Casper Electronics

The SK series have 4 voice polyphony, and this mod explains how to get these as separate audio outputs. It is most apparent on the SK-1 demo song where the Bass line can be extracted or the melody or Percussion.

All 4 channels ( Melody, Obbligato, Chord and Bass ) are the result of 8 separate signals from the CPU, it combines a pitch signal and an envelope signal to give an Audio signal per channel. All we need to do is identify where on the main PCB to get these signals. As you are connecting these signals to a line out socket and other gear, it is a good idea to de-couple the SK-1 signals so there is no DC voltage present on the sockets, which we do with a 4.7uF capacitor.

Looking at the PCB, the audio signal for CH1 where is leaves the bit of circuitry that combines the pitch and envelope signals is R98 which should be a 47K resistor. For CH2 it is R47, CH3 is R81 and finally CH4 is R98. If you look at these resistors and follow the trace, one end will go to an electrolytic capacitor and the other side of that capacitor connects to a 3K3 resistor. You need to solder a wire at the side of R98, R47, R81, R98 that connects to the Capacitor / 3K3 resistor circuit. The other end of this wire you solder to a negative side of a 4.7uF cap. The other side of the Cap goes to the hot or tip connector on your audio output jack. Then you wire all 4 Ground sides of the jacks together and connect to the negative battery terminal inside the SK-1. You now have 4 individual outputs.. play the demo song and connect your Amp to each jack in turn. As this is a hardwired mod, you can plug the headphones into the SK-1 to silence the internal speaker, but your 4 outputs will still be live.

Not forgetting, the Percussion. This exits the CPU at pin 97 ( SK-1 only ) and you have a choice. Get another bit of wire with a 4.7uF capacitor soldered to it ( -ve side ) on the one end, the free end you can connect to R46 (100K) where it joins with R45 (47K) to give Raw percussion from the CPU or connect it to either side of R43 (100K) to get the percussion audio though the SK-1ís existing percussion filter. Once again solder the +ve side of the 4.7uF cap to the Hot or tip connector on an audio Jack and connect the ground side of the jack to the negative battery terminal.