ICOM IC-730 CW FILTER Famous classic ICOM Transceiver alternative 455 kHz CW Filter Rolf Heine, DL6ZB
Abstract
It's a shame, folks. The gold
rush is on. Prices for used CW filters these days are through the roof.
Therefore, the cost-conscious homebrewer needs to get creative. Why not
use IF filters that are available in sufficient quantities at fair
prices?
Using the example of the over
40-year-old ICOM transceiver IC-730, I'd like to present the expansion
with a 455 kHz CW filter. Originally, this transceiver was not designed
for the use of narrowband 455 kHz CW filters. So, a few small changes
to the circuit are necessary. But don't worry, the changes are
marginal. However, the OM/YL still needs to pick up the soldering iron
because a few components are needed for the new filter.
The IC-730 by ICOM.
This transceiver is legendary for
its excellent receiver and DX modulation using the real IF voice
processor. Even today, in the well-known Sherwood list, the aged
transceiver outperforms much more famous devices. That's no wonder
because the circuit, as the knowledgeable person immediately
recognizes, is marked by pronounced sophistication and expertise of the
developers of that time. In short, it's worth taking a closer look at
the device. Incidentally, it's worth noting that this transceiver
embodies a timeless design that has not lost any of its charm after
over 40 years. No one would think that this IC-730 design from the late
70s has endured the times by ICOM designers.
The problem arises when you want
to acquire accessories for the device. After such a long time, there is
no support from the manufacturer. Especially the CW IF filter is now
worth its weight in gold. Opportunists take advantage of the shortages
for their devious activities. So, solutions are needed.
Which filters are worth considering?
One of these solutions is to
simply acquire and use used filters from other manufacturers. Simply
plugging them into the transceiver is not possible because the CW
filters intended for the transceiver are of the FL-45 and FL-54 types
with an IF of 9.0115 MHz - unfortunately, an odd and tricky frequency
for which there are no other filters.
So, why not bypass the problem by
using the 3rd IF? For the frequency 455 kHz, there are still relatively
inexpensive filters from various manufacturers on the market. A few
additional components, reconfiguring the mode switch on the front panel
in two places, two control wires, and two short pieces of coaxial cable
laid, and you've saved money and found an excellent solution. After
all, the CW filter in the 3rd IF now also allows for an even better
utilization of the IF shift. And since the 455 kHz filters are still
before the AGC generator, the user now has all the advantages at hand.
The crosstalk between the input
and output of IF filters at the frequency 455 kHz is much lower than at
the 2nd IF of 9 MHz. The adaptation is easier because many of the
offered 455 kHz filters have similar impedances. The insertion loss of
455 kHz filters is usually slightly lower, and the form factor is more
favorable than for filters at much higher frequencies.
I installed the additional CW
filter for the IC-730 in the location for the frequency marker since I
don't use a marker in the IC-730. Please refer to the attached photo
for this. The circuit can be found in the accompanying paper. It might
be a bit confusing at first, so please use the original circuit diagram
for reference and look at the circuit of the MAIN board. On the top
left of the MAIN board circuit diagram is the J13. On the far left of
the circuit diagram, the controls of the front panel are drawn,
including the MODE switch we're interested in.
And now you basically only need a
filter. I used the YAESU type XF-115C, a mechanical Collins filter with
an impedance of 2000 ohms, which fits into the circuit without further
measures. Add a small circuit for driving the 1N4148 switching diodes, and install the CW
filter as shown in the picture below.
It really works fine, and for the CW operator, the effort is worthwhile.
