If the Type 3 Preselector is just more than you want to build, consider the little brother "Type 4."
This is about as simple as it can get, while still incorporating all necessary, basic functions.
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
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.
The Mini-Type 4 Preselector
I have used this version with several radios: Kenwood TS940 and R5000, Icom IC746, R71A, R75 and
PCR1000, Japan Radio NRD-515, 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
When tuning, it is important to remember that you have an Attenuator control. A preselector helps
your receiver in two basic ways:
1 - The obvious of peaking (tuning) the desired frequency to help the radio's IF and other tuning
circuits do their jobs easier - better.
2 - To keep incoming signals down to a sane, safe, and sensible level for the very sensitive
(sometimes delicate) front end devices, thereby eliminating overload, intermod, and images.
Do not submit to the intuitive urge to "use as much signal as is available." It is not necessary and
certainly can be a detriment.
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.