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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  1. Quick question. I've bent several keyboards, but they've all been battery operated. How can I safely bend something like this and other drum machines/keyboards that have a power cord without electrocuting myself?

    1. To safely bend or modify a device that uses a cord, you have to be aware of where the power supply circuit on the machine is. For a device like the RX-15, you can locate the transformer and use a multimeter to make sure you are on the low voltage side. On most items that use a wall wart power supply, you are already operating at less than 15 volts. Again, just stay away from the power supply components, or use this as an opportunity to learn more about circuit design, voltage dividers, etc.

  2. nice, thanks for the documentation. found one these units at the market for 10 and since im not proud of my track record of permabending some yamahas im weary of wondering off course but curiosity permabends yamahas mwhahah so thought to ask, do all the top outer pins on IC10 cause results other than permabent? also, around IC15 there are 3 other identical looking IC12,13,14 in a bank/row you think these are also sound rom chips with others bends there? excited about getting home to make this a useable piece of kit.

    1. Pretty much everything other than what I listed here on IC 10 is a BAD IDEA to mess with. I've fried a couple of yamahas until I figured this out. As for IC12,13,& 14 there are other bends on these chips, just nothing spectacular. Event with some bending, the RX-15 is still pretty limited. Good luck.

    2. The issue is not the topic, but I really should. Nemogli you tell me what the output voltage of the supply transformer. I burned down and I can not identify. Help please.

  3. Hey anode8,

    do you think the same bent will work with a rx11?

    thx max

  4. @Joe Clay - I believe the transformer output voltage is 15 volts

    @Maximillian - These bends will also work on the RX-11. there are a few more on the RX-11 also, but i don't have them documented right now.

  5. Hi Anode8,
    I saw somewhere you've already bend the XR-17, which I'm currently modifing.
    I've already put a LTC1799 on it (pitch up knob and pitch down knob) , and played live yesterday with a friend during an Acid Jam :)
    Try it on other XR machines, it really really fun !

    So, if you can give me informations about the XR17, I'll be glad.
    Anyway, I'll still diging into it, especially to find sequencer glitches.



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