Yaesu FT-991 AM Issue Resolution

Yaesu FT-991 AM Issue Resolution

Glenn/WA3LAB found the fix below on the net to resolve the issue with the Yaesu FT-991 in AM mode. The techs at Yaesu they said they’re aware of it, but Yaesu has not put out a bulletin on it.

Both Glenn & I have followed this procedure and it appears to work. I wouldn’t want it put out there without letting people know that they should proceed with care, and not to change anything else unless they really know what they’re doing. Since there’s not many people running AM anymore most people have no idea there’s a problem.

(Warning: once you put the radio into the service mode you can really screw things up, so do this at your own risk.)

AM Transmission Audio Trouble
http://www.amateurtele.com/index.php?artikel=168&id=#752

The problem:

20160205 – 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.

Full article here :
http://www.paarc.net/yaesu-ft-991-am-issue-resolution/
Thanks

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.