Famous classic ICOM Transceiver alternative 455 kHz CW Filter
Rolf Heine, DL6ZB


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.

See here a video how it works on youtube.

About the circuit.

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.

YAESU XF-115c installed

Schematic of installation

MAIN PCB Schematic