Article:32166 of rec.radio.amateur.misc
From: kchen
Newsgroups: rec.radio.amateur.misc
Subject: Re:FT990 mod for extended tx
Message-ID: <76900
Date: 17 Jan 93 23:01:05 GMT
References:<1jcftdINNh42
Organization: Apple Computer Inc., Cupertino, CA
Lines:51
blw7149
>I received a mail from AA6SQ about Extended transmitter frequency range
>for FT990 and FT1000
>-Remove top and bottom covers. Remove top screw on each side which holds
>front panel to chassis. Pivot front panel down slowly. CAUTION :if you
>let it go, it will flop down rapidly and probably tear somethins loose.
>Behind the meter will be the CNTL unit. It has a metal cover about 4 * 8
>inches. Snap the cover off. Look to the top of the unit. You will see
>JP-3. Remove the solder connection on JP-3.
This appears similar to the mod given for the FT-1000 that is given
in "Radio/Tech Modifications, Volume 5B," ISBN 0-917963-011-3,
published by artsci, inc.
BUT! The mod for the FT-990 that appears in the same book is
completely different. I tend to believe the book, since the control
unit for the FT-990 is on a plug-in card to the motherboard, and not
a thing attached to the front panel.
I did not attempt to make the extended range mod, but I did make a
mod to my FT-990 (didn't read of this mod anywhere, I simply traced the
schematics) to reduce the minimum power output from 10W to about 3.5W,
to operate QRP using the last ARRL 10m contest.
(I just felt like getting 59 reports from everyone that weekend,
and, except for one truthful gentleman in NC, I did :-).
I didn't blow anything up, but do the following at your own risk.
Find the R.F. power control pot in the schematics. All this pot does
is to supply a variable voltage level to the ALC circuitry. Trace
the signal from the pot to the ALC goodies, and you will find a series
limiting resistor whose value is 1/10 that of the pot's max value.
This is what limits min power to about 10W. I simply paralleled this
fixed resistor (a surface mount resistor on the board with the ALC
sensing circuitry) with a smaller axial resistor.
You have to raise power to 8W or so to get the automatic antenna tuner
to buzz and click. That may be why the power control knob would not
go below 10W on a stock unit; would only confuse all the appliance
operators :-).
BTW, has any other FT-990 owners noticed that the built-in computing
SWR meter is quite inaccurate at powers below 25W?
73,
Kok Chen, AA6TY kchen
Apple Computer, Inc.
