Broken & Unreadable on Certain Frequencies

It sounds decent in transmit. But the tower transmissions sounded really weak and almost unreadable. Does your headset michrophone have a gain adjustment on the mike itself ?????. It sounded almost as though you were yelling into it.
 
Vaflier said:
It sounds decent in transmit. But the tower transmissions sounded really weak and almost unreadable. Does your headset michrophone have a gain adjustment on the mike itself ?????. It sounded almost as though you were yelling into it.
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The tower sounds weak because that was recorded off of a scanner that is far away from the airport and I was in the air so it could hear me better than the tower. None of my headsets have any kind of Mic gain and adjustment.
 
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itsjames2011 said:
We repeated the test with the airplane running and the RPM's at 2000. Now the frequencies that I know are bad are broken and unreadable, the frequencies that I know are good are clear and random frequencies are also clear.

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So the first time you did the test on the ground with engine running, you were at idle and all was well. The second time you did it, you were at 2000 RPM and the problem was there. The only difference in set up was engine RPM??
 
Could the frequency be just a tad off? Some interfacing radios may tolerate it an others not. Unicom here used to have frustration with one particular aircraft (broken, unreadable). His response was always "Well, I talked to so and so just fine". We swapped out the base station with the spare with no change. Eventually, his radio quit and was replaced. No more problem.
 
SkyDog58 said:
So the first time you did the test on the ground with engine running, you were at idle and all was well. The second time you did it, you were at 2000 RPM and the problem was there. The only difference in set up was engine RPM??
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Correct, the only difference was the RPM. Ive put three separate radios in that slot and I've checked them all with an external frequency counter.

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itsjames2011 said:
Correct, the only difference was the RPM. Ive put three separate radios in that slot and I've checked them all with an external frequency counter.

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I guess we need to define "checked them" as regards frequency. The fact of the matter is that the limit on frequency is very roughly +/- 3kHz (0.0025% or 25 Hz/MHz)at band center. Thus, Westfield Tower (118.9) would have to read between 118.897027 and 118.902972 to be within limits. Mostly we do 10 kHz. filters these days which is +/-5 kHz to give us the minus tolerance on one end and the plus tolerance on the other end.

I don't know if you (a) wrote down the results of the frequency counter and (b) verified the calibration of the counter itself. Please comment.

Thanks,

Jim
 
weirdjim said:
I guess we need to define "checked them" as regards frequency. The fact of the matter is that the limit on frequency is very roughly +/- 3kHz (0.0025% or 25 Hz/MHz)at band center. Thus, Westfield Tower (118.9) would have to read between 118.897027 and 118.902972 to be within limits. Mostly we do 10 kHz. filters these days which is +/-5 kHz to give us the minus tolerance on one end and the plus tolerance on the other end.

I don't know if you (a) wrote down the results of the frequency counter and (b) verified the calibration of the counter itself. Please comment.

Thanks,

Jim
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I did not write them down but I can tell you that the little deviation that I did see here and there was les than 1khz which is not bad at all given the age of these radios. To answer your second question I don't know when it was calibrated.

For what it's worth I bought and IC-A14 handheld and I will be hooking it up to one of the external comm antennas today so at least I can get in and out of controlled fields while we sort this out however it needs to be short lived because while being down to portable gear I have to remain VFR ><

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itsjames2011 said:
Here is a youtube video from worcester tower a few weeks ago. In the morning I will go flying and get some more since the audio archives don't go back far enough for me to find more :D

http://youtu.be/C8tYhfce3RE
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Ah, but Worcester Tower sounds just fine on the YouTube audio but they report broken at the tower itself. Methinks that perhaps we DO have a out of band problem, but why would three separate radios have the same problem. THis is getting worrisome.

I'm just wondering ... since we have no problem with the engine either off or at low idle, but a problem at higher RPM if something doesn't like a high battery voltage. Just a thought ... we might think about getting a battery charger onto the battery and see if charge voltage is the problem.

I'm also thinking ... what is there in that audio panel that would put crap out onto the mic line, but why would it do it at spot frequencies when the audio panel has nothing to do with frequency.

I'm still thinking a combination of frequency tolerance AND overmodulation is giving us a problem, especially since the YouTube audio was clean on the scanner that was feeding the YouTube audio.

