To all who e-mailed, this is faster!!!
>
>This mod greatly improves strong signal IMD in the FT-1000. I
used a two
>kHz test spacing in the measurements, so the 12 kHz wide 70
MHz first IF
>filter did not make results look artificially better.
>
>I ran two HP generators through a combiner into the front end,
and measured
>IMD with a selective level meter with 50 Hz BW and also with the
1000's
>internal meter (which is about 2 dB per S unit below S6, not six
dB as
>claimed). This was the primary source of IMD in my brand new FT-
1000, and
>the difference was dramatic. Remember this is a sample of one
unit.
>
>I carefully looked but I never found anything wrong other than the
fact the
>receiver forward biases the first FET in the noise blanker fully-on
when
>the blanker was shut off, and that IMD feeds directly back into
the 8 MHz
>IF stages. The result was a few volts of peak RF voltage on the
FET drain,
>and considerable amounts of that voltage feeding back to the IF
stage
>through stray coupling.
>
>I am working on better mods to the front end, but this is a
substantial
>change.
>
>Be sure to use common sense and turn off power when soldering,
etc.
>
>1.) Remove the bottom cover, locate the noise blanker on the left
rear
>corner of the IF board near the first group of 8 MHz filters.
>
>2.) Locate TP -2001. Connect a 1N914 or any other silicon
switching diode
>through a 10k ohm resistor to ground from that point, cathode
towards the
>resistor and ground, anode towards TP-2001. DO NOT use a high
leakage diode
>like a germanium diode.
>
>3.) Connect a pair of common cathode connected diodes (same
type) with one
>anode to pin 1 (brown wire) and the other to pin 2 of J2001. I just
pushed
>the wires down in the pin of the plug and tacked them in place
with a tiny
>bit of solder. You can verify these are the correct pins by
measuring the
>voltage when switching the NB and wide NB buttons. Either or
both NB
>switches "on" applies about 8 volts to either or both pins.
>
>4.) Connect the common to the diode cathode and 10k resistor
junction
>through a short hook-up wire.
>
>This mod removes forward AGC bias from Q2003 and 2004 when
the noise
>blanker is off, and prevents IM distortion fed back via C2041 and
foil
>traces from impacting the receiver when the noise blanker is off.
>It does not affect NB performance when installed.
>
>
>
>--
73, Tom W8JI
w8ji@contesting.com
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