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  #1  
Old 04-11-2021, 10:40 AM
rvdave rvdave is offline
 
Join Date: Jun 2005
Posts: 447
Default Comm failure

I have an ifd550 nav/com which I notice most recently has a comm failure at certain frequencies, I’ve narrowed it down to between 132.2 thru 135.6 will trigger comm failure after about 2 seconds of mic keying. I can repeat this continually when it is failing but sometimes it will allow me to get enough in when talking to center frequencies on around 132.9. At first I thought this could be a connection problem but now that I’ve got it down to a region of frequencies I’m thinking maybe antenna or swr problems. Any ideas how to troubleshoot further? All other frequencies will allow me to transmit fully.
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  #2  
Old 04-11-2021, 02:31 PM
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bjdecker bjdecker is offline
 
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 626
Default

I am not familiar with that COMM radio, but it might be a problem with SWR at those frequencies -- Has the cable, connectors, or antenna itself changed recently (become damaged, etc.?)

An easy thing to check is the SWR on the antenna/cabling -- see MFJ 259-D as an example test unit.
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2020 RV-14 QB -- Under construction - Tailcone & Empennage Complete.
2018 RV-7 QB -- Built, Flying
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  #3  
Old 04-11-2021, 03:43 PM
Steve Iacoviello Steve Iacoviello is offline
 
Join Date: Apr 2015
Location: The Woodlands, TX
Posts: 44
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Quote:
Originally Posted by rvdave View Post
I have an ifd550 nav/com which I notice most recently has a comm failure at certain frequencies, Iíve narrowed it down to between 132.2 thru 135.6 will trigger comm failure after about 2 seconds of mic keying. I can repeat this continually when it is failing but sometimes it will allow me to get enough in when talking to center frequencies on around 132.9. At first I thought this could be a connection problem but now that Iíve got it down to a region of frequencies Iím thinking maybe antenna or swr problems. Any ideas how to troubleshoot further? All other frequencies will allow me to transmit fully.
I saw something similar with a SL-40. It turned out that a high resistance avionics master switch was causing low voltage to the transmitter resulting in no transmit in a specific frequency range. Strange that it was not all frequencies but as I understand the transmitter can pull more amps on specific frequency ranges. Replaced the switch and problem went away. I agree that this situation was rare but easy to check.
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  #4  
Old 04-11-2021, 04:19 PM
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Carl Froehlich Carl Froehlich is offline
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
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Quote:
Originally Posted by Steve Iacoviello View Post
I saw something similar with a SL-40. It turned out that a high resistance avionics master switch was causing low voltage to the transmitter resulting in no transmit in a specific frequency range. Strange that it was not all frequencies but as I understand the transmitter can pull more amps on specific frequency ranges. Replaced the switch and problem went away. I agree that this situation was rare but easy to check.
Exact same symptom if either terminal screw is loose on the associated breaker terminals.

Carl
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