[Yaesu] FT-990 QRP Tests

[Darryl J. Kelly]

To any FT-990 owners and QRP operators:
I have been testing two methods of getting
the output power of the FT-990 down to 5
watts or slightly below. First method tried
was to inject a controlled negative voltage
into the Ext ALC connector. Best success
was with a 4.5+ 3-AA cell battery pack with
a 1K 15 turn cermet pot across the battery
pack and also through a 1K resistor. 
Resistance was about 875 ohms and voltage
about -3.55. Main problem with this setup
was that the radio would not maintain the
same power setting. It seems that the radio
was not designed to maintain consistent
power levels with this method. I have basically
given up on this method.
Second attempt was to make a pi-network
attentuator and reduce the output power from
10 to 5 watts. This calls for 3 db attentuation
and 5 watts power dissipation. I used the
values from the ARRL Handbook, with 1 300
ohm 3 watt metal film resistor for each leg
of the pi-network and 2 33 ohm resistors of
same type in parallel for top section. (I have
ordered 36 ohm resistors for final version.)
This works satisfactorily, and will pass a 
one minute key-down test with the resistors
getting only rather warm, not hot, to the 
touch. The resistor network in my setup is
between the output of radio and input of
antenna tuner. I can not tell that much 
signal loss on receive, as apparently the
two ACG circuits, one in radio on fast, and
other one in MFJ-784 digital filter, seem to
make up the difference. I don't want to mess
with bypassing the network on receive with
relays. This seems to work as a good
compromise. Any suggestions or comments
appreciated. Thanks for reading.
73,
Darryl, KK5IB

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