Yaesu FT 991 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.
