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I have an older VAL 760 TSO COM radio. Twice now the radio has blown an internal radio fuse on transmit. VAL has tested and certified the radio as "Good", but advice that the cable or antenna could be faulty. How does one test that the antenna or cable is adequate? Can one use an ohm meter to check for resistance or continuity? Any other advice is appreciated. I have a newer VHF antenna. It may be a CI-101, although not positive.

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  • $\begingroup$ Just a suggestion: If the cable/antenna was wrong, the fuse would probably have no time to blow, that would be the power amplifier transistors. In addition if they are protected against impedance mismatch, nothing should happen except a bad reception and bad transmission. So, as the unit has been checked, I would now check the other connections, specially any existing DC output (to power external devices or switch), as this could be a short-circuit between +Vcc and the ground. $\endgroup$ – mins May 2 '17 at 6:30
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Your radio shop may be able to test the SWR of the antenna working with the COM radio. That would be my starting point. However, sometimes problems like these are intermittent and difficult to isolate.

You can test the antenna with an ohm meter but that will not measure the impedance at transmit frequencies. An SWR meter will.

Unfortunately, unless your other hobby is ham radio, you may not have the equipment on hand to check out transmitter operation. But your radio shop will.

They will also have a dummy load for testing the transmitter.

I have to say that I am surprised that the unit blew a fuse. Most aviation radios I have experience with (I am not an avionics tech only a ham) have protection against shorts or opens on the output of the transmitter, which usually shut down the power of the transmitter, for example when a high reflected signal comes back. So blowing fuses is normally extremely rare.

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