FDK Multi 2700 CTCSS Fitting

For testing purposes the CTCSS tone board was wired to my FDK multi 2700 transceiver in the following method.

With the function of the board only being required when using repeaters, I decided it would be usefully included it in the – 600kcs shift. The method of fitting is also appropriate for the + 600kcs shift, or, if you wish, to make it available from the Simplex setting.

Crystal Balls

This picture illustrates the Crystal board within the FDK Multi 2700. The picture is click-able for a larger image if you need it. The board is located in approx the middle of the top of the radio. You’ll see it once you’ve taken the top cover off.

FDK Multi 2700 Crystal Board

Power Location on the FDK Multi 2700 Crystal Board

The required crystal is selected by the frequency shift switch from the front panel. The switch grounds the earthy end of the Crystal Oscillator thus ‘switching on’ the appropriate crystal circuit.

The Black croc clip in the photo is attached to a PCB wire wrap pin that connects to the selector switch. Therefore this pin goes to ground when the appropriate switch setting is selected from the front panel. There is one of these wire wrap pins available on ‘south side towards the front panel’ of each of the crystal socket where socket 1 is the simplex crystal. This makes for the same modification technique no matter what frequency offset you want from +/- to simplex operation. Just pick up on the appropriate pin. If in doubt substitute your multimeter on a dc volt range across where the Yellow/ Black wires are shown with the frequency shift switch on the setting you want the CTCSS to be live on, and where you pick up around 9 volts is exactly where you want to place your CTCSS power wires.

The Yellow croc clip wire is attached to a wire wrap pin just south of the small choke as shown. This is picking up a + 9 volt supply. Together these two wires provide power to the CTCSS board. The board is only in operation when the pin of the appropriate crystal oscillator is earthed which is selected from the front panel control.

The large can crystal that you see in the picture is non standard. My rig was missing a crystal so this one of the correct frequency was pressed into service, its legs being extended to reach the socket. Less than ideal but what I’ve had to do.

Dealing with the Audio

The CTCSS is injected into the audio circuit from the top board accessible from under the ‘users’ hatch on the top of the rig. You can see the preset pots white extension shafts in the picture.

I’ve picked up on the output side of the FM Mic Gain pot. As shown this is located towards the bottom right of the switch that handles the ‘Normal or Compression’ audio control.

Image of the FDK Multi 2700 Audio Control Board

The FDK Multi 2700 Audio Control Board

The Brown FDK wire is actually a shielded two core cable with the inner I think it was white cable handling the ‘live’ side of the audio signal and carries the audio down to the lower board. This method facilitates the FM Mic Gain pot to do that job while the CTCSS board has its own level control by way of a preset potentiometer.

On the lower board is the real FM Deviation pot. Its located just under the bundle of wires on the right of the picture. This would have been a better place to inject the tone but would require removal of these two boards and associated wiring to get a soldering iron in there.

This is a rather crap Oscilloscope screen capture of the CTCSS tone at 88.5hz. The complete fracture of the trace is all down to my mobile phone having been used to picture it – its not a fault of the CTCSS board – its the scan rate of the mobile phone image sensor.

Image of the FDK CTCSS Tone Scope

Waveform produced by the CTCSS board

On air test

I’ve not been able to get GB3SR to respond to me on this set up.

Using an FT290 receiving on the tx input frequency 145.000 I can hear myself and the tone. By adjusting the preset pot on the CTCSS board I can hear the tone level varying but so far no cigar on access to the box. :(

Using my Multimeter frequency meter it displays 88 or 89hz which is what you’d expect for 88.5hz CTCSS tone.

That’s a bummer isn’t it.

Have I got problems somewhere else?

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