Yaesu FT-991 Final Stage

For background information please refer to my previous posts and the comments received on the FT-991 ALC problem and HF PA issue.

The following procedure for setting the Yaesu FT-991 final stage idling current has not been verified against Yaesu’s technical supplement for the FT-991.  Use this procedure at your own risk.

Having noticed that the idling current measured on the working FT-991 was circa 2.5A and studying the FT-450 technical supplement for similarities (thanks to Dave Bird G6JED for that pointer), I noted that the FT-450 technical supplement specifies 1A (+/- 100ma) for each RD100HFF1.  If you set the FT-991’s idling current accordingly, max RF output is on the low side circa (90W) on 20m USB.  My conclusion from testing is that the idling current for each of the RD100HFFI’s should be circa 1.25A to achieve full output or at least it gets the radio to the total idle current of 2.5A originally set by Yaesu.  While I initially opted to set the idling current to 2.5A, but have subsequently decided to revisit that decision and have chosen to set each RD100HFF1 for 1A giving a total idling current of 2A.

Alignment Procedure Yaesu FT-991 Final Stage Idling Current

The following procedure assumes the top covers of the Yaesu FT-991 have been removed. Click on the image to enlarge to view test and adjustment points.

1. Ensure a dummy load is connected to the HF antenna port and power is disconnected (unplug the power cord).
2. Remove the microphone if connected.
3. Remove bridge across J4006 and J4007. Connect Amp meter, scale 10A across J4006 (+) and J4007 (-).
4. Plug in the power and power the radio on.
5. Set dial frequency to 14.000.00MHz, MIC Gain 0 and Mode to USB.
6. Select and press MOX on.  You should now see a current reading on the Amp meter of circa 2.5A if you did not turn turn VR4001 and VR4002 fully counter clockwise.
7. Press MOX off, power off the FT-991.
8. Remove jumper J4005.
9. Power on the FT-991 and press MOX on.  Adjust VR4001 for 1.25A 1A
10. Press MOX off and power off the FT-991. Remove jumper J4004 and replace jumper J4005.
11. Power on the FT-991 and press MOX on. Adjust VR4002 for 1.25A 1A
12. Press MOX off, power off the FT-991.
13. Replace jumper J4004.
14. Power on the FT-991 and press MOX on. The Amp meter should read 2.5A 2A
    

Plug in the microphone, MIC Gain 30 and test ALC and RF output on 14.000.00 and other bands e.g 40m. Be sure to test FM and AM output as well, all should be working.

Press MOX off, power off the FT-991, switch off the power supply and unplug the FT-991 from the power source.

Remove the Amp meter and replace the jumper across J4006 and J4007

Close up the FT-991, you are done for now..

Note 1: The idling current will tend to increase as the devices heat up. I have opted to set the idling current after the fans have kicked in.
The Vg for the RD100HFFI post adjustment were still found to be
4.24V  4.15V (RH RD100HFFI) and
4.4V4.39V (LH RD100HFFI) respectively.
Ideally the coupler balance should be adjusted, however I have not determined how to do that procedure as yet.
Note 2: Standard practice suggests otherwise, however,  prior to doing the procedure, I decided to verify the factory setting for the RD100HFFI idling current.  Idling current for the RD100HFFI associated with VR4001 was found to be 1.8A and for the other the reading was circa 0.7A, giving a total of 2.5A. The failed RD100HFFI was associated with VR4001 i.e. on the right when looking at the radio from the front toward the antenna ports. The idle current for the failed RD100HFFI was found to be factory set to circa 1.8A.
I will redo the FM as well  AM output test for the repaired FT-991 and log the results on the blog in due course.