Yaesu FT-991 Two Tone Test

The following images depict the results of a two tone test I ran on the repaired FT-991.  The rear data port was used to inject the signal from a PC running SpectrumLab. Tones where set to 1000Hz and 1575Hz respectively. The RF out of the Yaesu FT-991 was fed into the SM-220.
This image shows the transmitter flat topping, settings where 100W and ALC  increased slightly to peak RF output (a few bars visible).  The maximum output power achieved and observed on the external power meter was 90W (10% accuracy at full scale). Flat topping is a symptom of the PA being over-driven.

With reduced ALC, i.e. no visible ALC indication in fact one step (click) below zero observable ALC and with the FT-991 set to 100W, the following output was observed.

The maximum RF output observed on the external RF power meter was 60W.
Running the test using the microphone and audio drive via a speaker, provided similar albeit slightly better results.
The purpose with which this test was run was to verify if the bias voltage and current settings I chose for the RD100HFFI’s are at least in the ball park.  In a previous post I mentioned that Ids for each RD100HFFI was set to 1A giving a total of 2A as combined current for the pair of RD100HFFI’s.  The resulting bias voltage for each RD100HFFI was 4.24V (RH RD100HFFI) and
4.39V (LH RD100HFFI) respectively. The aforementioned bias voltages are effectively Vgs.
Looking at the RD100HFFI specifications I noticed that Ids flows at circa 3.7V and with Vgs at 4,2V about 500mA Ids is achieved.  At Vgs of about 4.3V 1A Ids is expected which reasonably correlates with my settings and those of the Yaesu FT-450

The original Vgs settings were 4.24V and 4.5V respectively suggesting an Ids between 500mA and 1A per RD100HFFI.
Next steps will be to vary the bias voltage to achieve Ids of 1.25A,  800ma and  500mA per device prior to measuring Vgs and running a two tone test for each setting.