Using an outboard CTCSS Unit
in an old FM TCR

The near universal application of CTCSS coding for accessing has sent many old transceivers to the recyclig bin. Mine have survived but have been relegated to a dark corner and have been gathering dust for years. On a whim I bought a non-working Trio TR2300 a few years ago. I enjoyed making it work and it took me back to the early 1980s when one was my first rig. Recently I have renewed my interest in working through a GB3WR (2m) and remembered that I had never commissioned DJS Electronics Ltd (cstech.co.uk) CTCSS encoder kit that I had bought over ten years ago. Now would be a good time to finish the job and use it.

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To avoid modifying my old radios, which have 4 pin mic sockets, the ctcss unit would have to be battery powered. I would use my existing base mic adapter leads (with DIN connectors) and I salvaged a 4 pin mic socket from a dead AM CB. Other bits were scavenged from old PCs. The plastic box was ordered from China... inexpensive, and attractive, but with some problems.

The first issue was to source screws to hold the halves of the box together! Mounting hardware on the line of the join between the 'lid' and 'base' of an enclosure is not a simple matter. I made a panel (from plastic salvaged from a dead laptop PSU) to carry the mic socket and switch. I had trouble getting the top part on after fitting the switch and had to cut sections from the ends of the panel.

There is not much space!

Control

The circuit board suspended on the thick wires between the mic sockets manages the battery consumption. A mechanical switch would do, but it would be easy to leave it on by mistake and run the battery flat for no good reason. I used the same circuit that I had devised for my base microphones.

My first method of switching the battery using the PTT line was not a success! It worked if the rig was switched 'on'. When the rig was switched 'off' the low PTT line switched the battery 'on'. Epic fail!

The answer is to separate the microphone PTT from the rig PTT line. The battery supply is switched by a PNP transistor and the switched battery line drives an NPN transistor to pull the rig PTT low (30 mA). I cannot detect any battery current flow when the system is inactive. The encoder draws about 10mA, but only on transmit. As a late afterthought I added a switch to disable the encoder. The battery is still needed though to make the NPN transistor pull the PTT line low and the rig transmit. The current drain is very small.

Connection

Injecting the CTCSS tone via the mic feed is not the best way of adding it. A certain amount of juggling the mic gain and deviation settings, in conjunction with the CTCSS level control, is required to get the system working acceptably. Although the CTCSS tones are low they are audible on a wide range audio system, even a LoFi one. Communications radios should be engineered to filter them out but I can hear it with HiFi headphones.

Removing an Unexpected Audible Tone (~750Hz)

Although the desired low tone is the only tone that can be heard if the tone encoder output is fed to an audio amplifier and headphones I have found that the CTCSS level control also affects a steady musical note rather like a commutator motor, on the transmitted audio. (This is heard over a repeater too... it is not the result of the CTCSS system.) I have come to the conclusion that this is a very low level unwelcome artifact produced by the encoder, the result of frequency synthesis of the code tone and less than perfect audio filtration. The level is some 20dB lower than the CTCSS tone, but several octaves higher in frequency and sounds as though it has significant harmonic content. I have not been able to view it on an oscilloscope.

You can play with a dummy load and a modern receiver with tone squelch enabled to test the system. With care the unwanted tone can can be reduced to an unobtrusive level. When I injected a sine wave from a dedicated audio generator (100hz) I was able set a radio to respond to it and open the squelch. It worked, and no higher tone was heard on the transmission. (At whatever level I set the CTCSS tone.) I have yet to try the preferred method of injecting the CTCSS tone directly at the modulating varicap diode or similar device. When injected at this point there is no frequency selective amplification and the artifact may not be audible on the transmitted dignal. Two pins of the four pin microphone socket are connected to ground. I may be able to re-purpose one to bypass the mic audio amplifier.

Simple, Additional, Low Pass Filter

It struck me that I could get a better idea of what was going on by digitally recording (with Audacity) the various audio streams as they came out of a receiver. When amplified by some 30dB it was very evident that there was a high frequency riding on the CTCSS tone. I could digitally remove it with a Low Pass filter (200Hz 24dB per octave) so I hooked up a simple RC combination to mimic this.

The resistor values were a guess (with hindsight the input resistor is superfluous, there is a 10k resistor in the output line). The capacitor was selected using a substitution box. My first guess was spot on! It works treat. Play the link(s) below and hear me switch the capacitor in and out. (The recording was made from a tone squelched receiver with the usual volume setting listening to the TR2300, with added CTCSS 94.8Hz, connected to a dummy load.)

Filtered and unfiltered ctcss.mp3   LOUDER



Screen shot of the LOUDER sound file. (About 30dB amplification.)

I have squeezed the filter in between the microphone sockets. For the moment this project is finished. I want to try it with my IC715 and my TS700 but I'm not expecting any problems. There is a small issue in that the tone does not start immediately power is applied to the tone generator. There is about half a second delay. I think that is the unavoidable price I have to pay to optimise the life of the battery. Its a small one and I think I will live with it.

Adjustment

There needs to be rather more than the minimum necessary CTCSS to open the tone squelch on the listening device (repeater). Too high a level may cause problems, too low and the squelch will close on speech peaks that a significant low frequency component. A high pass filter in line with the microphone, even a low(ish) value capacitor, may solve the problem. (Or, perhaps two capacitors in series, with a resistor to ground.)

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Tone Selection

The encoder can be addressed to produce many more than the eight tones in use by UK repeaters. The simple way to select them is by using jumper links. One frequency is enough for my purpose. If you want the whole range you will have to address the four binary coding lines, and (preferably) display what you have chosen. I have made a diode matrix (IC240 style) to use with a 12 way single pole rotary switch. It works. You can buy BCD switches. You can buy a module which reads a four place binary address and displays the result on a 7 segment LED display. You could combine this with a BCD counter and a push button. These all take up more space than I had available, and the last two need a significant amount of power. Perhaps one day I may make a 'full feature' unit to pair with my Trio TS700. My present solution is bigger than a modern HT but that doesn't matter. It is irelevant.

Follow Up

Do contact me via the email address below if you would like to make any comments or need any help regarding "legacy" rigs and repeater operation.

John Everingham. G4TRN
 
Webmaster.
 
email:- g4trn(at)saundrecs.co.uk
 
October 2025