Final Testing: By bench testing and actual usage I have determined that the 4:1 transformer option is
probably not desirable for most users. The coils that MFJ uses are rather low-Q and do NOT function
very efficiently at the lower frequencies, like below 1.7MHz. Because of that, I experienced a little more
loss in the BCB than I expected. Above 20MHz the 956 is of questionable utility, in my opinion. However,
for the maximum Q and filter response sharpness, the transformers are necessary. This is the kind of
compromise that happens when less-than optimal components are used in the basic design.Their use is
a value judgement based on what application the unit is going to have for you. Of course, the
transformers could be switched in or out via a 4PDT switch (maybe mounted on the back of the unit).
You may wish to experiment and see which configuration is right for you.
Note that when the function switch is in the "Bypass" position, the "Attenuator" is still active.
* S1 is a center-off toggle switch (a 3 position rotary switch may be substituted).
Shorten the electrical length of the antenna circuit.
Lengthen the electrical length of the antenna circuit.
Resonate the entire antenna circuit to a variable bandpass frequency.
This mod suggests the addition of two 4:1 RF transformers at each SO-239. If you are going to use them,
wire the transformers so that the 52 Ohm side is facing out and the 12 Ohm sides are facing in towards
the preselector's circuitry. Be sure the transformers you select have a bandwidth rating suitable to your
needs (Ref: Mini Circuits and Coilcraft).
The 12 Ohm internal impedance will increase the "Q" of the bandpass filter and tighten up the bandpass
NOTE: You may just do a partial mod and not utilize the 2 transformers. In that case, add a 470k Ohm
resistor from the "antenna" jack to ground to provide a constant static bleed.
This is a tinkerers project and will void the warranty with the manufacturer. Don't go where you don't belong when working on electrical devices or
using power tools. If you are not comfortable using soldering irons or power tools, find someone who is. Wear appropriate eye protection and be
careful to not burn the cat with the soldering iron. All other disclaimers, cautions, and warnings apply -- please use common sense and good
judgement. Enjoy this hobby; it's very rewarding.
1. Disassemble: Remove the cover and two PC board bracket screws. Measure for two holes for the
SPDT (center off) toggle switch, and the 5K Ohm pot. Securely mount the two new components.
2. Wiring: Solder three wires to the back of the
PC board as shown: blue to the rotary switch
wiper, violet to the capacitor input, and gray
to the common bus for the coils.
(Note disregard jumpers on the board by the
black arrow, as they were from past
experimentation) Leave the wires at least 4-5
inches long. They will be trimmed during
3. Assembly: Follow the schematic,
wire-by-wire, and connect the three
switch wires and the three pot wires.
Add the 470k Ohm bleeder resistor
across the antenna input to ground.
Test the assembly before closing the
1. You may remove the logo and model number
legends with a cloth dampened with lacquer thinner.
2. The 4:1 transformers shown in the schematic are
optional. I elected to live with the simple
modification first, for a while, to get a feel of its
capabilities. Initial testing indicates that the
transformers must be used to fully appreciate the
utility of a preselector. Also, I may replace the coil
for the .5-1.7 MHz position since it seems to have a
Q value too low for its function. (IMPORTANT: Please
read the test results above.)
3. The Lengthen/Shorten function is most
appreciated when using a wire antenna that is
shorter than one might like to have for the
frequencies of concern. Likewise with an end fed
antenna which is too long for the receiver. The L/S
functions are very important when tuning high
gain/low dynamic range receivers. This is very
apparent with a radio like the Icom PCR-1000 or small
It will become quite obvious that tuning the antenna
circuit greatly increases the radio's
performance-displaying its full potential.
Hints: Use a 100-160 Watt soldering iron for the two SO-239 connectors and a 25-40 Watt iron for the
PC board connections. Wires added should be 20-26 gage solid or stranded. Using the original wire
colors will help to make assembly easier.
A view of the finished unit. You will note from
the schematic below, that even in the "bypass"
position, the Attenuator is still in the circuit.
This can be a very valuable feature when using
a large outside antenna with a small table top or
portable radio that may be easily subjected to
signal overload. Remember, portable radios
usually have very high gain front-ends,
because they are designed to operate from a
whip antenna, and thus overload easily.
MFJ-956 Passive Preselector
If you own, or are about to own, the MFJ-956 passive
preselector, here are three simple modifications,which
will make this device considerably more flexible and a
valuable asset for the feeding of your receiver.
This project is a good alternative option to building
one of the real Betts Preselectors featured elsewhere
on this site.
The MFJ-956 works well, is about the best
"bang-for-the-buck" for a passive device, and allows
the ability to make further mods or experiments easily.
The stock preselector, with a source impedance and
load impedance of 52 Ohms will tune in the following
Position (min / max
3 200 KHz - 1.0 MHz
4 600 KHz - 3.0 MHz
5 2.5 MHz - 10.0 MHz
6 9.0 MHz - 35 MHz
Measurements are close-approximate and will vary with terminal
S1 = SPDT center off
(Single pole / double throw)
(The pot is carbon composition type, deposited, or film. Do not use