On the bench today is the Casio SA-8, picked this up in a charity shop 7 or 8 years ago and used in some musical jams throughout the year, mainly for novelty affect. This week it was rediscovered in a box full of percussion but on launching it, there was a fault which was making some very interesting glitchy sounds.
As it was still making sounds, albeit very sporadic and glitchy, a clocking problem was the main suspect as to the error. Looking through the parts box a crystal was located, however it only possessed half the clock speed of just over 11Mhz (the original crystal resonates at 22MHz). This was used to replace the crystal and, indeed, it worked just at half the usual speed … which sounded much better than factory. The original crystal was a cheap, plastic 3 leg version, so the xtal had to be modified by adding two 22pF ceramic (it’s very good practice to use ceramic at high frequencies) between the two legs and ground. The xtals are symmetrical so it doesn’t matter which leg is which. Following this two more xtals were ordered one at 22MHz and one at 6MHz, although it was later discovered that an LTC1799 precision oscillator (Etsy, 2019) could be used to give a continuously variable range of clock speeds.
Casio SA8 Main Board
LTC1799 Precision Oscillator
New Control Wiring
After collecting some information on the Casio SA series, the experimentation began, first adding an output jack to allow the audio to be routed into an amp, through pedals, etc. The first attempt used a switched jack to mute the speaker when the audio cable is plugged in. However, this meant that the audio to the speaker had to also go through the 100R resistor and volume pot which significantly reduced the volume of the onboard speaker. Therefore, the decision was made to implement this via switch instead (more toggle switched ordered). A lot of circuit bent instruments do away with the onboard speakers and battery compartment but I wanted the SA-8 to remain portable for easy of implementation.
The second and third mods were a Random Glitch control, which appears to fire off random sounds, although at the current clock speed, seems to favour the samba whistle which can be annoying until you implement the third circuit bend, the feedback control at which point the samba whistle turns into a distorted 8-Bit ‘level up’ glitch sound. As illustrated on my Instagram videos (right).
Continuing modifications on the Casio SA-8. During my initial repair and subsequent experiments (see Circuit Bending – Part 1), it was discovered that under-clocking the chip created some interesting sounds. Research was done into ways of implementing a continuously variable clock and the LTC1799 Precision Oscillator was discovered. Available as either a single SOT-233, 5 pin SMT (Surface Mount Technology) package or on a populated board from circuitbenders.co.uk, the later was ordered.
The decision was made to implement a switch with a replacement 22MHz Xtal on one side and the oscillator on the other. In order to achieve this a stable source of 5 volts needed to be obtained. The Casio SA-8 is supplied by four AA batteries giving around 6 volts to the circuit. One method would be to implement a couple of diodes to drop the voltage by between 1.2 and 1.4 volts. However, this was unnecessary as, on measuring voltages at different points on the existing circuit board a 4.95 volt source was found that was stable and could handle the additional circuitry.
Before implementing the circuit into the synth, it was first breadboarded and measured on the Rigol DSO1054E Oscilloscope to test how the Casio reacted to different clock frequencies. The original clock ran at 22MHz and the results started to get sonically interesting around a quarter of the original clock speed at 5.5MHz. The LTC1799 daughterboard was then adjusted by testing different external potentiometer values and adjusting the trimmer to result in a range of 5MHz-11MHz. The external potentiometer value implemented was 10K ohms linear pot, a reverse logarithmic response would give better control in the higher range but the linear type for adequate for circuit bending.
Implementing the circuit bends into the synth was fairly easy, as it has a “lip” which overhangs the back of the shell, which is perfect for mounting switches and potentiometers in (see photos above). Although the internal speaker was retained the battery compartment had to be sacrificed to fit the wires and oscillator board inside the synth.
The circuit bends made, so far, to this Casio SA-8 are:
- Variable Clock, 22MHz & 5MHz to 11MZHz switchable
- Feedback, switchable in/out
- Random glitch control, switchable in/out
- Fifths switch, high and low, switchable on/off/on
- Button which activates hidden sounds when used with button 1-5
- 3.5mm jack output for feeding into other synth systems
If you are into music and electronics, it is definitely worth looking out for these synths in charity shops, car boot and jumble sales. Future experiments will include experiments with starving the circuit of power and looking for touch points which might be sonically interesting.
References & Links
DigiChip.com (n.d.) LT1799 DatasheetAccessed 21st Jun 2019Available at: https://www.digchip.com/datasheets/parts/datasheet/262/LT1799.php
Etsy (2019) circuitbenders LTC1799 Precision Oscillator PCB module or on DIP6 adapter – for circuit bending or moddingAccessed 21st Jun 2019Available at:https://www.etsy.com/uk/listing/230105357/ltc1799-precision-oscillator-pcb-module