My thanks to everyone who replied to my query about this. Apparently,
the problem was that I wasn't aggressive enough. The FT-847 manual
describes the control voltage range on the ALC input as 0 to -4.0V, and
as I described in my earlier note -4.0V made no difference at all.
However, bolstered by the encouragement of those who responded to my
earlier note, tonight I tried greater levels of negative voltage (my TIB
will go to ~ -8.5V) and was greeted with success. At -5.0V the FT-847
power output is reduced to a truly neglible level (meaning the wattmeter
needle barely wriggles). Somewhere between -4.5V and -4.6V the radio's
power output looked appropriate for my purpose of driving a DEM transverter.
However, while searching the web for clues about this, I discovered that
the FT-847 has a hidden "alignment menu" which allows reducing the
transmitter gain (and hence power output) of the radio on a band-by-band
basis. This is described in the FT-847 service manual (available on
line) and might be an even better solution.
Ed, K1ZE wrote that he had problems with leading-edge power spikes while
trying to back down the FT-847's output to drive an older Mirage brick,
so I may need to drag out the oscilloscope and check that out in my
setup as well. He did say this was with a very early FT-847. I think
mine is about 2004 vintage and I used it in the January VHF SS with the
power control set to minimum on 144 MHz to drive my 903 DEM transverter
with no problems. Hmmm...
Thanks again for everyone's helpful comments.
73, Dave/K8CC
-------- Original Message --------
Subject: Use of negative voltage to reduce power with FT-847
Date: Mon, 01 Feb 2010 19:21:59 -0500
From: David Pruett <k8cc@comcast.net>
To: vhfcontesting@contesting.com
All:
Has anybody tried using the trick of applying a negative voltage to a
FT-847's ALC input to reduce power to drive a transverter? This
technique worked really well with my IC-756 with only -1.5V applied, but
applying even -4.0 volts to my FT-847 didn't seem to make any difference
at all.
I'm trying to use a Downeast Microwave "Transverter Interface Box" (TIB)
to provide the negative voltage to cut back the 144 MHz power to my 903
MHz transverter when I enable the transverter.
TIA for any advice or observations.
73, Dave/K8CC
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