Bizarre Intercom Problem

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Andy Young
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Bizarre Intercom Problem

Post by Andy Young »

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
Last edited by Andy Young on Sun Aug 24, 2014 7:43 pm, edited 1 time in total.

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Andy Young
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Post by Andy Young »

One point I missed in the above: I can't honestly say if the problem existed with the original intercom or not. I didn't have it in there long enough to tell, and it was so long ago. I don't remember it doing it, but....

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maules.com
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Post by maules.com »

I have had problems with various ANR/ANC headsets when turning head to window. I believe a gap is opened between the bottom of the headset earseal and the prominent vertical muscle on side of neck.
Another item to look at is the copilots mike jack for contacts connecting at the correct terminalson the mike pin.
Same thing re the rear unused jacks which become loose and contacts foul the back of the side panels the are fastened to.
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Andy Young
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Post by Andy Young »

Thanks Jeremy,

It's happening with both ANR and non-ANR headsets. It also does it regardless of which jacks it's plugged into. Position in the plane seems to be the important part. I know NLA checked the pilot and copilot jacks. Not sure about the rear ones, so I'll have a look at them.

Andy Young

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TomD
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intercom

Post by TomD »

This may be totally off base but I put new radio and intercom in and had problems with feedback and it was traced to the side tone setting on radio. Does not sound like same problem but possibly a straw to grasp.

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taildragger
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more to think about

Post by taildragger »

Hi,

I have a PS intercom also. Was in already when I purchased my M5. I have noticed the
exact same thing you are describing although not as bad. I always thought it was my wife turning her head and getting the mic close to the vent. I was at one time having it break squelch a lot and getting worse until I noticed it doing on full power t/o then getting a little better in cruise. Replaced the mag filters and now it is slightly noticible like you describe but not bad. Good luck.

Scott

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Post by pilot »

Andy, it sounds to me like you have done everything to isolate this problem. You have narrowed it down to either a specific noise level or noise frequency that is being picked up in that location OR a specific interference (RF/EMF/static discharge) causing you to break squelch.

Where are your antennas located? Have you looked at their wiring lately? Are your static wicks in good shape? How old are your p leads and what shape are your mag harnesses in? Also it could be a resonance picked up from an air noise in a different location - but the whole door removed thing probably ruled that out.
I can't remember if I fired six shots, or only five.....


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Andy Young
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Re: intercom

Post by Andy Young »

TomD wrote:This may be totally off base but I put new radio and intercom in and had problems with feedback and it was traced to the side tone setting on radio. Does not sound like same problem but possibly a straw to grasp.

Thanks. I turned all the radios off, and the problem didn't change, so I'm thinking that eliminates that.

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Re: more to think about

Post by Andy Young »

taildragger wrote:Hi,

I have a PS intercom also. Was in already when I purchased my M5. I have noticed the
exact same thing you are describing although not as bad. I always thought it was my wife turning her head and getting the mic close to the vent. I was at one time having it break squelch a lot and getting worse until I noticed it doing on full power t/o then getting a little better in cruise. Replaced the mag filters and now it is slightly noticible like you describe but not bad. Good luck.

Scott

Thanks Scott,

That's a good clue! I'll have a look at the mag filters.

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Andy Young
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Post by Andy Young »

pilot wrote:Andy, it sounds to me like you have done everything to isolate this problem. You have narrowed it down to either a specific noise level or noise frequency that is being picked up in that location OR a specific interference (RF/EMF/static discharge) causing you to break squelch.

Where are your antennas located? Have you looked at their wiring lately? Are your static wicks in good shape? How old are your p leads and what shape are your mag harnesses in? Also it could be a resonance picked up from an air noise in a different location - but the whole door removed thing probably ruled that out.
Thanks. I thought of the antennas, and there are two right above that door, but I turned off all the radios and the GPS, so I'm thinking that eliminates that. Am I wrong there? P leads and mag harnesses seem good, but I'll give them a more detailed check. Or perhaps I could try shielding the headset with aluminum foil to see if it's an electronic interference issue.

Static wicks? On a Maule? Does anyone have these?

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Post by aero101 »

Dumb question, but I assume all mics have mic muff installed?
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Andy Young
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Post by Andy Young »

aero101 wrote:Dumb question, but I assume all mics have mic muff installed?
Not a dumb question at all. They do, and I've even tried a the really nice leather ones on my girlfriend's Bose, with no difference noticed.

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Post by pilot »

Mine has them(wicks), not sure if they were factory. If nothing else works fashion the aluminum foil into a pyramid-shaped hat and see if that may help :o
I can't remember if I fired six shots, or only five.....


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Post by Andy Young »

One more key clue that I forgot to mention:

Unplugging the mic plug from the jack eliminates the problem. So something (either sound/pressure impulse or electronic interference) is coming through the mic circuit. Can the mic even pick up electronic interference?

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Post by maules.com »

Was your cellphone charger plugged into power socket?
Still suspicious of the mike socket contacts!
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