The Mini - Type 4    Preselector
If the Type 3 Preselector is just more than you want to build, consider the little brother "Type 4."
This is the basic "peak" band pass circuit with "lengthen" and "shorten" option. Also included is the input attenuator and a stage of preamplification.
This version is probably the most utilitarian version, providing wide bandwidth with decent band pass "Q" and enough gain to help your receiver's AGC circuit with the weak signals.
150 - 900 kHz, 300 - 1600 kHz, 0.6 - 3 MHz, 0.9 - 5 MHz, 1.6 - 8 MHz, 3 - 16 MHz
Tuning tips:
For highest band pass "Q" use the greatest amount of "L" and tune "C"  for resonance.
For the highest signal level, use the least amount of "L" and then tune "C" for resonance.
I have used this version with several radios: Kenwood TS940 and R5000, Icom IC746, R71A, R75 and PCR1000, Japan Radio NRD535 and NRD545, Yaesu FRG7700. This little sweetheart (8" x 1.75") really helps dig out the weak ones!
There's great satisfaction in seeing the "S-meter" peak-up on a station, while adjacent stations go away.
Builders' Hint: If you want, you could buy the MFJ-956 and modify it to this kind of circuit. See the page on that Preselector.
Rear view with the cover off: Note the 6 coils wired to the rotary switch in the same pattern as the switch contacts. Also the use of RG174 coax for the input, output and bypass connections of the preamp. This is the Ramsey SA7 mounted to a terminal strip by soldering the ground plane of the PC board to the terminal lugs. Power is via a coaxial jack and an RCA phono jack to allow for 'loop-thru' power to other accessories. I used very hi-Q coils (100+) and have found it not absolutely necessary to add the 4:1 impedance reduction transformers. In actual use, this unit is quite sharp tuning. In time I will try the transformers and measure the difference.
Last updated: March 19, 2006
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