I have just finished modifying an old IC-720A HF transceiver to replace
the rotary relay filter control with a set of reed relays and a small
digital circuit to select the appropriate relay and provide feedback
to the existing CPU in the radio so it thinks it's still on the rotary
relay. The cost for parts was about $25; the cost of my time was slightly
more than that, but it was "fun".
The modification is so far successful, and the radio is much quieter in
band switching as well as being more reliable (the old rotary relay was
impossible to get mechanically aligned correctly, leading to poor connectivity
between the rx/tx and the antenna, as well as many missed stepping pulses).
Summary: Use the step-pulse line to drive a small SPDT relay; use the SPDT
relay to drive an R/S flip-flop (built from two nand gates) to provide a
clean step signal (the existing step signal has some kind of very high speed
clock train superimposed on it and therefore was not usable directly). Feed
the pulse train into a CMOS decade counter type 4017. Take the appropriate
decade outputs (active high) and recreate the switch-position feedback signals
using a small diode matrix ala the original design. Also use the decade outputs
through 33K resistors to turn on 2N3904 NPN transistors, which are used to
select the appropriate (1 of 7) DPST reed relays. The CMOS chips (the nand
gates and the decade counter) can be had at Radio Shack and must be powered
from 9V (or so) because the radio does not use 5V logic (of course...)
I mounted the reed relays inside the original low-pass filter subassembly,
and the rest of the circuitry on a small perf-board near the SWR module
(connected by ribbon cable).
This is not a detailed design description but if you are interested I will
provide more details. This project did ssalvage an otherwise-worthless IC-720A,
but I don't think I'd do it again (rather time consuming, but perhaps you
can benefit from my experience and save some time).
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