The Syntom drum synthesiser, described in the April '81 issue of E&MM, is a very versatile instrument capable of producing a wide range of useful effects. In its original form, however, the unit can only be triggered by way of the internal microphone, no trigger input for use with a synthesiser being provided.
Fortunately, it is quite easy to add a trigger input to the Syntom, and a simple method for doing this is given here. The trigger signal must be a positive pulse between 3 and 15 volts in amplitude, and have a duration of a few milliseconds or more.
Figure 1 shows the small additional circuit required to provide the trigger input. TR2 has its collector and emitter terminals connected across the input of the main Syntom circuit and effectively feeds a small, negative pulse to this input if it is switched on briefly. Due to the high gain of the amplifier at the input of the Syntom, this small input pulse is sufficient to send the output of IC1 fully positive, and thus activate the envelope shaper and other circuits (see Figure 1 of the original article).
TR2 will in fact be switched on if a positive input pulse of adequate amplitude is applied to JK2. C5 ensures that TR2 is only pulsed on briefly regardless how long the input pulse may happen to be. R30 is a current limiting resistor which protects the base-emitter junction of TR2.
As there is very little excess space inside the Syntom unit, it is strongly recommended that the method of construction described here is followed precisely or it may be found that the additional components cannot be accommodated inside the unit.
Start by mounting the input jack on the rear panel in the position shown in Figure 2. Note that with the addition of this socket there is not enough space for the battery to fit horizontally across the rear of the unit so it must be fitted vertically into the case, as close to the microphone as possible.
The other components are mounted on a small piece of 0.1in. matrix Veroboard which measures 5 strips by 7 holes. The component layout is shown in Figure 3. There are no breaks in any of the copper strips and the leadout wires of TR2 should be trimmed quite short so that the component panel is as compact as possible.
A Veropin or a short piece of thick, non-insulated wire is soldered to the board at the point where it must connect to JK1. When this pin or wire is soldered to the appropriate tag of JK1, it should be found that the board is held firmly in place. Be careful to position the board where it will not come into contact with the jack plug when inserted into JK1.
The other two leads from the component panel are then wired in place; these can conveniently connect to the Syntom printed circuit board at the two pins which connect to XI but be careful to connect the leads the right way round. After connecting the earthing lead from JK1 to JK2, as shown in Figure 3, the unit is ready for testing and use.