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Side tone

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gordona23(at)earthlink.ne
Guest

Posted: Sun Apr 19, 2009 6:48 am Post subject: Side tone

My recollection of the Sigtronics is that they just allow the audio from 

a comm radio to be mixed in wiring with the intercom audio. No switching 

is provided "because I sleep better that way". You should be able to use 

the transmitter for a test with the intercom turned off. That way you 

would be absolutely sure any sidetone is coming fro the transmitter. If 

that doesn't work you should also be able to connect the mike to a 

"standard" mike jack and the phones to a "standard" phone jack but you 

might need a mike with a push-to-talk switch on it. You can make a PTT 

by grounding the tip of the mike connector. That's the tip of "ring, 

tip, and sleeve of the three terminal connector. The CP125 does not 

generate any sidetone by itself. The Sigtronics drives the headphones 

but, although it's quite the same, it's not called sidetone. The war 

department's problem with distortion could well be the presence of audio 

from the Sigtronics being mixed out of phase with sidetone from the 

transmitter.

--



Thanks, that's helpful.  Here are a few more questions:



When I talk to the Sigtronics (comm radio off), I hear myself 

(sidetone).  Is that coming from the Sigtronics ?  If so, then I'm 

guessing that both the intercom and the COMM radio are putting out a 

sidetone, when I tranmit.  If they are "out of phase", that would 

explain the distortion.  Is there a reason why I shouldn't disable the 

radio's sidetone and just rely on the intercom sidetone?  I almost 

always fly with the intercom on.

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douglist(at)macnauchtan.c
Guest

Posted: Sun Apr 19, 2009 8:34 am Post subject: Side tone

At 10:47 -0400 4/19/09, Gordon Arbeitman wrote:

 	  Quote: 	
		  Thanks, that's helpful.  Here are a few more questions:



When I talk to the Sigtronics (comm radio off), I hear myself 

(sidetone).  Is that coming from the Sigtronics ?  If so, then I'm 

guessing that both the intercom and the COMM radio are putting out a 

sidetone, when I tranmit.  If they are "out of phase", that would 

explain the distortion.  Is there a reason why I shouldn't disable 

the radio's sidetone and just rely on the intercom sidetone?  I 

almost always fly with the intercom on.


	


The intercom really ought to be turning off its output to the 

headphones while a transmitter is keyed. It appears that it is not 

doing that. Intercom audio is being sent to all headphones at the 

same time. Strictly speaking, sidetone would be sent only to the 

headset that has its microphone activated.



Are you , perhaps, keying the transmitter without  telling the 

intercom about it? A push to talk switch on a control yoke might be 

doing that.



The sidetone from the transmitter is derived from a point very late 

in the circuitry and is an indicator that the transmitter is working 

pretty much properly. If you turn that off you would lose a failure 

indication.



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.



-- 

-> Stocks are getting pelloreid <-

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douglist(at)macnauchtan.c
Guest

Posted: Tue Apr 21, 2009 8:33 am Post subject: Side tone

At 10:32 -0600 4/19/09, I wrote:

 	  Quote: 	
		  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(at)macnauchtan.com if you want to talk in more detail about the schematics. I'm not sure about copyright and non-disclosure issues.

-- 



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