Today's Message Index:
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1. 08:18 AM - 757 mic bias voltage (Greg Young)
2. 09:33 AM - Re: Side tone (Doug McNutt)
3. 10:29 AM - Avionics cooling fans - yea or nay?? (McFarland, Randy)
Message 1
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| Subject: | 757 mic bias voltage |
I've got a friend that flys 757's using a Bose headset and has the mic get
hot (warm) during the course of a flight, not burn-your-lips-hot, but too
hot. Talked to the Bose service mgr here at Sun n Fun and he wants to know
what the mic bias voltage is since a replacement mic didn't fix it. FWIW,
the mic is live continuously because of the CVR. I thought there might be
someone in the brain trust here that might know the answer or have any other
ideas. It's not a problem in GA aircraft - he flew with me to SnF in my
Navion for 5 hrs and had no problem. Likewise none in his GA airplanes.
Regards,
Greg Young
Message 2
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At 10:32 -0600 4/19/09, I wrote:
>It's been almost two decades since I have had a Sigtronics apart and I never owned
one. I'll have a look at my records later today. There might be a schematic
around.
I found a schematic.
The SPA-400 uses diode isolation to activate the transmitter key. Activation of
the transmitter without going through SPA-400 pins 2 (pilot) or 10 (copilot)
will not be recognized by the intercom.
The intercom normally removes power from its headphone amplifier when either the
copilot or pilot push-to-talk key is pressed and diodes see that the aircraft's
push-to-talk system is also activated.
The output of the audio amplifier goes through 1 1 microfarad capacitor and a 100
ohm resistor to the aircraft's headphone bus, The 100 ohms allows for the audio
from other sources - like a radio or audio panel - to be connected to the
same bus where the audio signals are mixed.
A part of the on-off switch connects the aircraft microphone bus to the SPA-400
when it is turned on but reconnects mic bus directly to the pilot microphone
when off.
I'm now betting that the Gordon's problem has to do with his microphone jack being
wired so that he grounds the aircraft push-to-talk wire directly instead of
going through pin 2 of the intercom connector.
If I'm wrong there is a diode failure inside of the intercom, probably CR-7.
Off line at dmcnutt@macnauchtan.com if you want to talk in more detail about the
schematics. I'm not sure about copyright and non-disclosure issues.
--
--> Life begins at ovulation. Ladies should endeavor to get every young life fertilized.
<--
Message 3
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| Subject: | Avionics cooling fans - yea or nay?? |
I have a Garmin 327 transponder and Dynon D-10 that I suspect become
unreliable and freeze or lock up due to high behind the panel
temperatures. Have any of the group experienced avionics issues related
to heat, and will/does cooling fans provide enough cooling to eliminate
the problems?
Randy
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