MFJ-956 Tuner Enhancement

MFJ-956 Tuner Enhancement

Post by Fcathe » Wed, 31 Jan 2001 08:59:55

For those of you using the MFJ-956 Tuner and a long wire who are not entirely
satisfied with its performance, here is a mod that will improve its shortwave
performance w.r.t. intermod and effective tuning Q.  As designed the tuner
works OK for the longwave band but at higher freqs. (> 2MHz) the series
resonant tuner becomes problematic because the impedance of the series L/C
approaches the input and/or output impedances and the Q is so poor its
performance is marginal to none depending on the length of the antenna used and
its overall configuration. The existing bandswitch is a double-pole, six
position with only one of the sections being used so one can take advantage of
the other section to switch in some additional inductors that modify the
antenna's input impedance.  You will need to get a 1 uH, a 6.8 uH, and a 100 uH
RF ***(Mouser Electronics and probably Radio Shack in one of their "grab
bags"). The values aren't critical but should be within 20% or so.  The
"RECEIVER" and "ANTENNA" (input and output) ports will have to be reversed also
so the tuning cap is on the output (rcvr) side.  

Further improvement can be made on the LW range by changing the 2.5 mH ***to
a 4.7, 5,  or 6.8 mH choke.  This will extend the lower end down to the
specified 150 kHz for antennas less than 100 feet.

If you are interested in this mod please E-mail me and I will send the
schematic to you.

Frank