Tuesday, April 3, 2012

Circuit bending the Yamaha RX-15 Drum Machine

RX-15 Modifications:
To open up the Yamaha RX-15, first remove the five screws holding on the plastic rear panel. Next, turn the machine face down, and remove the eight screws along the top and bottom. To have clear access to the circuit board, you should disconnect the white wire harness located by the output jacks. You should now see something like this :
We’ll start with IC 10 on the circuit, which is a large chip with alternating small and large pins which reads YM 2154 on top. This is the CPU of the drum machine, and handles most of the functions. In the picture below, I have marked 4 pins with a sharpie. If you connect pin 59(with the solitary mark) to either of the other pins I have marked, you will get a harsh sounding distorted synth effect to varying degrees depending on the pin choice. It is imperative that you DO NOT experiment with the pins on the other side of this chip or you will do PERMANENT DAMAGE to this machine. Trust me; I’ve learned the hard way.
Next, we’ll move on to IC 15, the YM2190 chip. I believe these are the ROM chips which contain the sounds for this model and the RX-21 models. By connecting pin 7 to pin 12, a decent distortion is created. Other connections on this chip result in muting of sounds, and clicking variations. There are not any negative consequences to experimenting here, but not much else to be found either.
Next, we’ll move on to IC11, the YM 3012 chip. This is the D/A converter for this machine. By connecting pin 4 to pin 11, some digital signal is fed into the analog output, resulting in noisy distortion. This connection works best when filtered by a capacitor. Experiment with values, I chose a 1.5 picofarad (102) metal film capacitor to suit my tastes.
Finally, we will work on IC 16, labeled YM4556D. This chip is one of the output opamps. Connect pin 3 to pin 7 for an overdrive type of distortion. I used a 10K potentiometer for variable results.

After these mods are completed to your satisfaction, you may drill the housing for the RX-15 near the top for the switches and potentiometer. Solder all of your connections, test, and reassemble the machine. You have now made a crappy drum machine slightly less crappy, or more if you see it that way.
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