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  #1  
Old 08-25-2017, 01:07 PM
todehnal todehnal is offline
 
Join Date: Jan 2005
Location: Kentucky Lakes area in KY
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Default Noisy Comm "Buzz"

To start simple, if I leave the SL40 off, the intercom is dead quiet with no buzz and works great. When I turn the SL40 on I immediately get a buzz that is sympathetic with RPM. In fact, at low RPM the buzz is loud and at a low frequency. As I increase the RPM, the buzz is less noisy, but the frequency increases with the RPM. My voltage stays constant at 13.8 volts. In flight transmission are acceptable, but at idle, I get a very strong side tone feedback. Also, I have been told that this buzz is actually transmitted along with my voice.

I first thought that the RPM associated buzz was coming from the tach, so I added a 30K resister in the feed line from the RPM trigger coil, but it had no effect. My guess is that it is coming through the charging circuit, but I'm not sure how to isolate it as a test. I thought of pulling the 30 amp generator main buss fuse, but it seems like I read, or heard that this could damage the VR. My thoughts are that the main battery would still run everything while I test for the buzz. Could the VR be going bad, even though I still have solid voltage and charging, and it physically looks healthy? Any experience, or thoughts would be greatly appreciated..........Tom
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  #2  
Old 08-25-2017, 02:10 PM
rv7charlie rv7charlie is offline
 
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Location: Pocahontas MS
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Default

Unplug the alternator connector. Unplug the regulator connector as well, if you're worried about damaging it. Crank the engine & go through the steps you've mentioned, to see if the noise is still there, or gone.

That will tell you if the charging system is the source of the noise, but it won't give a definitive answer on whether it's defective. If you have a bad ground in your antenna circuit, it could be allowing 'normal' alternator noise to enter the radio's circuitry.

You can also remove the antenna coax from the radio, & substitute a known-good run of coax to a 'rubber ducky' antenna from a handheld radio, & repeat the test.

It's possible the comm itself could have an issue, but that's not as likely as the other items. Substituting a known-good comm would be the easy way to check that.

Oh, make sure that the radio is fully seated in the tray, and that no pins have backed out of the connector on the back (hard to tell by just looking).

Charlie
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  #3  
Old 08-25-2017, 02:48 PM
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Flyin'Bryan Flyin'Bryan is offline
 
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Location: Littleton, Colorado
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Default Another test....

if your charging circuit tests do not identify or solve the problem, here is one more thing to check. I recently experienced a major comm problem in a C-182 that turned out to be a bad P-lead connection/ground to the ignition switch. So if you have dual mags, switch to one or the other and see if anything different happens. The point being that this type of problem could also be coming from a bad ground situation with the ignition switch. In my situation the noise got so bad that you could hardly hear any of the radios through all the noise.

Just a thought....
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  #4  
Old 08-25-2017, 03:23 PM
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Raymo Raymo is offline
 
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The fact that it only affects the SL40 tells me it is not a charging system problem, per se. The idea of turning other things off to diagnose is a good idea. I'd suspect a weak ground relative to that radio. Ensure all grounds have < .5 Ohms. Also try swapping antenna inputs (assuming you have 2 radios) to see if the problem moves.

I'm sure other ideas will come along soon.
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  #5  
Old 08-25-2017, 03:55 PM
todehnal todehnal is offline
 
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Location: Kentucky Lakes area in KY
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Quote:
Originally Posted by rv7charlie View Post
Unplug the alternator connector. Unplug the regulator connector as well, if you're worried about damaging it. Crank the engine & go through the steps you've mentioned, to see if the noise is still there, or gone.

That will tell you if the charging system is the source of the noise, but it won't give a definitive answer on whether it's defective. If you have a bad ground in your antenna circuit, it could be allowing 'normal' alternator noise to enter the radio's circuitry.

You can also remove the antenna coax from the radio, & substitute a known-good run of coax to a 'rubber ducky' antenna from a handheld radio, & repeat the test.

It's possible the comm itself could have an issue, but that's not as likely as the other items. Substituting a known-good comm would be the easy way to check that.

Oh, make sure that the radio is fully seated in the tray, and that no pins have backed out of the connector on the back (hard to tell by just looking).

Charlie
Okay! My intent in the morning is to disconnect the 2 leads from the engine trigger coil. That should isolate the charging circuit. then start the engine and check out the noise. All that should be left is the battery power, unless my problem is in the SL40. Sound like a plan? Tom
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