do you really need that cap on pins 1+8?? Using other headsets and counting on thumb, I estimated needed gain like 10, not 200. Take the cap off. If it is now too soft, replace cap with 2k then 1k resistor for gain like 30 or so. The input volume pot should be full up for slightly too much loudness.

I would feel mildly better if the '386 were wired non-inverting. If I an reading the stripboard right (doubtful) that means moving several jumpers. If it squeals middle of the audio band, it might be worth a try. If it squeals way up high, it probably is not phase.

While the 4-conductor plug offered the ability to totally separate mike and headphone circuits, you show it with 1 and 3 jumpered IN the headset. That seems wrong to me, but I assume you do not want to re-wire good headsets.

There's no strong need to use "good" headsets in classroom. Motorcyclists use intercom systems driver to passenger and driver to driver. I recall wired headsets but BlueTooth has taken over, and prices beat-down to not extravagant. OTOH you can buy PC gamer headsets (as cheap as a penny!! $20 decent $50 nice) and use with your '386 amp.
 

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Many thanks for the info, it has given me some choices for a rebuild. I think the original headsets must be for the Peltor system. I think another reason why my friend wants it all to work around his headsets is so he can quickly try them out before going onto the aircraft apron. The wiring is indeed strange and difficult to measure the mic lines as the multimeter starts very high in resistance and slowly drops, rather the inverse to the effect of charging a capacitor. I notice the large schematic uses inverted input to the amplifier, that does make sense and I thought about reversing it myself. This I will try, thank you once again.