New Install - Comm and Intercom issue

[viking750]

Search turned up nothing to address my issues, so thought I'd ask the community of experts. I have HAM radio experience but no avionics experience other than being a user not an installer.

Followed the wiring diagram in the installation guide specifically for the T-25 comm radio.

All wiring between intercom and jacks, Bose LEMO plug, and comm radio are with shielded cables and all shields are bonded at the intercom and to pin 1 and ground.

Intercom working between pilot-copilot using either the Bose or a regular headset plugged into standard jacks on pilot's side.

PTT switches are control stick mounted and one wire is grounded to airframe below control stick, other wire in correct pins for pilot-copilot PTT (10 & 16).

PTT working as intercom led turns orange and Skyview/Comm Radio displays switch from RX to TX.

Issue: when talking with PTT engaged I can't hear myself in earphones - mic appears to cut out. Dynon support said to play with squelch and sidetone values in comm setup but had no effect.

 

[GKC Aviation]

Sidetone generally comes out of the radio on the audio lines. If you break squelch, can you hear the radio? If you can, then I would suspect that microphone input to the radio from the intercom.
Are you sure the radio is transmitting voice?

 

[viking750]

I can hear transmissions from the radio when someone flies close to my location on the local airport frequency (aircraft is inside a large garage/workshop). Not sure if transmissions are getting out, sold my handheld so can't easily test. Pin 25 Intercom Mic Out is connected to Comm Pin 23 Mic In, pretty simple connection. Couldn't find anything in Dynon documentation that says you won't hear yourself when transmitting so pretty sure I should be hearing mic output through headphones during transmission.

 

[GKC Aviation]

Yeah, you should hear yourself on transmit (sidetone). Push it outside if you can and try for a radio check to another aircraft if possible, or borrow a handheld.
Yes, it's a simple connection, check your mic hi and lo twisted pair aren't reversed at the radio (23 and 9), and you have continuity between 25 on the intercom and 23 on the radio.
I know it sounds like I'm telling you to sucks eggs, but it's usually the simple things that trip us up.
Failing that, if all the wiring is OK, then I'd be looking at the radio or intercom unit having a problem internally.

 

[viking750]

Comm 23 and 9 are correct and continuity between 23 and IC 25 is good.

With sidetone turned way up (75) when I engage PTT I can hear an increase in background volume and a slight hum so I think the radio is transmitting but still no mic output from IC to Comm. Mic output with intercom cuts out when PTT engaged (redirected to radio?).

At this point I'm wondering if the Intercom has an issue with mic out on pin 25?

 

[GKC Aviation]

Since your wiring is all good, it's either going to be the intercom isn't switching audio through to the radio when PTT is active, or the radio isn't outputting sidetone.
You could make up a small harness from your mic jack ring terminal (mic hi), straight to the radio input pin 23, this would take the intercom switching out of the equation. Drop pin 23 out of the radio connector and make up a single wire with a pin on the end that goes from the mic jack and put that pin in 23. This will bypass the intercom. This will do for a test.

 

[viking750]

Great minds think alike - although I approached it from the other end. I pulled pin 23 and touched it to my headphones plug (and grounded the plug's barrel) figuring PTT would send mic output audio through the wire from IC pin 25. Engaging PTT yielded no audio on pin 25 from IC. I'll try your idea to verify the radio is working and if it is, the IC goes back to Dynon. Unfortunately it's a one month turn-around since UPS takes 10 days each way to get it back and forth to WA from NH plus a week there. Hope they have an advance replacement option since this is new out of the box.

 

[GKC Aviation]

Yeah, putting the mic output from the IC straight into the audio input (your headset) won't work due to the impedance mismatch between the microphone, and the headset speakers.

Try the mic audio from the jack straight into the radio input pin 23, and listen to your normal headset output as described above.

My experience with Dynon, even with brand new gear, is that they will want it back and won't send you a replacement until they have run it over the bench and confirmed a fault.

 

[viking750]

Mic jack patched direct to pin 23 works - mic output heard on sidetone. So, Intercom is fubar on pin 25. Will call tomorrow and send back. Good thing I'm still building and not flying or I'd be grounded for a month. Thanks for the troubleshoot assist, Dynon support geared more toward using not installing.

 

[GKC Aviation]

Good stuff mate! What are you building?

 

[viking750]

Zenith 750 STOL with a VIking 130 Engine. Panel done, tail feathers installed, control cables next then on to wings.