Here is the message I mentioned that was on the TopBand reflector. Have not
had time to try it yet but looks like a very easy job.
Rob, 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 and HNY!
Tom
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