Intermittent hot mike

Clark1961

Clark1961

Touchdown! Greaser!
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Intermittent hot mike, difficult to reproduce on the ground. Shut down audio panel and still a problem so seems to be "upstream" of the panel. New pilot's side PTT and switched headset. Problem remains on last flight test. Inspected wiring and nothing obvious but lots of wire impossible to see.

Rewire both PTT and all headset jacks? What's the best way to fix an intermittent problem like this?
 
When you say "hot mic" do you mean you're transmitting?

PTT is usually handled by a closure to ground. Look for chafed PTT wires that can contact metal.
 
denverpilot said:
When you say "hot mic" do you mean you're transmitting?

PTT is usually handled by a closure to ground. Look for chafed PTT wires that can contact metal.
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Yup, that's what we're trying to solve. Bare wires aren't apparent.

timwinters said:
Frankenkota
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And I thought we'd solved the problem of the aircraft doing strange things when the autopilot was overhauled...maybe "it's alive!"
 
Have you tried hitting it? Really hard? With a mag light?

If yes to all, I think you're SOL. Sorry.
 
The most aggravating one I've seen was in a new-to-me airplane, I had removed a side upholstery panel for the annual or something, and when it went back in I did not put all the mostly-identical 1" screws back where they came from. You see, one upholstery screw was 1/8" shorter than the others.
Use a long screw in the hole next to the pax mic jack and you ground the...is it tip? anyway, hot mic (just intercom) until you back that screw out and get it off the receptacle!
 
denverpilot said:
LOL. Is this SOP for the Piaggio? Percussive maintenance? ;)
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Hey, if preventative doesn't work for you, try percussive!

This goes back to the 1900 days. Light out? Gauge stuck (or 0)? Panel falls off? Generator drops to zero in flight? They're all fixed the same way, or they're not fixed at all!

One day in AUG I was showing a new guy around the Beech. His first day flying is tomorrow. It's his first time getting hands on the airplane since the sim, so we're doing a first-flight walk around and all that. Part of the first flight checks includes all the exterior lights. So I turn them all on and we start walking around. Right wing inspection light is out.
New guy: "So do we have to write it up or something?"
Me: [Punches cowling just above inop light. Light comes on.] "Nope!"
New guy: :yikes:


[/7500]
 
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Unplug hand mike(s)? Only have one thing plugged in at at time to diagnose? We had a problem like this once. We solved the immediate problem by unplugging everything except one hand mike. The problem was traced to the right side PTT switch but since someone else did the diagnosing I can't say how they figured it out. Makes sense though, given what the symptoms were.
 
Everskyward said:
Unplug hand mike(s)? Only have one thing plugged in at at time to diagnose? We had a problem like this once. We solved the immediate problem by unplugging everything except one hand mike. The problem was traced to the right side PTT switch but since someone else did the diagnosing I can't say how they figured it out. Makes sense though, given what the symptoms were.
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Everything unplugged, still transmitted. Pressed each PTT and it still transmitted. Turned off audio panel and it still transmitted. Turning radios off seems to help. It seems to me that'd have to be in the wiring or switches but nothing obvious is jumping out at us. Just a bit frustrating...
 
Teller1900 said:
Have you tried hitting it? Really hard? With a mag light?

If yes to all, I think you're SOL. Sorry.
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I'm not quite sure that I should admit that I don't own a mag light.

I was close to attempting percussive maintenance yesterday.
 
I'm guessing you already checked where the wires come out of the yoke shaft.
 
Everskyward said:
:confused: Hmmm... The airplane has an inner voice which wants to be heard? :idea:
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Quite possible. Maybe it should be renamed Teufel Frankenkota?

Henning said:
I'm guessing you already checked where the wires come out of the yoke shaft.
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The right side wire looks fine, what I can see of it. The left side is in a coil and is tougher to judge. Just voting on looks, I'd replace the coil before the other. Of course looks can be deceiving.

Moving the yoke does not reproduce the inadvertent TX on the ground. The TX sometimes happens while taxiing and sometimes on very short final and touchdown. It seems more common when slowing to flap speeds. I'm assuming that more vibration at low speeds causes the problem rather than just the flaps themselves since retracting flaps doesn't seem to affect the TX. There shouldn't be any system wires near the flap linkages anyway.
 
Clark1961 said:
Everything unplugged, still transmitted. Pressed each PTT and it still transmitted. Turned off audio panel and it still transmitted. Turning radios off seems to help. It seems to me that'd have to be in the wiring or switches but nothing obvious is jumping out at us. Just a bit frustrating...
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Wasn't your panel just all apart? Bent pin in the audio panel tray or if tray less, something loose back there? They had to wire Ms. Sweetness into the audio panel, right? Shorted wire to ground at the back of the audio panel?

