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Dave a écrit:Posted 03 September 2006 - 05:08
Inside R22 headset incompatibility:
I have documentation from David Clark on the R22 hot-mic or simultaneous PTT conflict. It's a REAL PROBLEM if the two headsets in the aircraft do not have the same microphone impedance--and, worse yet, it's not a low-vs-high impedance problem, but an even closer match, so the year in which each microphone was manufactured strongly influences whether one mic defeats the other!
History: aviation microphones are wired to simulate the old carbon-cartridge mike of 60 years ago. So each microphone requires a "bias" current to pass through the circuit. David Clark adheres to the idea that the airframe manufacturer provides the standard method of having a Separate Bias Resistor for each microphone outlet, so that each mic can get the right current.
Robinson, for whatever reason, chose to imbed a Single Bias Resistor serving Both microphone leads--with the result that when you're on hot mic, or both IC switches are depressed, BOTH microphones use the SAME bias resistor--with the result that the microphone with the lower resistance of the two wins the battle, and takes all the current, and defeats the other microphone!!
I delivered the David Clark documentation to Pat Cox in about March of 2005, and he put this issue in the problems-to-be-solved hopper for me (what, they didn't notice the problem for 27 years after initial certification?!?), but there must be lots of other more important issues, nothing has happened on it since.
I have poked and probed and asked around, but there seems to be no simple rewiring fix short of installing a separate intercom electronics system interfaced to the factory wiring; on the R22 circuit diagram, the PTT relays connect to a single wire running to the radio audio, with no way to separate out the two microphones and install separate bias resistors for each. I still have $3.50 of Radio Shack resistor-capacitor combos if only I could figure out a way to wire them in!
Meanwhile, the solution is to keep trying headset combinations until you find two that work well together.
Happy to provide copies of the David Clark comments on wiring (a .pdf file, too) to techies more ingenious than I who want to figure out a fix.
Dave
W2DLS, Amateur Extra
Chief Flight Instructor, HelicopterAcademy.com
Luc Lion a écrit:Donc radio avec intercom incorporé.
Quel modèle ?
Luc
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