Bizarre Intercom Problem
Posted: Sun Aug 24, 2014 7:36 pm
Hi everyone,
I have a long-term intercom problem that is proving rather baffling to resolve. I'd love it if someone from this very savvy, experienced group had some useful insight. I've done a fair bit of diagnosis so far, so the full explanation will take a while. If you've decided to read on, you might want to grab a cup of coffee first, and settle in....
Shortly after I bought my M-6-235 (about three years ago) I installed a PS Engineering PM3000 four-place intercom. I did the install, with the assistance of an avionics shop in Colorado. I ran all the wiring harnesses, drilled holes, mounted the box, etc. They made the actual wire connections to the pins and the jacks. More or less immediately, I noticed that there was an issue with the co-pilot position. That position would break squelch much more easily than the pilot position, and when it did, the background noise was much higher. It's important to know that the PM3000 does NOT have separate squelch controls for each position; it has only one squelch knob. That adjusts for all four positions. Since it was pretty much always my girlfriend in the co-pilot seat, I assumed it was due to a difference in the impedance of our headsets. I have a Lightspeed Zulu (original, not "2") and she has a Bose X10. If I turn the squelch up really high, it mostly, but not completely eliminates the problem (it will still occasionally break squelch and have the loud background hiss). The downside of doing that is that it's really hard for me to break squelch to talk. I have to more or less yell. I did come up with a work-around that sort of works: I turn the gain all the way up on my Zulu mic and turn the squelch way up. That way I can break squelch to talk, and she is less likely to break squelch inadvertently. However, with my mic gain all the way up, the voice sound quality suffers measurably. I'd love to turn her mic gain down, but it's not adjustable. We sent her headset in to be checked, and Bose said it's fine.
Over time, we noticed that the problem mostly happens when she turns her head to the right, and/or gets it near the door/window frame. I do have opening windows. This spring, a friend from this list rode with me to Alaska and we noticed the same problem, though not quite as bad. He was using a Lightspeed Zulu 2, so the theory that it's her headset seemed to go out the window at that point. Shortly after arriving in Alaska, another friend rode with me and had the same problem; he was using a Bose A20. I've since confirmed the existence of the problem with an old set of non-ANR Telexes, and a set of Denalis (can't remember who made those).
I started thinking it must be a bad intercom. I described the problem to Northern Lights Avionics in Anchorage and they agreed. They helped me convince the factory to swap my unit out with a new one (or it might have been a refurbished one) but that didn't change the problem at all. I then brought the plane to Northern Lights for them to look it over while I was out of state for a while. They checked the jack connections, etc., but they didn't find anything. They ran the engine on the ground, but didn't fly it, so of course were not able to see the problem in action.
Today, I went out with a co-worker to run a bunch of tests to see if I could narrow it down a little. He was using the Denali headset. Again, we found that the problem mostly occurred when he turned his head to the right, or leaned close to the window. Turning right was more likely to produce the problem than just leaning right. Below are the various things we tried, and the results:
1. Swapped headsets (co-pilot wearing my Zulu and vice versa): mostly eliminated the problem.
2. Swapped jacks left to right (co-pilot wearing the Denali, but plugged into the pilot's jacks): no improvement
3. Pulled door and window tighter shut: this was ambiguous; it seemed to lessen the problem, though we could not consistently eliminate it. It certainly was true that if I had him position such that it wasn't breaking squelch and then pushed out on the top front of the door frame, it would break squelch.
4. Taped over top and front edge if door frame to airframe interface: no improvement.
5. Removed door completely: no or possibly slight improvement. One thing I did notice was that the background hiss I'd heard when it broke squelch was not the same as wind noise. With the door off, when he leaned or turned right a little, it would break squelch and have the same old loud, clean hiss it had always had. When he leaned out a little further, I could then hear the turbulent wind noise IN ADDITION to the background hiss.
6. Turned off lights, radios, GPS, transponder, pulled breakers for cigarette lighter ports: no improvement.
7. Removed OAT probe from upper right of windshield: no improvement.
The problem is, in general, worse at higher speeds. It will not happen at all when not moving. I also noticed that with the door off, it seems worse at higher RPM, even if I pitch up to keep the speed from increasing. I'm assuming this is due to the greater airflow past the airframe from the prop, but we didn't test this extensively. All this seems to point to some sort of wind noise problem, but #5 calls that into question, and it still doesn't answer the question of the loud background hiss.
So that's the story. All input is welcome. I am experienced mechanic, consider myself good with electricity, and am an A&P/IA, though not an avionics specialist. I mention that just so you can feel free to be as technical as you like; I can probably figure out what it means
.