(And turning the audio panel off probably won't stop it because the PTT will still be in "fail-safe" configuration to key COM 1 in most setups if the audio panel fails.)
 
Clark1961 said:
The TX sometimes happens while taxiing and sometimes on very short final and touchdown. It seems more common when slowing to flap speeds.
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It sounds like back pressure on the yoke is the common thread.
 
Some ideas:

Many airplanes have a mic jack wired "behind" the intercom, often left over from the days before headsets and intercoms were common. If your airplane has one of these I'd take a close look at it and it's wiring.

I didn't see anything in your post to indicate if one or both radios transmit inadvertently. If both do at the same time the problem almost has to be in the audio panel or "upstream" of that. Same if only the radio selected on the audio panel for transmitting acts wrong. If only one radio screws up regardless of which is selected on the audio panel, chances are the problem is downstream of the audio panel (i.e. between it and the radio).

Try pulling the audio panel out of it's tray (or disconnect the connectors attached to it) and see if you can get the problem to recur. If it still happens the problem is in the wiring between the audio panel and the radio.
 
denverpilot said:
Wasn't your panel just all apart? Bent pin in the audio panel tray or if tray less, something loose back there? They had to wire Ms. Sweetness into the audio panel, right? Shorted wire to ground at the back of the audio panel?

(And turning the audio panel off probably won't stop it because the PTT will still be in "fail-safe" configuration to key COM 1 in most setups if the audio panel fails.)
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I don't know if you panel contains your headset jacks but if it does and if it has indeed been apart recently check to see if the phenolic isolating washers are on both mike jack plugs.

Just something else to check...
 
If you've been able to get it consistently to happen on the ground and stay keyed, it would seem that right at that point some quality time with a continuity test would find out which PTT lead is shorted to ground.
 
denverpilot said:
If you've been able to get it consistently to happen on the ground and stay keyed, it would seem that right at that point some quality time with a continuity test would find out which PTT lead is shorted to ground.
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The major problem is that so far we (Ron & Bill and I) have not been able to reproduce it on the ground.

The inadvertent TX can be switched from Com 1 to Com 2 and it keeps doing it with the audio panel switched off.

The problem developed pretty much out of the blue. Have had recent maintenance but nothing immediately before the problem started.

Anybody got a pin diagram for an 8000B audio panel? I'm thinking that it's time to disconnect PTT switch wires to see if the problem can be isolated that way.
 
Clark1961 said:
The major problem is that so far we (Ron & Bill and I) have not been able to reproduce it on the ground.

The inadvertent TX can be switched from Com 1 to Com 2 and it keeps doing it with the audio panel switched off.

The problem developed pretty much out of the blue. Have had recent maintenance but nothing immediately before the problem started.

Anybody got a pin diagram for an 8000B audio panel? I'm thinking that it's time to disconnect PTT switch wires to see if the problem can be isolated that way.
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Go to PS Engineering's website, has everything. If you still need more help, call them. One of the owners used to hang out on rec.aviation.* and answered questions, took ideas for new features.
 
Talked problem over with avionics tech. Went through the list of symptoms. Tech was fairly certain the problem is in either the coiled bundle on the pilot's yoke or in the handheld mike plug wire.

Key symptom seemed to be that the hot mike condition continued even with the audio panel switched off.

Visit to avionic tech scheduled for next Monday.
 
Nope. They'll never get my business again.
 
denverpilot said:
Ooh. They ticked you off?
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That'll happen when I get charged twice to fix a transponder problem and it still wasn't fixed. They had the gall to blame ATC radar. The next shop said the problem was obvious...
 
Clark1961 said:
That'll happen when I get charged twice to fix a transponder problem and it still wasn't fixed. They had the gall to blame ATC radar. The next shop said the problem was obvious...
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Ahh. Yeah. Hmm.

I did try to ring ya and left a VM. Feel free to disregard. ;)
 
Problem fixed. Turned out to be the wire from the pilot's side mic jack to the audio panel. The wire was installed new just over a year ago.

Had trouble locating problem (of course) and ended up replacing the coil cord on the yoke. Nearly an AMU total. Intermittent problems suck...
 
Ahh good.

The airplane is ready to go to Gastons now, so your plans have changed, right? ;)

Poke. Poke. ;)
 
Meetings in WWW on Thursday. 3M0 is unlikely...
 
kimberlyanne546 said:
We were supposed to fly together in Michigan, silly.
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At least the 'kota is back to being airworthy these days...

(it's in pretty good shape right now, a couple gripes but nothing major - still carrying lots of batteries for the handheld though)
 
Clark1961 said:
At least the 'kota is back to being airworthy these days...

(it's in pretty good shape right now, a couple gripes but nothing major - still carrying lots of batteries for the handheld though)
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So then you're flying to California - when ?
 
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