The audio system in my plane has developed a series of audio problems and while we (the shop and I) have done a fair amount of troubleshooting we still don't have a good theory as to the cause(s). So I thought I'd ask here for help.
The first problem started last February when random "crackles" every few seconds developed in the headsets. I traced that to the transponder being on. The shop repaired some grounds and the crackle went away for a month. It then returned and the shop had planned to recheck and expand on that work, but I can live with it and they are swamped, so I've been waiting.
Recently, though, another more severe audio issue has developed. When I first start up for a flight and call for a radio check, there is a loud squeal in the plane's headsets during transmission on Comm1 (GNS-530w). The squeal on transmission sounds like that which happened in the past when one of the headset jacks got shorted. But with the shorted jack, the squeal would happen when trying to transmit on either C1 or C2 (KX-155). Now it only happens when transmitting on C1 (does not happen transmitting on C2). Also, the current squeal does not show up in the transmission (always get a loud and clear response no matter how badly I hear squealing). So it seems like the squeal is in the side tone, not the radio output. I will still refer to it as Radio Squeal. After several minutes the squeal would go away and communications seemed normal.
Also, my intercom (Sigtronics SAS-440) has an automatic squelch button. When I pressed the squelch button the intercom used to go silent for a second, then give a quiet pop as the squelch set itself. About the time that the radio squeal developed, the intercom started to produce a squeal (a much quieter squeal) whenever the squelch button is pushed (instead of silence and a "pop"). I refer to it as intercom squeal.
Over the past 2 days the shop specified a series of tests. All were done with the engine off, breakers/fuses pulled for the T&B, GI-275s, and Engine Analyzer. The USB cigarette lighter adapter was removed from the socket. The radio squeal only happened when Comm1 was on our local CTAF freq. Even 0.025 MHz above or below avoids the radio squeal. Also C2 must be tuned to close to 121.5 MHz (121.6 = no squeal, 119.0 = no squeal..anything closer to 121.5 squealed....although it almost seemed like things were getting worse as I continued testing more frequencies on C2). Squeal is in all 4 headset positions. It happens using either the pilot or copilot push to talk. The intercom squeal happens even when the radios are off. Turning off the intercom eliminates the intercom squeal, but not the radio squeal. When turning the intercom back on to ALL from OFF, there is no intercom squeal when initially pressing the squelch button, but after about 5 seconds the intercom squeal returns. Turning off either C1 or C2 eliminates the radio squeal (not the intercom squeal). Setting the Audio Panel (EDO AM-550) Comm1 output to OFF does not prevent the radio or intercom squeal. Nor does setting it to output to the Speaker.???
Disconnecting the ELT antenna coax at the ELT did not eliminate either squeal. Disconnecting the C2 antenna coax at the Icom Switchbox (a socket that allows a handheld to use the external C2 antenna instead of C2) did eliminate the radio squeal???
And, all of this testing was done over a few hours. So unlike normal operation when the squeal went away in a few minutes, during this testing it did not go away or even diminish for hours.
Does any of this make sense?
The first problem started last February when random "crackles" every few seconds developed in the headsets. I traced that to the transponder being on. The shop repaired some grounds and the crackle went away for a month. It then returned and the shop had planned to recheck and expand on that work, but I can live with it and they are swamped, so I've been waiting.
Recently, though, another more severe audio issue has developed. When I first start up for a flight and call for a radio check, there is a loud squeal in the plane's headsets during transmission on Comm1 (GNS-530w). The squeal on transmission sounds like that which happened in the past when one of the headset jacks got shorted. But with the shorted jack, the squeal would happen when trying to transmit on either C1 or C2 (KX-155). Now it only happens when transmitting on C1 (does not happen transmitting on C2). Also, the current squeal does not show up in the transmission (always get a loud and clear response no matter how badly I hear squealing). So it seems like the squeal is in the side tone, not the radio output. I will still refer to it as Radio Squeal. After several minutes the squeal would go away and communications seemed normal.
Also, my intercom (Sigtronics SAS-440) has an automatic squelch button. When I pressed the squelch button the intercom used to go silent for a second, then give a quiet pop as the squelch set itself. About the time that the radio squeal developed, the intercom started to produce a squeal (a much quieter squeal) whenever the squelch button is pushed (instead of silence and a "pop"). I refer to it as intercom squeal.
Over the past 2 days the shop specified a series of tests. All were done with the engine off, breakers/fuses pulled for the T&B, GI-275s, and Engine Analyzer. The USB cigarette lighter adapter was removed from the socket. The radio squeal only happened when Comm1 was on our local CTAF freq. Even 0.025 MHz above or below avoids the radio squeal. Also C2 must be tuned to close to 121.5 MHz (121.6 = no squeal, 119.0 = no squeal..anything closer to 121.5 squealed....although it almost seemed like things were getting worse as I continued testing more frequencies on C2). Squeal is in all 4 headset positions. It happens using either the pilot or copilot push to talk. The intercom squeal happens even when the radios are off. Turning off the intercom eliminates the intercom squeal, but not the radio squeal. When turning the intercom back on to ALL from OFF, there is no intercom squeal when initially pressing the squelch button, but after about 5 seconds the intercom squeal returns. Turning off either C1 or C2 eliminates the radio squeal (not the intercom squeal). Setting the Audio Panel (EDO AM-550) Comm1 output to OFF does not prevent the radio or intercom squeal. Nor does setting it to output to the Speaker.???
Disconnecting the ELT antenna coax at the ELT did not eliminate either squeal. Disconnecting the C2 antenna coax at the Icom Switchbox (a socket that allows a handheld to use the external C2 antenna instead of C2) did eliminate the radio squeal???
And, all of this testing was done over a few hours. So unlike normal operation when the squeal went away in a few minutes, during this testing it did not go away or even diminish for hours.
Does any of this make sense?
