Wednesday, March 18, 2020

Electro Harmonix Rhythm 12 Schematic

I was able to get another obscure drum machine, an Electro Harmonix Rhythm 12.
It has a somewhat unique sound, and I wanted to understand the voices better. There is a schematic available out there, but it's not the clearest. As usual, I decided to redraw it in Eagle CAD.

There are basically 4 voices, but they can be manipulated a bit. Parts of the sounds can be muted or even cut short.

Here is the portion that actually creates the sound. The sequencing and amplification parts are not shown. 

There are four "Twin-T" oscillators present. Three are damped, so that they decay naturally. One is self oscillating, and generates a tone used in the cymbal.

The cymbal and snare noise go through single transistor VCAs. These are pretty crude, and seem to work by offsetting the input into (and out of) the active region of the transistor. This heavily distorts the signal, but it does work.

Most of the other transistor serve to mute drums for pattern variations.

Here is the beginning of a layout. It's very easy to route, and could be done single-sided with some jumpers.

Saturday, March 14, 2020

Radio Shack Electronic Drums Schematic 60-2249

One more attempt to preserve and understand a drum circuit...
This time we have the Radio Shack Electronic Drums from some time in the 80s.
It features four velocity responsive drum pads, and four sounds. There sounds are bass, snare, tom tom, and phaser.

I took a few good picture of the board, front and back. Then I traced them in Gimp. I make layers for things like jumpers, components, holes, top/bottom copper, etc. I also color code things like power rails and audio buses. That makes it easy to view exactly what I need while creating the schematic.

Here's a rough copy of the board in Eagle. Some of the transistors didn't have equivalent footprints, so liberties were taken.

I recognized the configuration as a variation on the classic "Bridged-T" oscillator, or a damped tuned resonator. Three of the voices are basically the same, while the last is gated noise. Nothing terribly interesting, but here it is.

Eagle files and high resolution images available on my github.