Yamaha RX-21 L Modifications
| Here is a description of the modification process of the Yamaha RX-21L digital rhythm programmer. This is a relatively simple drum machine from the late 1980's,with a Latin Percussion kit(congas, bongos, agogos, whistle, timbales, and cuicas).The RX-21L has MIDI in and out, stereo outputs, and headphone outputs. This particular unit was purchased on E-Bay for $15.00, from a very tall cross dressing man in the Denver area. This is the first RX-21 L that I have modified, although I've done several RX-21 models with the normal drum set. The Chip set in the RX-21 is the same, but the IC's are labelled differently on the PCB. To begin the mods, remove the 6 screws on the metal baseplate, and remove the baseplate. This is what you will see.... |
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| There is not very much room
to work with inside this machine for mounting switches, pots, etc., so
I have mounted a small box (2 x 4 inches) to the side of this machine.
Once inside the machine, there are three IC chips of primary importance.
On the printed circuit board these are labelled IC 112, IC 113, and IC
120.
-IC 113 (printed with YM 3012 on the chip) -IC 112(printed with YM21866) -IC 120 (printed with YM2188) |
| First, you should program
a pattern with a beat of your liking. Hit start, let the beat play, and
turn the machine over.
On IC 113, bridge pins 4 and 12 with an LED of your choice. This produces an overdrive type distortion. I added a green LED and a switch. The anode :) should be connected to pin 12. The only difficulty of installing this mod is the large heat sink right next to this chip. This particular IC (the yamaha YM 3012 chip) can be found in many yamaha drum machines and synths including the RX-15, RX-17, RX-21 and the FB-01 sound generator(and possibly others). This mod works in all of these machines, the LED can be replaced with a resistor, potentiometer, or capacitor of different values for slightly different effects. In the area around this IC, you should see a small ceramic capacitor labelled C 112. By connecting an audio input jack(RCA jack in this model) with a resistor or potentiometer(10k Audio in this model) to the legs of C 112, you can share the output of this machine. This is useful as an effect or to save mixer space. |
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On IC 112 a LED was used to bridge pins #43 to 59, the anode at 43, then connected to a switch. This produces a slight clipping of the signal and a dancing LED. |
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On IC 120 two capacitors(experiment with different values for varied level of effect) from my parts bins, connected in series, and attached to a switch. This completely transforms some of the sounds into crazy digital noise. |
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After determining the layout, it was time to wire it up and put it back together. I used a small project box bolted directly to the upper left hand corner of the machine for my new control center. I have included the 2 distortion LED's,audio input level, and switches in the box. All of these modification functions have no effect on MIDI performances, only the audio output. After completing a project I like to relax with a beer or two. I currently recommend RedPoint Amber Ale. |
