Noise Blanker Modification for FT1000MP by K0ZZ
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. Hope this helps. Tom
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