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Circuit Maker

Modify your Casio keyboard

Casio Modifications

Glenn Rogers

Internal modification to VL-Tone.

It happened in the early 70s with the launch of the first pocket calculators and it has happened now, in the early 80s, with the portable keyboard. Every so often a company launches a new product which uses the most up-to-date technology to enable millions of people to become involved in what to them is a new field.

Casio began by adding musical sounds to their calculators and now the musical content has taken over. The first Casio keyboard, the VL-Tone, also doubles as a calculator. This unit is probably their most successful because of its low price. It offers preset rhythms, preset sounds, a sequencer and a programmable sound where you can set each of the waveform, attack, decay, sustain level, sustain, release, vibrato speed and tremolo speed controls to a volume between 0 and 9; thus enabling a wide variety of sounds to be produced. The sequencer can be programmed in real time or step mode. The VL-Tone is ideal as a pocket size composition aid but does it fit into the studio environment?

The instrument has a digitally set tempo clock which runs the rhythm section and the sequencer. A sequence of up to 100 notes can be recorded. By use of the DEL key the sequence can be repeated 4 times but that is all.

If you want to use your VL-1 in the studio as a sequencer it is likely that you will want to repeat the sequence more than 4 times and you will want precise control of the tempo.

Figure 1. Trigger interface for the Casio VL-1.

The heart of the VL-1 is a single 64 pin flat pack integrated circuit which carries out all the functions. This makes modification of the instrument rather difficult. Our aim is to modify the circuit so that an external clock can be used to drive the sequence continuously. We can achieve this crudely by using the 'One Key Play' buttons. These keys will step through each note in the sequence then when it gets to the end it inserts a rest (unfortunately) and then repeats. If the circuit shown in Figure 1 is connected across the 'One Key Play' button we can trigger the sequencer from an external clock. The length of each note of the sequence is related to the trigger pulse length. A short trigger pulse will only give a short note - possibly inaudible - so to make the VL-1 more universal as a sequencer, a monostable with variable pulse length can be connected between the trigger source and the external clock input. It is just about possible to fit the additional circuitry inside the VL-1 but ensure that the supply connections are correctly made so that the extra circuitry only comes on with the synthesiser, therefore prolonging battery life.

If you don't want to tamper with your VL-1 innards too much then the extra circuitry can be outside the case and powered from its own 9V battery. The two wires from across the 'One Key Play' button can then be brought out via a socket. This is the best approach to avoid damaging the VL-1 or running down the batteries.

To take the VL-1 apart you need to remove the slider knobs and batteries and then unscrew the fixings in the base. The two parts of the case can be pulled apart and the two circuit boards removed - care is required here to ensure that none of the switches in the top are lost. The wires can then be fitted to the top circuit board with extreme care (see photo). The connections to the lower PCB can be made and the keyboard reassembled. The trigger input socket can be fitted with care and gentle persuasion. The draw back with this method is that the rhythm section cannot be synchronised to the external clock.

Interior of VL-5.

If you have a VL-5 you may like to use it as an externally clocked sequencer. The VL-5 is basically an uprated polyphonic version of the VL-1 with a few bonus extras such as the bar code music storage system. The VL-5 allows slightly more scope for modifications because it contains more discrete circuitry. The same modification as that given for the VL-1 can be implemented. However, if we use a higher external clock frequency we can substitute it for the internal clock. This means that both the rhythm and sequencer sections can be synchronised with other equipment. This may mean that you will need a clock multiplier and we hope to publish one in the near future. The modification for this is very simple and is shown in the photo. The external clock should ideally be TTL or CMOS compatible. A 4.7 volt zener diode is connected across the input to protect it from trigger pulses exceeding 5V. The clock frequency required is approximately 100 to 600 Hz.

Figure 2. Trigger interface for the Casio VL-5.

The construction of the VL-5 is very similar to the VL-1 but the case is slightly larger. It is not so difficult to find room for the external trigger socket. A 3.5mm switched jack socket is used so that when no plug is inserted the internal clock triggers the unit and when an external trigger is used the internal clock is disconnected. The connections are shown in the photograph.

Figure 3. Trigger interface for the Casio 701.

Casio's largest keyboard, the 701, can also be modified to accept an external clock to synchronise both the rhythm section and the music store. The clock required is somewhat slower than that needed for the VL-5 but a clock multiplier will still be required. Another useful modification that can be made to the 701 if you wish to use it in a studio situation is to split the output. One output for the rhythm and chords and the other for the melody. This allows the selective use of external effects such as echo and reverb etc. Figure 4 shows this modification. The output at this point are of a very high level and should be fed into a high impedance buffer or attenuator. This modification will also allow you to use the foot controller for expression on the melody while taking the rhythm and accompaniment directly out at the fixed level.

Figure 4. Split output for rhythm and chords.

Dismantling the CT701 is a matter of removing the 3 bolts at each end and the 5 screws along the back edge and then lifting off the control panel. The modification of the tempo circuit can be carried out without removing any circuit boards. R47 and the adjacent diode need to be cut at point X, then the two components joined to a lead and the other ends in the board joined to another lead. These two leads, together with an earth lead, are connected to the socket (Figure 3). A 4.7V zener is fitted to protect the input.

To split the outputs the audio output board needs to be removed. Two resistors R180 and R15 need to be removed at one end as shown in the photograph and in Figure 4. A screened stereo lead is needed to connect each of the outputs to a switched jack socket.

Internal modification to CT701.

Due to the way that the Casios are designed using custom integrated circuits it is not possible to use the keyboard to transpose the recorded sequences. This is a limitation if you wish to use your Casio as a bass or repetitive sequencer and need to be able to change key. However, the mods given here should allow you to improve the versatility of your Casio system.

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jun 1983

Feature by Glenn Rogers

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