The G8TMV CTCSS encoder board
Many older FM rigs don't have CTCSS capability, they were built in the days
when repeaters were accessed using a 1750Hz tone burst at the start of each transmission.
I wanted to add CTCSS tone encoding to my Kenwood TS-790E so it could be
used on my local repeaters.
The European version of this rig can't have the Kenwood module fitted because
the header connector is not fitted on the main board of the radio.
I wanted a system that didn't require opening up the case of the radio to
change the selected tone. I also wanted to make the modification easy to revert
and didn't want to lose any existing functionality. Some internet research gave me some ideas
and I decided to design and build my own CTCSS tone encoder board.
The Rev 4 board (made to my design by the nice people at Hackvana) is 35mm by 28mm
and 5mm high, it is a major update to the Rev 3 board with
a smaller size and lower profile.
The board comprises a voltage regulator, an MSP430 micro controller to generate the tone output
and control the functionality of the system, an active filter to turn the voltage
ladder into a sine wave and a trim pot to allow setting of the output level, plus a few
ancillary parts to make it all work.
There is provision for a control button, an led and a PTT input.
The encoder is designed to run from 5 - 15v and draws approx 9mA. It makes sense to have
the supply line switched so that tones are not generated during simplex operation.
The tone output is only active when the PTT input is grounded.
The tone encoder will generate any of the 10 standard CTCSS tones used
by the U.K. repeater system. Other tones are possible, please contact me for details.
Each time the control push button is pressed the encoder moves to the next tone
in the sequence. Each time a new tone is selected, and at power up, the LED annunciates
the code letter for the tone in morse code.
A tone can be made the default by pressing and holding the push button until the LED
flashes once. This stores the currently selected value in the micro controller
non-volatile memory so that it will be used the next time the encoder is powered up.
The tones and their code letters are:
A || 67.0Hz |
B || 71.9Hz |
C || 77.0Hz |
D || 82.5Hz |
E || 88.5Hz |
F || 94.8Hz |
G || 103.5Hz |
H || 110.9Hz |
J || 118.8Hz |
S || 100.0Hz |
The CTCSS board can not drive a Piezo sounder for audible Morse code output directly due to
microprocessor drive current limitations, however if audio output is prefered then the simple
circuit shown overcomes this.
The transistor can be any NPN type that is rated for the current required by the sounder.
The value of 'R' determines the volume and will depend on the sounder type and the supply voltage,
for a 5V sounder and a 12v supply 560 ohms is a good starting point.
Nick Barnes, G4KQK has written some very good articles about updating the
Icom IC-290D, the
Yaesu FT290R-Mk1 and the
which include fitting the CTCSS boards.
Here is my Kenwood TS-790E with the ctcss board installed. The Led is fitted beside the S-meter
so that it shines through the translucent body of the meter. Power is provided by a modification
to use the '144 ATT' switch and the command button uses a spare pole on the '1200 ALT' button.
Below is a photo of a rev 2 module installed in a TS-780, note the screened cable used for the
tone output connection and the extra button on a stand-off which is accessible
through the small backup battery flap on the top of the radio. The 'tone' button on the front
panel has been re-purposed for the ctcss on/off function. As on the TS-790 the Led is
fitted beside the S-meter so that it shines through the translucent body of the meter.