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Yaesu Support recently confirmed the following procedure I’ve been using for calibrating the FT-991/FT-991A frequency accuracy against WWV:

(1)  Turn on the transceiver for at least 30 minutes

(2)  Turn off both CLAR and SPLIT

(3)  In VFO-A (not MEMORY) and in AM mode, tune in a strong WWV signal during carrier-only transmission (minutes 43-53)

(4)  Be sure all digits trailing the MHz value are set to zero

(5)  Change mode to CW-LSB

(6)  Tap ZIN to initially zero rig against WWV

(7)  Tap LOCK to avoid changing frequency

(8)  Turn off both BK-IN and KEYER

(9)  Key the sidetone (straight key or paddle) and adjust AF gain so incoming WWV signal is equal to the sidetone volume

(10)  Go to REF FREQ ADJ (menu 39) and press SELECT

(11)  Press the key to generate the sidetone and turn MULTI knob back and forth while noting the wavering of the beat note

(12)  Some value between -25 and +25 in menu 39 will result in a non-wavering beat note indicating the radio is zeroed with WWV

(13)  Press ENTER to lock in the setting, and press BACK to exit the menus

I re-run the procedure every six months or so.

WWV minutes other than 43-53 can be used as long as you are sure you are beating against the WWV carrier and not one of the other tones.


Kent Trimble  K9ZTV

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