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Comm failure

rvdave

Well Known Member
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.
 
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.
 
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.

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.
 
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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