1750 KHz Toneburst

To access most repeaters in the UK, a 500 millisecond 1750 KHz tone burst is required. When the repeater hears the tone, it switches on the transmitter and re-broadcasts anything heard on the input frequency.

There are numerous commercially made tone burst modules available for the modification of ex PMR equipment.

The Wood & Douglas TB2 tone module, shown here, can be easily made yourself with readily available components.

The adjustments are for output level, set at quite a low volume for best results, and the of the tone burst frequency. One advantage of this module is that it actually times the length of the burst to 500mS itself. This means that it can be wired across the input and when power is applied, it self times the tone burst, quite useful!

If you are a clever sod, like some of us, you can make your own tone-burst module with the LPWS design below.

This is a simple, cheap, and effective 1750KHz toneburst circuit, essential for "opening" repeaters in the UK.

This circuit has the added advantage that it can be left powered as it has an inbuilt 500 mS tone-burst timer. [i.e. when power is applied, the tone-burst only sounds for 500mS, after than a mere few µAmps are needed on standby]. If you are using a common ground (0V) you may be able to use just the output to the "hot" side of the TX input, check for polarity, otherwise it can be wired permanently across the input in parallel with your audio source.

The IC is a Motorola MCI 4001BCP or equivalent.

The only important adjustment is RV1, this controls the output frequency. There are several ways to adjust this, but assuming you don't have a frequency counter, you can use trial and error by testing it on your target repeater. Start with the trimmer in the centre position, most repeaters are very tolerant! [unlike the repeater groups!]
Another tried and tested way to set up the tone is by ear. Use a hand-held or a known good radio, and monitor it on a scanner, with your tone-burst connected to your transmitter apply the power and adjust for the same tone. RV2 adjusts the output level, in general, we have found the lower the level - the better it works.

If you want to get a constant output from the LPWS tone-burst circuit for testing etc. simply apply 12 volts via a 100K resistor to pin 6. This will disable the internal timer. 

This can be built on a board, or to save space, we have one actually built entirely on the back of the MCI 4001BCP chip. Another space saving way, is to use an IC socket, solder the components to the pins before plugging the chip in. After testing, both RV1 & RV2 can be substituted for fixed resistors to save space.

Tried, tested & it works! (well ours do anyway)