Modifying the Icom IC-290D (CTCSS, Deviation & Filtering)

Front faceThe Icom IC290D is a compact 2 metre multimode transceiver dating from the mid 1980s. In common with many rigs of the age it suffers from a lack of CTCSS for repeater access, and it was designed for 25kHz channel spacing. The FM receive I.F. ceramic filter bandwidth is too wide and the FM transmit deviation is excessive for use these days with 12.5 kHz channel spacing.

My recently acquired 290D has been modified to address the above issues:

Installation of CTCSS

There are many designs for add-on CTCSS modules for retro-fitting to older equipment, ranging from very simple single frequency designs to sophisticated selectable multi-tone modules. I have previously used several PIC based multi-tone modules successfully. One issue which can be a problem is the selection of the required tone. In one Kenwood transceiver I installed a 9 way rotary switch on the rear panel. In another the selection is made by altering jumpers inside the rig- not the most convenient method, especially when you lose a jumper inside the transceiver!  Some amateurs have installed CTCSS circuitry in an outboard enclosure which requires potentially untidy connecting cables to the transceiver.

I recently became aware of a very elegant CTCSS board designed by Colin Tuckley G8TMV ( His design uses a microcontroller to generate the tones. It measures only 35mm by 35mm and it has the great advantage that a single control wire is used to select the required tone. Grounding the control wire switches the tone sequentially through the 10 tones. Grounding the line for several seconds sets that tone as the default until deliberately changed. There is a connection for an LED which flashes the code in Morse code corresponding to the tone selected. A PPT line switched to ground activates the tone.

My IC290D is a new/old stock item and is perfectly original. I wished to avoid any unnecessary irreversible alterations. Ideally, the board would only generate a tone when in FM mode, in repeater mode, (called duplex mode on the IC290D) and only on transmit. After studying the circuit diagram of the 290D, it was clear that the mode switch would be the ideal place to steal an 8V supply when the rig was switched to FM. The PTT connection on the G8TMV board could be connected to the transceiver’s PTT line as it is activated by connecting to ground. The Simplex /Repeater selection was a little more problematical. The circuit diagram shows that a line is connected to ground in repeat mode, which grounds an LED and series resistor fed from a 5V supply. This is the front panel ‘Duplex’ LED.  My solution was to switch another LED (in the form of the input side of an opto-coupler) and series resistor from the same point. The opto-coupler is therefore activated by operation of the simplex/duplex switch. The output side of the opto-coupler is connected in series with the 8V supply from the mode switch, so that the subtone is generated only on FM, in repeat mode and on Tx.

The LED annunciator presented two slight difficulties for me. Firstly, there was no obvious place to install a new LED and secondly, I find Morse much easier to read as an audible signal- after all that’s how we all learn to use Morse code. My simple solution was to use the LED output to switch on an NPN transistor which can easily handle the current needed to run a Piezzo buzzer. This was mounted inside the transceiver and is easily audible.

Opto circuit
Circuit to enable Simplex / Duplex switching and audible annunciation
Relevant circuit areas

Next, the sub-tone output needed to be connected. The usual place to make a connection is directly following the deviation adjustment control, (trimmer R31 in the 290D), that is, after any audio frequency shaping circuitry.  I found that by connecting to the end of R32, at its junction with C16, 250Hz deviation (recommended) was easily achieved.

audio+arrow copy
Point of tone insertion

Finally, the control line needed to be installed. After much consideration my solution was to connect the control wire and ground to two of the unused tags on the rear panel accessory socket. A small tactile switch was mounted on a piece of Veroboard which was mounted inside the B9A accessory plug supplied with the rig. The switch actuator protrudes through the cable access hole in the centre of the plug. Even the B9A plug remains undamaged after the conversion!

Unfortunately, I could find no practical way of firmly mounting the PCB despite its small size. Eventually I decided to install it wrapped in thin expanded polythene foam and wedged between the main PCB and the bottom cover- not, perhaps, a military standard installation, but it works!


Reduction of FM transmit Deviation

This adjustment ideally requires access to a modulation meter. While monitoring the transmit output of normal speech, the deviation trimmer R31 is adjusted to give 2.5-3.0kHz deviation.

Trimmer tool inserted in the deviation control R31. Also visible in foreground, blue heat-shrink covering buzzer, opto-coupler and resistor.


FM I.F. Filter Replacement

The filter originally fitted in the FM receive I.F. was a Murata CFW455E ceramic filter with a 6dB bandwidth of 15kHz. I chose, as a replacement, an LT455GW. This is easily available at reasonable price. Like the original, it is a six pole ceramic filter but has a narrower bandwidth of 9kHz at 6dB down. It is pin for pin compatible making a swap easy. The only other difference is that the new filter has an input and output impedance of 2000 Ohms as opposed to the original’s 1500 Ohms. In practice this appears to make no material difference. Indeed Murata advise that a mismatch of up to +/- 50% will have little adverse effect.

