Crackling noise from transpond

[bkspero]

A few weeks ago a random crackling developed in the audio system of my Tiger. The 1090es King KT-74 transponder is a big factor in the sound, and maybe the cause. In an attempt to isolate the cause I did the following test:

1. Airplane in closed metal hangar at a non-towered airport
2. Engine off.
3. Ignitions (1 magneto, 1 Surefly electronic) grounded through ignition switch.
4. Circuit breakers pulled for engine analyzer, electronic GI-275 attitude indicator, GI-275 electronic HSI, audio amp, and alternator field circuit.
5. Fuse pulled for turn and bank indicator.
6. Power turned off to Com1/Nav1 (GNS-530w), Com2/Nav2 (KX-155), nav lights, strobes, fuel pump, interior lights, and panel lights.
7. Headset microphone jack unplugged from panel and power to headset was off (Lightspeed Zulu 3).
8. Headset earphone jack plugged into pilot’s location and audio recorder plugged into right rear seat audio output.
9. Test sequence:
1. Turned on Master switch,
2. Started recorder,
3. Turned transponder to Alt and crackle begins (for some reason the recording has about 5 seconds of dead time prior to sound starting),
4. After about 22-24 more seconds the transponder was switched to Stby (crackle stops),
5. After about 8 seconds the transponder was switched back to On (crackle resumes),
6. Recording then was stopped.

Anyone have a similar experience and/or suggestions on next steps. I have made arrangements to have the avionics shop check and repair the ground connections, but they are resisting troubleshooting. The only other thing they've entertained is just sending the transponder Mid-Continent Instruments (who does the servicing of these units for BK/Trig). I'd rather do more troubleshooting first.

 

[Bell206]

suggestions on next steps.

Was there any avionics work performed in the months prior to hearing the crackling noise? When you switch to ALT it starts transmitting so you may have a shielding or ground issue. Perhaps rig up a 2nd xspndr coax/antenna and replace the original or simply temp reorientate your existing antenna/coax to a different location and see what you hear.

 

[bkspero]

Was there any avionics work performed in the months prior to hearing the crackling noise? When you switch to ALT it starts transmitting so you may have a shielding or ground issue. Perhaps rig up a 2nd xspndr coax/antenna and replace the original or simply temp reorientate your existing antenna/coax to a different location and see what you hear.

Last avionics work was completed last November (the GI-275s). Several months passed before the crackle showed up.

The shop will look at the grounds and other connections. I like the suggestion of testing with a separate coax and antenna. I forgot to mention that in the past it had developed a crackle but the shop would put a drop of Stablint contact cleaner in the connection between the coax and the antenna and the crackle would go away. Recently that didn't work. Neither did doing the same thing to the connection inside of the transponder tray. The separate coax and antenna test would isolate those items as the cause.

Thanks.

 

[Bell206]

but the shop would put a drop of Stablint contact cleaner in the connection between the coax and the antenna and the crackle would go away.

In detective work that would be called a clue. Stabilant 22 is about as close to a mechanic-in-a-can as it gets. Good luck.