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Intercom disrupting radio transmission

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aelbrown

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I'm having an issue on my RV-4 where the radio doesn't transmit while the intercom is turned on. I'm struggling to figure out why. This is a new to me plane, so I'm still familiarizing myself with the systems, but wanted to check if this is an issue anyone else has seen?
 
You're going to have to give people more information than that, like what radio, what intercom, how are they wired together, etc.
 
I'm having an issue on my RV-4 where the radio doesn't transmit while the intercom is turned on. I'm struggling to figure out why. This is a new to me plane, so I'm still familiarizing myself with the systems, but wanted to check if this is an issue anyone else has seen?

Do you have an Sigtronics intercom by chance? Some radios don't play well with Sigtronics because of how the PTT circuit works.

https://www.sigtronics.com/_pdf/air_pdf/air_intercom_pdf/Radio_Keyline_Mod.pdf

The problem is that some newer digital radios have a keyline that looks for a 0vdc potential at ground. But, the Sigtronics has a slight (millivolt) drop acrossed a diode in the keyline circuit.

The fix is to wire a relay into the PTT switch that connects the radio keyline directly to aircraft ground.

I've seen this problem on an aircraft that had an SPA-400 intercom and a Icom IC-A200 radio.
 
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Yup

+1 on the Sigtronics PTT wiring. I chased a problem with a Sigtronics intercom wired to an Icom. I’m an A&P, not an avionics tech. I verified it was all wired up per the Sigtronics diagram. The local avionics guy blamed the intercom and advised replacing the intercom, which we did with no effect. Owner’s buddy, a real avionics whiz, told me the way Sigtronics said to wire the thing is messed up. (not his exact terminology). Said the answer is to wire the PTT button from the radio directly to ground, the way Icom says, bypassing the wire to the intercom…. the blue wire, to be exact. I just capped off the blue wire. Downside, the owner thinks I’m an avionics whiz now. Well, I found a problem with the coax to the transponder antenna before that. But anyway, the radio and intercom play nice now.
 
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