[INRAD] Icom Ic-756 Cascade Filters Mod

[Charlie Mazoch Jr.]

Hi All:
  I incorporated the INRAD FT-920 cascade filter board with two # 110 
INRAD 2.1 Khz filters onto the rear of my IC-756. It is enclosed in a 
Deltron diecast box from Mouser. The part # I used is a 400-4591 but the 
lid had to be machined so everything  would fit. Using the 400-4587 box 
which is 2.2 inches depth would eliminate having to machine the lid. The 
circuit board has to be trimmed to fit the enclosure. Using a pair of 
tin snips I removed all circuit board above the filters. The corners 
were also trimmed and the board is mounted in the enclosure by new screw 
holes between the filters. Remember to file the trimmed edges of the PC 
board to prevent shorts.  The enclosure is mounted by a couple of 1/2 X 
3/4 inch  aluminum angle brackets to the rear of the transceiver. I 
carefully drilled and tapped a couple of 4-40 holes into the 
transceivers rear casting.  Photo's are available on request.  This 
board is designed for Yaesu filters so I had to capacitive couple the 
#110 filters. I removed L1 and L2 from the circuit board. Then I added a 
0.1 mf capacitor between the junction of R1, Q1 base and the first 
filter's output. The coax braids are soldered to the ground plane and 
the input center goes to C2. The output center goes to the junction of 
C16 and C18. The coax cables are soldered to 2 pin headers which plug 
into the optional filter slot. The outer pin is the coax center. The 
rig's 2nd IF crystal filters, F152 and OPT 1 are used in the receive 
mode only.  There is no external switching involved as the transceivers 
soft touch switch in its narrow mode selects the cascade filter unit. 
The red lead is soldered to  +12 volts on top of the relay RL 601 on the 
main board. I set the cascade circuit's gain to be same as in normal 
mode. While listening on a quiet band my S meter read S4+ and when the 
cascade filters are switched in the S meter falls below S1! I'm using an 
INRAD 2.1 Khz filter for the third IF in narrow mode and have 
incorporated an INRAD IC-765 1st IF roofing filter into the rig also. 
Crystal filters are still far superior to digital filtering when it 
comes to selectivity in a rig!  All those cute colored displays of 
digital filter skirts amount to is a lot of smoke and mirrors.         
Charlie  W5VIN