Transmit problems With my Radio set up

[Mark Browning]

HELP ! Can anyone offer any advice with aproblem i have thats driving me mad
I have had this fault since my RV-7 first flew last September.

Ten minutes after take off I find that air traffic control can only hear my transmissions as carrier wave.
I initially thought that the problem was with the Radio its self or antenna/ coax. Having checked the coax and antenna including earthing etc. I removed the radio and sent it back to Narco for test and repair; Narco returned the unit to me with no fault found with a $300 invoice.

I had an Air force avionics technician check my installation last week he carried out extensive testing with specialised equipment ( on the ground) but couldn’t find anything Wrong. The modulation was OK, Power out put ok etc ...

He suggested that I replace the radio with another Narco to test.
I did this but again ten minutes after takeoff my transmissions were carrier wave only.This confirms to me that my radio is OK.

Yesterday I flew my RV-7 until my transmissions became carrier wave. At this point I was 15 miles from the tower, so I pulled the circuit breaker for the Sigtronics SCI-4 intercom and made a radio call to ATC. They immediately came back with radio check at strength five. My radio transmissions remained perfect until I switched the intercom back on.
I'm waiting for a reply back from Sigtronics before i go ahead an buy a replacement.
Any ideas ? or advice would be really helpful
My Radio is a Narco 810+
Intercom Sigtronics SCI-4

Cheers

Mark
RV-7 G-LNNE
England

[breister]

If you've ruled out the radio by using another, then you know two things for certain:

1. It's not the radio
2. The wiring from the mic switch to the radio is ok

Given the above, it sounds like either a fault in the mic in the headset itself (the physical mic, or the wiring between mic and plug) or a fault in the wiring between the mic receptacle and the radio.

[Steve]

If you can xmit & rec to your wingman but ATC still can't hear you, you've got antenna problems.

[az_gila]

Yesterday I flew my RV-7 until my transmissions became carrier wave. At this point I was 15 miles from the tower, so I pulled the circuit breaker for the Sigtronics SCI-4 intercom and made a radio call to ATC. They immediately came back with radio check at strength five. My radio transmissions remained perfect until I switched the intercom back on.

Most intercoms route the mic. line directly to comm 1 when power is off as a safety measure.

Sigtronics calls this "Back Up" for their SA-400 intercom.

Sounds like a bad intercom

[Mike S]

Originally Posted by az_gila

[i] Sounds like a bad intercom

What he said

[Mark Browning]

Thanks Gents appreciate your advice , I know my antenna and coax is good and i have used different head sets previously to rule these out.

I got an e-mail from Sigtronics they also think it could be a bad intercom with the description of the problem.
so long as the harness is good. They said they would need the inetcom back at their shop to test.

I am pulling the Narco com and intercom wiring harness out tomorrow and rechecking the whole thing and also replacing the intercom with a new SPA400... Will let youk now hopw i go with it
Cheers

Mark

[Mark Browning]

Its Fixed
i pulled the wiring out to double check the pin call out and then wired in the spa 400 new intercom and re installed every thing took it flying for an hour
Perfect