I have recently bought second hand MFJ-939Y for FT-450/950. I have Yaesu FT-450 (plain, without D or AT). Since the tuner did not come with a cable, I have built my own. (I am 33yo IT engineer with experience in building custom electronics, so no problem in quality here.)

The tuner was reset to factory defaults.

Jumpers were configured for: Yaesu, pull up resistor, external power.

Now I am facing strange issue:

  • If the antenna is NOT YET tuned and I press the TUNE button on the FT-450, the tuning sequence starts and finishes sucesfully.
  • If the antenna is ALREADY tuned and I press the TUNE button on the FT-450, the tuner correctly does not do anything. However on the FT-450 the TUNE symbol keeps blinking and eventually disappears. After that, the tuner cannot be enabled on the FT-450 by the TUNE button until I powercycle the tuner.

My assumption is that when the tuning sequence in the tuner is running, the tuner communicates back to the radio over the serial line. When there is nothing to be tuned, the tuner remains silent on the serial line. That makes the radio think the tuner communication was lost and disables the tuner as faulty.

Thank you for any suggestions how to overcome this issue.

EDIT: Just noticed that when I turn the tuner off and press the TUNE button on the radio in order to disable the function, the TUNE will start blinking exactly the same way. If anytime during the blinking I power the tuner back on again, the TUNE will disappear right away. So simply put the TUNE blinking means the serial communication is lost.

  • $\begingroup$ Are you sure your FT-450 does not have the internal tuner? $\endgroup$
    – Brian K1LI
    Commented Mar 19, 2021 at 21:01
  • $\begingroup$ 100% sure it has not $\endgroup$ Commented Mar 21, 2021 at 7:40

1 Answer 1


I found a workaround for this problem:

You can assign SWR function to your CS key, pressing it will trigger an SWR test, which sends a low wattage carrier signal to the tuner and triggers an auto tune. This basically does the exact same thing, with one exception, on the FT-450 with the cable installed disables the onboard tune button of the tuner. Using the CS key leaves the manual tune function enabled on the external tuner.

If there are better solutions, I'm curious to hear about them,


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