FT-991 AM transmission audio trouble

AM transmission audio trouble

the problem
Information can be found about the bad transmitted AM audio quality on the internet. To be sure that’s true, I verified this with my FT-991. And the AM audio quality is indeed quite bad. The FET’s from the HF power amplifier of my FT-991 are already replaced (at the Yaesu service centre) and therefore there isn’t a relation between the bad finals and bad AM
audio quality. Based on my “research” I can conclude that the solution is quite simple and can be done yourself.

verification of the facts
As a start I wanted to be sure that the AM audio quality is indeed bad. The FT-2000 was set in receive mode and a dummy load is connected to the HF antenna port of the FT-991. Both radios are set to the same frequency. On all the HF bands, the AM audio quality was bad. Transmission power and other settings didn’t contribute to better audio.



the cause
The cause of bad AM audio turned out to be a incorrect level setting in the service menu. Service menu item 18-2 is the level setting for the AM carrier level. If you have bad transmitted AM audio quality with your FT-991, it’s likely that the level setting is too high. (My radio was set to “066”.) If the original service procedure is applied, the resulting AM audio is not good. It’s likely that the adjustment of the radio is done correctly at the factory by the instruction, but that the instruction isn’t correct. (According to the service information, the S-meter should be set to “7” during transmission in the service mode at 14.200MHz without modulating.) If the AM carrier level setting is too high, the result is bad audio. I think that some FT-991 circuit is overdriven with bad AM transmission as a result.

the solution

The solution is quite simple and can be done yourself. The transmitted AM signal (and resulting audio) will be good if the AM carrier level of the FT-991 is reduced. The procedure I recommend is described below. Beware that the described method is different than Yaesu describes. According to me, it works great, but keep in mind that I’m not a authorised service engineer. Note, if you change a wrong setting in the service menu (by accident), press the on/off button to turn off the radio. The settings won’t be saved unless the [MENU/SETUP] button is pressed.

- Turn the FT-991 off and connect a dummy load to the HF antenna port;
- Press the [A/B]+[A=B]+[FAST] buttons together and hold them;
- Turn on the FT-991 while still pressing the three buttons;
- The FT-991 is now in service mode;
- Beware not to change anything except mentioned here.

- Rotated the [MULTI] knob clockwise until option 18-02 (AM carrier level) is shown;
- Make a (backup) note of the original value in your owners manual;
- Change the frequency to 14.200MHz by rotating the main dial;
- Press the PTT knob on the microphone without speaking in the microphone;
- Watch the S-meter for the reading, the original setting should be approximately S-7;
- Release the PTT knob.
- The carrier level of menu item 18-02 can be changed by the [CLAR/VFO-B] knob;
- Press the PTT button and speak in the microphone;
- Listen to the transmitted audio signal on the receiver;
- It’s likely that the audio sounds bad (distorted/overmodulated);
- Whistle into the microphone (to achieve maximum modulation) and reduce the AM carrier level by rotating the [CLAR/VFO-B] counterclockwise untill the S-meter doesn’t show anything (S-0);
- Speak into the microphone, the audio signal should be sounding good/clear;
- The PTT button can now be released;
- Make “for the record” a note of the new value into the operator manual;
- Press the [MENU/SETUP] button to save the new service menu values and restart the FT-991;
- My advice is to make a note of this procedure in you operator manual as a log of the radio.

in conclusion
The service manager of Yaesu is informed of the findings above. Hopefully this will contribute to better products and happier radio operators. I like the FT-991 very much and It’s a great radio! Hopefully the “AM carrier incident” doesn’t harm Yaesu too much, since the FT-991 is a great product. If you have a FT-991, my advice is to check (and change) the AM carrier level as described above. If you haven’t bought a FT-991 and want one, don’t be scared off, just buy one and have fun with it!"

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