FT-747GX: No TX above 21.5 MHz

[W3KEV]

I'm having an issue with my Yaesu FT-747 GX. It will not transmit above 21.5 MHz. It works fine on all other frequencies. Before I crack it open and poke around I was wondering if anyone has been down this road before. Thanks. ???javascript:void(0);

[NO2A]

The band ends at .450. Why do you want to transmit above .500?

[KA5N]

I assumed he meant the bands above 21.5.  Often posts don't include nearly enough
information to enable readers to give useful information.

Allen

[WA0ZZG]

You left out some needed information.  There are two places you need to look:
1. If the frequency display is flashing, check the voltage controlled oscillator calibration for the band not working.  It's part of the synthizer.
2. If the receiver is OK, and display not blinking, start checking the low pass filter for the band not working. Especially the two relays that will switch that filter in line.
I don't have this radio, but some parts of moden day HF radios are common.
Good hunting.
Dave
WA0ZZG

[W3KEV]

thanks Allen and Dave. I'll start looking there.

[W3KEV]

well, i hear the relay click when i transition between bands (eg. 21.498, 21.499, "click" 21.500.) The RX is OK on all freqs. That tells me something. I'm assuming there's a leaky pipe somewhere (hi hi) I still haven't cracked the sucker open yet. Maybe this weekend if it rains.

[KB9JXZ]

 Sounds like it may be the low pass filter board. I would check relay 11 and 12 for 12 volts when above 21.5 Mhz  ( P-03 pin 7 ), also you could check from the low pass filter board input to the normally open contact at relay 13 ( or the proper side of TC-01 or C-38 to see if you have continuity through the filter / or from the input of the low pass filter board to ground and see if a cap shorted.
 You could also test for output right at J-02 of the 100 watt PA board.

 This link may help.  http://www.radiomanual.eu/schemi/FT747_sch.pdf

 Let us know what you find.
 73

[KB9JXZ]

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