Improving the FT1000 Receiver close spaced signal performance
This mod is part of a plug-in package.
WARNING: The last batch of these mods will be built in this run Sept 2015. This product is obsolete.
order see the
Keyclick mod page.
I’ve tested over 15 FT-1000 and 1000D’s
with this mod. This modification will reduce close-spaced
dynamic range in every FT1000 or
1000D by a substantial amount!
The IM problem
is created because the FT1000 heavily forward-biases the noise-blanker’s first
FET whenever the blanker is turned OFF. This causes the
FET hanging on the IF system to have high gain. Strong signals within roofing
filter BW saturate the FET’s drain, causing mixing products (IM).
These new signals feed directly
back into the 8MHz IF section, creating interference to any desired weak signals.
The FET’s IM creates new “phantom” signals that sound like normal
signals. On SSB, it creates IM that sounds exactly like splatter!
The cause is the
same as in the
FT-1000MP MK V. You
can see a schematic
and explanation of
the problem on the
modification removes harmful forward FET bias
that is applied even
when the noise blanker is
prevents the noise-blanker from creating interference to weak signals when the
blanker is turned off. The NB functions normally when on.
1.) Remove the
2.) Locate the
noise blanker on the left rear corner of the IF board near the first group of 8
4.) Connect a
small silicon switching diode (1N4148 is one example, although other
silicon-type signal or switching diodes will work) through a 10k ohm resistor to
ground from TP-2001. The diode cathode (banded end) goes towards the resistor and ground, anode towards
DO NOT use a high leakage germanium diodes. Use low-current silicon switching or
5.) Construct a
pair of diodes with the cathode (banded end) tied together. Leave the leads long
6.) Connect one
of this diode pair’s anode to pin 1 (the brown wire) of J2001.
the other diode anode to pin 2 of J2001.
8.) Connect the
common-cathode of these two diodes to the junction of the 10K resistor and the
cathode of the diode installed in step 4.
Note: I just
pushed the wires down in the pin of the plug (P2001) 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. Turning either or both NB
switches “on” applies about 8 volts to either or both pins. 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. Other than improving IM performance when the
blanker is off, it has no effect on NB performance.