Thanks!
Andy Young
I have a long-term intercom problem that is proving rather baffling to resolve. I'd love it if someone from this very savvy, experienced group had some useful insight. I've done a fair bit of diagnosis so far, so the full explanation will take a while. If you've decided to read on, you might want to grab a cup of coffee first, and settle in....
Shortly after I bought my M-6-235 (about three years ago) I installed a PS Engineering PM3000 four-place intercom. I did the install, with the assistance of an avionics shop in Colorado. I ran all the wiring harnesses, drilled holes, mounted the box, etc. They made the actual wire connections to the pins and the jacks. More or less immediately, I noticed that there was an issue with the co-pilot position. That position would break squelch much more easily than the pilot position, and when it did, the background noise was much higher. It's important to know that the PM3000 does NOT have separate squelch controls for each position; it has only one squelch knob. That adjusts for all four positions. Since it was pretty much always my girlfriend in the co-pilot seat, I assumed it was due to a difference in the impedance of our headsets. I have a Lightspeed Zulu (original, not "2") and she has a Bose X10. If I turn the squelch up really high, it mostly, but not completely eliminates the problem (it will still occasionally break squelch and have the loud background hiss). The downside of doing that is that it's really hard for me to break squelch to talk. I have to more or less yell. I did come up with a work-around that sort of works: I turn the gain all the way up on my Zulu mic and turn the squelch way up. That way I can break squelch to talk, and she is less likely to break squelch inadvertently. However, with my mic gain all the way up, the voice sound quality suffers measurably. I'd love to turn her mic gain down, but it's not adjustable. We sent her headset in to be checked, and Bose said it's fine.
Over time, we noticed that the problem mostly happens when she turns her head to the right, and/or gets it near the door/window frame. I do have opening windows. This spring, a friend from this list rode with me to Alaska and we noticed the same problem, though not quite as bad. He was using a Lightspeed Zulu 2, so the theory that it's her headset seemed to go out the window at that point. Shortly after arriving in Alaska, another friend rode with me and had the same problem; he was using a Bose A20. I've since confirmed the existence of the problem with an old set of non-ANR Telexes, and a set of Denalis (can't remember who made those).
I started thinking it must be a bad intercom. I described the problem to Northern Lights Avionics in Anchorage and they agreed. They helped me convince the factory to swap my unit out with a new one (or it might have been a refurbished one) but that didn't change the problem at all. I then brought the plane to Northern Lights for them to look it over while I was out of state for a while. They checked the jack connections, etc., but they didn't find anything. They ran the engine on the ground, but didn't fly it, so of course were not able to see the problem in action.
Today, I went out with a co-worker to run a bunch of tests to see if I could narrow it down a little. He was using the Denali headset. Again, we found that the problem mostly occurred when he turned his head to the right, or leaned close to the window. Turning right was more likely to produce the problem than just leaning right. Below are the various things we tried, and the results:
1. Swapped headsets (co-pilot wearing my Zulu and vice versa): mostly eliminated the problem.
2. Swapped jacks left to right (co-pilot wearing the Denali, but plugged into the pilot's jacks): no improvement
3. Pulled door and window tighter shut: this was ambiguous; it seemed to lessen the problem, though we could not consistently eliminate it. It certainly was true that if I had him position such that it wasn't breaking squelch and then pushed out on the top front of the door frame, it would break squelch.
4. Taped over top and front edge if door frame to airframe interface: no improvement.
5. Removed door completely: no or possibly slight improvement. One thing I did notice was that the background hiss I'd heard when it broke squelch was not the same as wind noise. With the door off, when he leaned or turned right a little, it would break squelch and have the same old loud, clean hiss it had always had. When he leaned out a little further, I could then hear the turbulent wind noise IN ADDITION to the background hiss.
6. Turned off lights, radios, GPS, transponder, pulled breakers for cigarette lighter ports: no improvement.
7. Removed OAT probe from upper right of windshield: no improvement.
The problem is, in general, worse at higher speeds. It will not happen at all when not moving. I also noticed that with the door off, it seems worse at higher RPM, even if I pitch up to keep the speed from increasing. I'm assuming this is due to the greater airflow past the airframe from the prop, but we didn't test this extensively. All this seems to point to some sort of wind noise problem, but #5 calls that into question, and it still doesn't answer the question of the loud background hiss.
So that's the story. All input is welcome. I am experienced mechanic, consider myself good with electricity, and am an A&P/IA, though not an avionics specialist. I mention that just so you can feel free to be as technical as you like; I can probably figure out what it means
.Thanks!
Andy Young
