If it were installed the way most audio panels are, while either Pilots or Copilot's PTT is keyed, the bias applied to the respective mic would be coming from the radio; not the NAT! The fact that the mic bias disappears when the PTT is pushed means that either the Marconi transmitter is bad, or there is missing connection between the NAT and the COMM, or there is a dc blocking capacitor in the way! Pin 12 of the NAT should go directly to MIC INPUT of the Marconi transmitter.Interesting.... as I said, I am trying to install the Northern Airborne Technologies (NAT) aa12-022. The install diagram is here:
https://www.electro-tech-online.com/custompdfs/2009/07/804-0410-1.pdf
As in most audio panels there is a pilot isolate switch on the NAT intercom/audio panel that, as you said, connects the mike and headset of the pilot directly to the radio. I notice when you flip that switch the mic bias voltage drops to zero from 4 volts. My question is how does the old system manage to pass voice to the radio and why is the NAT unable to do the same? When not in isolate mode there is a voltage on the mic when transmit is pressed and still nothing.
Yes, the NAT install manual shows the direct connection. It is possible that the resistors and capacitor was someone's attempt at reducing the transmitted noise level because you have a high ambient noise level aircraft (Jet?)Is the solution as simple as taking out the capacitor and resistors? I have to wonder why they are in the circuit? It seems to me the capacitor is either protection for the radio or to isolate the radio from dc voltage while the resistor is to match impedance? The last thing I want to do is blow up the radio! Should I connect the audio panel mic out directly to the radio mic in?
BTW, I notice you say current not voltage. Is that an important distinction?
Would an Electret capsule develop enough AC volts (even shouting into it) to drive a "Carbon mic" input?
1) So why is this 'specific' voltage range listed?
2) Dynamic mics don't require bias voltages to 'operate' - do they?
3) Am I able to bias an electret microphone with a lower voltage without damaging it? (eg: +5Vdc). (NB: This is for personal use - I'm not modifying the aircraft radio).
4) I assume changing the bias voltage will change the microphones impedance, but does this really matter, if I can just 'turn up input gain'?
5) Are carbon mics actually still used in aviation these days?
... old UK telephones used a carbon mike, and they later brought out a fully compatible electronic replacement - just connected on the same two wires.
OK, new mikes making signal loud enough for earphones, amplifier in the NAT working too. Intercoms talk to eachother, good. Almost there.... a new NAT AA12-200 audio panel/intercom for ..... and a patch cable to connect it to the aircraft in place of the existing unit....
... I find the intercom works great...
OK, sound getting from the Radio to the intercom. (during non-PTT mode)... plus you can hear the radio....
I presume that during Radio PTT, the side tone comes from the radio itself, through the same audio output. (Rather than the side tone locally within the NAT, during intercom) I'd think it'd be the latter, so the radio would sound 'dead' if it wasn't receiving voice OK during PTT.However, I cannot transmit, or hear a side tone...
Sounds like a pot-down attenuator, with DC blocking. I can see why you considered running a bypass between the NAT and the Radio. Again, MikeMI is on the ball here.[paraphrased]
...two resistors and a capacitor in the mic audio line, from the audio panel to the radio.....an R & C in series... and an R to ground. .... on the aircraft 'somewhere', buried.
This is where I think it's going wrong.My guess is the capacitor is blocking dc voltage getting to <...> the Lightspeed electret mic. I tried apply a voltage of around 4volts but still nothing.
Per what you said about not know what the capacitor was for, NAT make a product the aa34-300 which is an interface between various radios and headsets. I'm told it would solve my problem; for a mere $500+! Here is the installation manual:
**broken link removed**
If you look at page 2-4 it says half way down the page under "bypass switch for blocking capacitor in mic out circuit":
"....if there are any concerns about dc interaction between the aa34 and the connected radio"
So there are apparently radios that may require a capacitor to avoid damage. Thus you see my concern in my installation. However, I think I'm going to try connecting the radio directly to the intercom. Wish me luck!
Would an Electret capsule develop enough AC volts (even shouting into it) to drive a "Carbon mic" input?
Marc,...
This is where I think it's going wrong.
I'm not sure that the radio really needs a DC voltage from the microphones. Does it?
The "LS electret mic" works fine (see earlier) during intercom mode, but does it lose it's 'bias' voltage when the Radio's PTT operated? If so, does the NAT do something like "transfer" the electret mic circuit over entirely to the Radio?
I would expect maybe any locally generated side tone (for the intercom) to be deactivated during radio PTT, so that the sidetone from the radio is heard instead of from the intercom?
Bottom line is: not enough information. Gather more data!
...
So how is it that the headsets with the carbon mics and the old Gemelli audio panel/intercom manage to work with that capacitor and resistors in the way?
Although the Marconi radio is operating on civilian frequencies, it is worth noting that the aircraft (it's an SF260, a 2 seat trainer) was in service with the Italian military back in the 90's. In addition to the Marconi VHF radio it also had installed a military UHF radio. Both radios fed through the Gemelli audio panel to the headsets. Perhaps this is why the headsets required carbon mics?
If I put a 500 ohm resistor across the mic in of the radio, I should see 4 volts, right? If that's the case then I think I could do no damage to radio if I jump the radio's "mic in" directly to pin 12 of the NAT.
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