The new LT455GW filter fits perfectly

Bill of Materials

G8TMV tone board  (

NPN transistor (PN2222 or similar)

Piezzo buzzer –must be rated to work below 8V e.g. Pro-Signal ABI-009-RC (CPC or Farnell)

220 Ohm resistor

Opto-coupler (4pin e.g. Sharp PC817 series)

Thin multi-strand wire in various colours

Heat-shrink tubing

Foam insulation

LT455GW, LTW455G or similar ceramic filter (Optional)


Instructions for Modifications
1. Remove top and bottom covers. The screws are brass, I think, and so chew up easily. Use an appropriate screwdriver (PH1). Remove two screws at the front and two at the rear on each cover. The speaker is mounted inside the lower cover and you will need to disconnect a 2 pin Molex connector on the main PCB.

2. With the top side uppermost, locate and disconnect the 2 pin PCB plug at the front right of the board. (Arrowed below)


3. Turn the rig over and remove the 7 remaining Molex connectors from the main PCB.

4. Remove 5 crosshead screws holding the main PCB in place- one in each corner and one in the centre.

5. Gently manoeuvre the main PCB with the grey lead attached (which was disconnected earlier) and remove from the rig.

6. Carefully solder thin multi-strand wires to the solder side of the main PCB. The tone output lead is thin screened audio lead. I trimmed the screen back and applied heatshrink sleeve at the PCB end. The screen was grounded at the other end, at the G8TMV board. The other connections needed are for +8V, PTT, and ground. There is a vacant PCB hole just behind connector J2 at the front right corner of the main board. I used this for the connection to ground, passing the wire through the PCB from the component side. Make a note of the colour of each wire to aid identification when the board is reinstalled.

Tone inject
Point of tone insertion


PTT, 8V and Ground connection points

7. Loosen and remove the four countersunk crosshead screws holding the sides to the front panel. The front panel can then be gently eased away and tilted to expose the back of the F SW Board which carries the simplex/ duplex switch. Connect a further multi-strand wire as shown below.

Simplex/Duplex Switch Connection

8. Reattach the front panel to the sides. Turn the rig top side uppermost and solder a wire as shown for the 5V supply to the opto-coupler. Thread this wire past the front of the main chassis to the underside of the transceiver.

5V Connection
PCB1 copy
Connections to main board as seen from component side

PCB2 copy

5V supply copy

9. Turn the rig right side up and remove the four screws holding the rear panel to the sides. Tilt the rear panel away to expose the rear of the 9 pin accessory socket. This is easier if the two RF phono plugs are disconnected temporarily. I soldered two wires onto pins 8 and 9 of the socket. (You could get away with one wire and link the other pin to ground on pin 4). Route these leads to the underside of the chassis, reattach the two phono plugs and reattach the rear panel to the sides.

Connections to Accessory Socket

10. Now is the time to change the I.F. ceramic filter if you wish. Desolder the 5 pins and remove the original filter. The new filter should fit without any alteration.

filter posn
Filter Position

11. Now carefully refit the main board, paying attention to the routing of the various connecting wires. Reconnect the grey cable. Refit the 5 screws which hold the main PCB in place then reconnect all the Molex connectors.

12. Connect the tone-out cable, ground, control, and PTT leads to the appropriate points on the CTCSS board.

g6tmv bd
13. Note the transistor (PN2222, but any general purpose NPN device would do). The base connects to the land marked ‘LED’, the emitter to the land marked ‘Gnd’ and the collector is soldered to a wire, the junction insulated with heat-shrink.

14. To complete the wiring connect the buzzer, opto-coupler and 220Ohm resistor-


The wire from the transistor collector connects to the sounder negative. The sounder positive connects to the 8V supply wire and to the opto-coupler pin 4.

Pin 3 of the opto-coupler connects to the  +ve terminal on the CTCSS board.

Pin 2 connects to the 220 Ohm resistor, the other end of which is connected to the wire from the simplex/duplex switch.

Pin 1, marked with a dot, is soldered to the wire carrying the 5V supply.

Heat-shrink tubing is used to ensure that the connections cannot short and the whole is then covered with a suitable heat-shrink sleeve. The expanded polythene insulation is then applied as shown below.


15. All that remains is to set the level of tone injected. Power up the transceiver, switch to duplex and FM. Connect an aerial or 50 Ohm dummy load. If a modulation meter is available adjust the trimmer on the CTCSS board to give a deviation of 250Hz on transmit. Otherwise adjust the trimmer to allow reliable access to a local repeater- having first selected your repeater’s appropriate tone.

16. The CTCSS board was installed as shown below. With the transceiver face down, the board was fitted in the right hand front corner. There is no obvious way of firmly fixing the CTCSS board without major surgery to the rig, so it was wrapped in thin expanded polythene foam, solder side uppermost i.e. facing the bottom cover. The foam insulation is held in place with adhesive tape (insulation tape) and the board positioned so that it fits comfortably when the bottom cover is refitted. Remember to reconnect the speaker cable when fitting the bottom cover. Refit the cover screws top and bottom.


17. Cut a disc of veroboard or perforated plain board so that it fits snugly inside the cover of the B9A plug. If you ensure that there is a hole at the exact centre of the disc, the switch will mount so the actuator protrudes centrally through the access hole. Carefully solder the switch terminals across pins 8 and 9. Ensure that there is nothing that can make contact with any other plug pins.



Note that when in use, the plug-mounted switch only needs to be inserted if there is a need to change the transmitted tone.


Sharp PC817 Opto-coupler Datasheet





March 2016



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