In theory low level modulation

Interesting. So where do you run the carrier control knob now that you've tweaked the "power in AM" in the service menu? Did you leave it at max and adjust it down to 23 in the menu? Thanks for the extra info.

Sounds like you have it doing right at or just a smidge over 100% positive peaks, which is great for a modern HF rig. I'm curious if it still swings or twitches backwards on an avg/rms reading watt meter with modulation or if it twitches slightly forward?

First i downloaded the service menu manual from online and went through that to find a solution.
Then found on the net someone already did that, and followed their description using the FT2000-D with dummy load as receiver.
First, you can f*ck up a lot of things going into the service menu and change settings you do so at your own risk.
Note down (all) or the setting(s) you change ORIGINAL radio values.
So in case something gets wrong you can go back to the original settings.

Now, for the change of setting in the service menu, again, be carefull and check 2 times before changing anything and NOTE DOWN original setting first.
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.

Just for info.
My original setting was 200, when following the above procedure to the letter my radio went to 46.... quite a bit of difference, every radio might have it's own sweet spot so don't take my numbers for the holy grail.
I left the radio on the 100 watt setting, and then changed the setting in the engineers menu.
I ended up with 23 watt carrier and good clean modulation and close to 100 watt out, in SSB the radio does around 110 watts, not bothered by the last few watts out, more for a clean clear audio on AM.

As said just turning back the P.A. output by regulating back the output power did not help a bit, there is not the fault.
It is in the driver section gain where the low level AM audio is added and which you can set back to good audio this way.
Lookin at the scope i now have a clean AM signal, no more distortion, and almost full output.
23 watt carrier and 90 to 95 watt max audio whistling in the mike.
After the modification the meter just gives positive modulation, no downwards modulation and no distortion as before.
Meter used Daiwa CN 801 HP with active PEP metering.
I might have used a bit more drive but i opted for the cleanest audio, the last 10 watts won't change anything.
The problem is already known since the introduction of the 991 in 2015, the current 991A of me was produced in March this year and still had the wrong setting...
Again, be carefull what you do.

Justme,

Thanks for the detailed description of the procedure you used.

The fault is not in the P.A, the fault is in the (pre) driver where the low level AM modulation is fed into, the gain setting of that driver section is set too high overdriving the rest of the P.A. chain, and adding the distortion on the driver level.

Setting your P.A. to 25 watts does not help because the distortion is already added into the driver section, not the P.A.

The only way to set the driver gain is in the engineer menu, you can only regulate the power output of the P.A. which does not help.

That setting of the driver gain is described in the engineer menu, hence the 14.200 frequency and rest of the allignment as per Yaesu instructions.
The S7 gives you the level of drive for that driver section, when you bring that down that level will drop as well.

After you did the mod, set the mic gain in AM to 100.( using original hand microphone)

Normally the carrier is 25% of the output when fully driven with audio in AM.
But if the distortion is already there from the too high gain in the driver section, it does nothing if you set the P.A. to 25 watt carrier and the distortion just stays because it is already there driving the P.A. as i described in my post, even setting the carrier to 5 watt from the P.A. did nothing to help.

So, you need to get to the bottom of the problem, the driver section that is factory alligned with too much gain overdriving the P.A.
Whatever is send into the P.A. gets just amplified, if that is a clean signal or distorted signal, just lowering the P.A. output does not help there, the problem needs be adjusted in the section/driver that causes the distortion.
Since every transceiver is different, has stages with different gain through spread in transistor gain etc every section is adjustable in the engineer menu.
That way they can all be alligned within specs and different tansceivers will have different settings in the engineers menu for that specific transceiver, so DO NOTE DOWN ORIGINAL SETTINGS FIRST.
Never change a setting if you don't understand it's function first.
My transciever was alligned to deliver 110 to 115 watts on most bands, almost 60 watts on 2 and 55 watt on 70.
For HF i did not change anything the 2 x 100 watt Fets will have a ball with that, new higher capacity fan keeps the transceiver more cool as the original with lower noise.

I did lower the output to 50 watt on 2 and 70 though, if i need more power i wil add a P.A.
50 watts from the 70 watt FET in that P.A. i rather keep healthy.

With the scope i found the pre driver was at fault and there the AM modulation was added and distortion came from.
Now i might have just 90 to 95 Watts AM but it is clean and crisp .as it should be.
i have a Heathkit SB-1000 rebuild so if i need more power i use that one.
Don't stare yourself blind on the last 5 or 10 watts, set the driver section for the cleanest and crisp audio without distortion.

Use other good receiver to monitor your audio when doing the changes,if you don't have a scope to do that.

I find it fascinating that the adjustments in the engineering menu can do stuff like that. I would have thought that the tech would have to plug a computer into it to get to those things. But I guess the computer needed to do it would go obsolete along with the needed software at some point and render the radio useless if those adjustments had to be made. This often happens with industrial electronics as we have some old software for certain systems that runs on DOS. Haha!

I don't know how the makers do their adjustments, but i guess they take a radio have a set menu values and some measuring equipment and adjust each setting per transceiver till it fits the specifications.
And not too much time to spend....

Strange thing is this AM modulation fault was noted to Yaesu in 2015 with the 991 so you would expect them to adjust their factory settings, but no, as said mine was made in March this year and still had the same fault.

One would expect the technician adjusting the transceiver to note the distortion in AM, but maybe they don't test that.
I only researched for the solution hence the downloading and careful reading of the technical manual describing the adjustment procedure for each step.
I already used the 100 MHz scope and saw the distortion didn't come from the P.A. but from the driver section.

Then i found the internet story and followed up that taking care noting down the original setting and checking progress on the scope.
Add FT2000-D as control receiver to hear the changes.
I might have used a bit higher setting, but my goal was a clean and crisp audio with absolutely no distortion and have a few watts less out I won't lose sleep over.

As said making more power is easy once it is clean.

AM has always been a tinkerer's game. Not many appliances come out of the box doing a good job. Most CB sets do better than the typical ham rig. The new sdr rigs with knobs are getting better but the audio bandwidth is still too limited. My ftdx1200 can make 4k ssb but is still limited to 6k total bandwidth on AM. That's probably why it doesn't get much use.

Same for the FT 2000-D i used before the FT 991A, but out of the box AM from the FT 2000-D was good without need of tinkering, 50 watt carrier and 225 watts fully modulated.
Always got good reports from the AM nets on 160/80 from that radio.
It ran the 4 K "mod"for SSB as well, nice for the rounds.
The FT991A will do 3200 max, not seen a mod to change that to 4 K.
still does sound nice using the equaliser and EQplus.
But i would like to have the 4 K for the round table rounds here.
If not, my bad luck.
At least AM is now working well for such radio.