Omnichord OM-100
Omnichord OM-100
The Suzuki Omnichord is a one-man-band instrument that combines bass, chords, rhythm, and a lead sound that is activated by a strum plate (it's the diagonal metallic plate in the photo). This particular model is designated the OM-100.
Pressing one of the chord buttons (the 3 rows of buttons at the bottom) will start a preset bass, chord, and rhythm pattern. If you glide your finger across the strum-plate, it will play a cascading sequence of notes, harmonically related to the chord button you are pressing down.
It may sound very preset and boring, but it's mesmerizing, and hard to put down once you've picked it up.
Opening The Omnichord
Opening The Omnichord
All Omnichords from the model OM-27, to the model OM-300, can be considered analog instruments. However, as the models progressed, more and more digital control was added. This gave the capabilities of chord storage and playback, and MIDI.
One thing that is not widely known, however, is that all Omnichord models use digital circuits to create their waveforms. The technology is somewhat related to the organ industry. All sounds in the Omnichord start as square waves generated by a digital timer chip. They are then filtered and sent through a VCA, which gives it that warm analog sound.
Omnichord Chipset
Omnichord Chipset
There are 3 digital chips at the heart of the OM-100. The chip in the upper right is a general purpose counter and timing chip, originally designed for use in the IBM PC. In the Omnichord it's used to create precisely tuned square waves. It has 3 outputs, one each for bass, chords, and the strum-plate.
In the middle is a chord generator chip. It gets fed a square wave at the root frequency (eg. C1), and can generate the other waves to create a specific chord (eg. for a major chord: C1, E1, G1, C2).
The chip with the paper label affixed to it is a ROM chip, holding all of the preset bass, chord, and rhythm patterns.
The empty space in the lower right (the dotted outline) is a placeholder for an 8-bit microcontroller. The OM-100 and OM-200 share the same circuit board, but some extra components needed for the OM-200 are not soldered in place. If you drilled holes for the chip along the dotted outline, they would match up with pads on the back side of the circuit board. The biggest difference between the two models is that the OM-200 has MIDI output.
My client contacted me on the phone, and told me his Omnichord was only outputting a low buzzing sound, but it was working fine a few days earlier. I told him to bring it over, and I would take a look at it.
I asked him if he could remember the first time it started to malfunction. He said he may have used the wrong power adapter, but he couldn't exactly remember. Uh-oh.
If a power supply fails, either from age, or by using the wrong power adapter, it can often blow other components in the equipment.
Getting To The Power Supply
Getting To The Power Supply
The Omnichord needs a 12 volt DC supply for its analog circuits, and a 5 volt DC supply for its digital circuits. When I turned it on, I could hear a faint buzzing that could be increased with the volume knob. This indicated that at least the analog components were getting power.
Burnt Current Limiting Resistor
Burnt Current Limiting Resistor
Once I had it open, I could read 12 volts feeding the amp, but nothing feeding the digital chips.
At first I though it might be the 5 volt voltage regulator, but when I looked under the power supply board, I found a burnt out current limiting resistor.
A current limiting resistor is designed to burn out when the current running through the circuit exceeds a safe level, either due to a bad power supply, or some other failure. It sacrifices itself to save other more expensive components.
Something interesting to note, is that this was not in the original design of the OM-100. If you look just to the left of the burnt out resistor, you'll see that a trace has been cut. As well, the resistor looks hand soldered, and not machine soldered like the rest of the components. This was probably done manually in the factory, as one of the last steps in manufacturing.
Once I replaced the resistor, everything powered up, and I spent the next several hours strumming the Omnichord. Eventually I phoned my client back, to have him come over and pick it up.
