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TL;DR Laptop keeps switching between Signal Link audio output and speakers. Problem goes away when I turn TX knob down far enough on Signal Link.

I just bought a SignalLink and have it connected between my laptop and my radio (a Yaesu FT-450D). I've successfully made a couple QSO's on FT8, but keep running in to a problem.

When I tune the tx knob too high, the computer seems to change the audio output from the Signal Link back to the speakers on the laptop. As a result, the Signal Link doesn't get an audio signal any more and it turns off the PTT. After a moment (well less than a second), audio switches back to the Signal Link.

I can avoid the problem by keeping the TX knob turned way down, but them my actual transmitted power is way lower than intended.

Details: Radio: Yaesu FT-450D Laptop: old Dell running Ubuntu 18.04

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  • $\begingroup$ The latest WSJT-X FT4 software has a bug that messes with the audio levels. If you are using the FT4 compatible software, I suggest you use the earlier released version. $\endgroup$ – Cecil - W5DXP May 31 at 18:51
  • $\begingroup$ I will try that. What's odd is that I don't have the problem testing with PSK31 in FLDigi or when using the Tune button in WSJT-X. $\endgroup$ – Adam KC0DAD May 31 at 20:50
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I'm not familiar with the failure modes of SignaLink devices, but in general, a USB device disappearing temporarily likely means that its internal microprocessor or USB interface chip has been reset, or the USB connection itself failed.

The obvious hypothesis, then, is that your transmitted RF power is itself causing the problem by interfering with the digital electronics when it gets too high.

To collect evidence for whether or not this is true:

  1. Check if the radio's transmit power setting also changes whether or not the problem occurs.
  2. Make a manual transmission using your radio's own controls, not the computer (using CW or AM, not SSB, so that you don't need to change microphone connections to get any output power). If you observe your computer switching audio devices in the same way, this shows that the problem is triggered by the radio and not by the SignaLink unit.

If this is the case, then solutions depend on how the problematic RF is getting in. That can be difficult to measure, so practically it's a matter of trying fixes in whatever order is convenient:

  • Problem: The transmitted power from the antenna is too high.
    Solution: Relocate your antenna farther from the rest of your station.
  • Problem: The antenna is putting common-mode current onto the feed line.
    Solution: Ensure that it is fed properly (e.g. a dipole antenna fed by coax must have a balun).
  • Problem: The transmitter is leaking RF onto its audio/control cables.
    Solution: Add ferrites to the cables between the radio and the SignaLink.
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  • $\begingroup$ Also, if you can by any means, try switching out the USB cable and try using another USB port. If the device is powered via USB, but can also be powered through an external plug, try that. $\endgroup$ – Marcus Müller May 31 at 7:51
  • $\begingroup$ This happened to me (radio interfering with usb). I fixed it by wrapping the usb cable 4 times around a #43 toroid. Alternately, they make usb cables with ferrite chokes built in. $\endgroup$ – Chris K8NVH May 31 at 11:12
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    $\begingroup$ I've checked item 1. Bumping power up on the radio allows a higher setting on the TX knob on the Signal Link. I would have expected the reverse. I'll try to test number 2 in the next couple days. $\endgroup$ – Adam KC0DAD May 31 at 21:05
  • $\begingroup$ I’ve confirmed that it is related to transmit power. Anything over 10W will definitely make it cut out. Seems to vary between 5 and 10. Still working on the source. $\endgroup$ – Adam KC0DAD Jun 7 at 18:09

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