Modifying Casio PT-82

A Casio PT-82 was considered as a classic of Casio 8 bit technology before the famous SK-1 came in market. A LushOne kit can be used to modify simple keyboards. But PT-82 does not has a circuit bend as the SK-1.The useful modifications that can be made are quite limited. The sliding switches can be stripped back to individual components, plastics can be removed from the debris and the springs can be adjusted on the sliding switches.

In one of the tutorials of Lushproject, base can be used to add a filter with LFO input to the cut-off frequency and a ring modulator with second oscillator input to the PT-82. The key parts of PT-82’s circuit are main and secondary waveform outputs from the oscillator IC in the PT-82, Percussion waveforms, envelope waveforms that control the note dynamics, Input to the PT-82’s amp, touch points that bend the PT-82’s clock frequency and modify the note pitch.

One can easily locate the circuit signals in case manual is not available.The PT-82’s audio is not complicated. The different instruments are all square waves with different patterns in all. There is an envelope control that controls the volume of the square wave but this seems to only have about 8 levels.

Modifying casio PT 82

Once all the signals are found bring them all to a 25 way D connector on the back of the PT-82’s case.Trace all the signal from the input to the PT-82’s amplifier and route it through theLushOne’s filter before giving it back to the PT-82. The signal is at a lower level than the LushOne required so a diode gain limiter can be used in the filter to match the lower level. The LFO from the LushOne should be hard wired to a control voltage input of the filter.

This kind of arrangement is needed to modify the PT-82 sound using the LushOne filter and LFO. To provide an opening for the LFO, the envelope signal can be passed through a comparator (actually constructed as a schmitt trigger) to determine if the PT-82 was playing a note.

PT-82 has square wave signals which requires a ring modulator that can be build by exclusive-oring with another square wave signal. The software and hardware are modified for the LushOne’s main oscillator to generate square waves that are similar to the patterns used by the different instruments in the PT-82. This is then passed via a switch in to an ex-or function which is inserted in to the digital signal path of the main oscillator in the PT-82.

For a neat fitting of hardware, both the old and new components are joined to a common plywood base to keep everything together. All the new electronics should be fixed to the front panel. It can be complicated. To make everything fit, a circuit board had to be mounted containing the new ex-or function underneath the keypad in a 3D stack. Everything should be linked with connectors to help manage the complexities of the wiring and assembly.

On viewing the photograph above you can see the back of the front panel with the LushOne board on the right and the additional electronics and keypad on the left. In the middle at the bottom is the D connector to the PT-82.