We have a interphone problem that's been kicking our ass trying to track down so I figured I'd post here and see if anyone has any ideas we haven't tried.
The Issue...
We have a C-130 H2, Tail 87-9281 with a loud buzzing tone that is heard only in the Pilots, Copilots and Flight Intstructors stations. All other stations sound clear.
The tone is heard regardless of which radios/nav systems are selected on the control box and volume panels. The tone in the bad stations goes away only when you press that call button, and only on the station which the call button is pressed.
Troubleshooting...
We have already pulled and swapped all the boxes and comm cords, doesn't look like an interphone control box or volume box. The tone is still heard even with one known good box plugged into the pilots station, all other stations unplugged.
We suspected a bad ground so we checked the grounds and audio commons to every station and they checked fine. We also ohmed out the audio lines and didnt' find any shorts.
We suspect noise coming from another system so we pulled every single circuit breaker to see if one made the noise stop, nothing made it go away.
We pulled wires from every terminal on the interphone junction box one banded group at a time. No Effect.
We suspected noise coming from the TRs so we pulled the breakers for them and ran off battery power. As long as AC power is applied ot the plane the noise persists.
The ONLY thing other than the call button that makes the noise go away is running the interphone off battery power with no AC power applied to the plane.
???????
We are running out of ideas. Can anyone here think of anything?