---------------------------------------------------------- Avionics-List Digest Archive --- Total Messages Posted Tue 04/21/09: 3 ---------------------------------------------------------- Today's Message Index: ---------------------- 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 _____________________________________ Time: 08:18:49 AM PST US From: "Greg Young" Subject: Avionics-List: 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 _____________________________________ Time: 09:33:55 AM PST US From: Doug McNutt Subject: Re: Avionics-List: Side tone 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 _____________________________________ Time: 10:29:02 AM PST US Subject: Avionics-List: Avionics cooling fans - yea or nay?? From: "McFarland, Randy" 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 ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message avionics-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/Avionics-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/avionics-list Browse Digests http://www.matronics.com/digest/avionics-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.