BUT, you tell us that the handheld was reporting the same problem, which may well be related to charge voltage ... strange problem...

Jim
 
weirdjim said:
Ah, but Worcester Tower sounds just fine on the YouTube audio but they report broken at the tower itself. Methinks that perhaps we DO have a out of band problem, but why would three separate radios have the same problem. THis is getting worrisome.

I'm just wondering ... since we have no problem with the engine either off or at low idle, but a problem at higher RPM if something doesn't like a high battery voltage. Just a thought ... we might think about getting a battery charger onto the battery and see if charge voltage is the problem.

I'm also thinking ... what is there in that audio panel that would put crap out onto the mic line, but why would it do it at spot frequencies when the audio panel has nothing to do with frequency.

I'm still thinking a combination of frequency tolerance AND overmodulation is giving us a problem, especially since the YouTube audio was clean on the scanner that was feeding the YouTube audio.

BUT, you tell us that the handheld was reporting the same problem, which may well be related to charge voltage ... strange problem...

Jim
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I don't know if it's of any relevance and its probably a longshot but the Collins AMR-350 audio panel has a local oscillator in the marker beacon receiver. If strong VHF audio was mixing with the local oscillator of the marker beacon receiver, couldn't that intermodulation cause frequency-dependent audio distortion?

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itsjames2011 said:
I don't know if it's of any relevance and its probably a longshot but the Collins AMR-350 audio panel has a local oscillator in the marker beacon receiver. If strong VHF audio was mixing with the local oscillator of the marker beacon receiver, couldn't that intermodulation cause frequency-dependent audio distortion?

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But you took care of that problem when you turned off ALL the electronics in the airplane except the one radio. Or can you not turn the marker beacon off if the audio panel is powered?

AND, I keep coming back to the YouTube audio that was just fine on the scanner but broken/inaudible in the tower AND broken/inaudible on your handheld at the far end of the field test. That's the crux of this issue right now for me.

Jim
 
weirdjim said:
But you took care of that problem when you turned off ALL the electronics in the airplane except the one radio. Or can you not turn the marker beacon off if the audio panel is powered?

AND, I keep coming back to the YouTube audio that was just fine on the scanner but broken/inaudible in the tower AND broken/inaudible on your handheld at the far end of the field test. That's the crux of this issue right now for me.

Jim
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No way to turn off the audio panel :(

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itsjames2011 said:
No way to turn off the audio panel :(

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There has to be, even if it means pulling a breaker. Now if the radio is also connected to that breaker, you may have a problem.

HOWEVER, I'm waiting for more audio to listen to AND whether or not you can get a battery charger to put out the voltage that your aircraft alternator puts out when charging.

Jim
 
I went flying tonight and unfortunately my radio was not captured by liveATC it all. I guess I'm going to have to go back to Worcester which is about 40 miles from here because the live ATC scanner is close to the airport.

In other news I bought an Icom IC-A14, hooked it up to the external antenna that used to be connected to comm2, and it works great. Not a fix but at least makes the airplane usable while we continue to diagnose. If I didn't have the intention of using the airplane IFR, this would be the end of the radio concern :)


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Reading through this, I'd want to bypass that audio panel completely and as Jim said, power it off. All roads other than the different frequency thing seem to lead back to it.

Additional questions: Hand mic... Is it an old carbon style? And was the hand mic plugged into the same jack the headset usually is?

I'm leaning toward "bad voltage regulator in the audio panel", since that would make an electret mic go quite screwy. That guess is less strong than "related to audio panel".

Most audio panels have a failsafe mode if off that's supposed to directly connect the audio paths to Comm 1. Does that one do it when off?

Haven't seen any notes if you've simply had a look at the ship's power in any way. Voltage right? Got a scope handy to see if there's a mess of noise on the DC bus?

(Granted having a scope handy isn't common, but I have more than one, so that's an easy check for me and eliminates a lot of stuff quickly when it comes to noises in things that shouldn't be noisy.)
 
Sorry for my lack of upkeep with this thread, I was out of town for a bit. I do have an oscilloscope and I'm going to go conduct all of the rest of the tests in the next day or two and I will keep you posted

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