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Omnichord Power Burnout

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.

A Digital Secret About Analog Omnichords

Opening The Omnichord

Opening The Omnichord

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

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.

Oops!

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

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

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.

67 comments on Omnichord Power Burnout

1 2 3 4 5 > [last]
Max Sentry
What was the value of the Current Limiting Resistor that you replaced On the Omnichord? (I can't quite make it out).
April 3rd 2014 13:15 EDT
Keith
The resistor in the photo is hard to read because it is the burnt out one, and has char marks over some of the bands. The replacement is a 22 ohm resistor, which allows a maximum of about 500 mA. There is an error in the OM-100 schematics that you can find online. They show a 2.2 ohm resistor, but ... Read More
April 3rd 2014 19:14 EDT
l
are you sure its 22 ohm ?? Im about to replace my burnt out resistor
April 24th 2014 09:23 EDT
Keith
The 22 ohm resistor worked fine for my client's OM-100.

Do you have the original, correct 12v DC adapter? If you don't have the correct adapter it may burn out the resistor again, or cause other problems.

Let me know how it works out.
April 24th 2014 18:54 EDT
lawrence
the resistor is burnt out but I can just read it as RED RED BROWN. ie 220 ohm. im just double checking that 22 ohm is not too a low value. To confuse matters my schematic I got off the web shows A 2.2 K ohm resistor. I think my mate used the wrong polarity 12v adaptor, that could explain the ... Read More
April 25th 2014 03:54 EDT
lawrence
thinking about it. Its hard to see but the burnt area maybe GOLD BAND. So divisor 10 and IT A 2.2 ohm resistor I've just removed from omnichord. It's hard to tell. IN this case your 22 ohm option to repair is safe. Would you go along with this CHAIN OF THOUGHT ?? Lawrence
April 25th 2014 07:33 EDT
Keith
Hi Lawrence.

The schematic is wrong. The correct value is 22 ohms. This resistor was added at the last minute, during the manufacturing process, so I'm not surprised that the schematic has an error in it.

The most important thing is to use the original adapter (12v DC) with the correct polarity.
April 25th 2014 18:47 EDT
Don
Great info. But I have one question. Is the resistor 1/4watt or 1/2watt? Also the OM-100 I'm working on has a bad cap. The 3300uf 25v cap is domed or bulging at the top. Folks might want to check theirs if they are using this repair.
December 10th 2017 21:05 EST
lawrence
Hi Kieth

I fitted an r22 resistor. This HAS worked

I did speak to Suzuki UK and they said there are errors on various schematics. So your help was real handy

Cheers Lawrence
April 27th 2014 07:08 EDT
Keith
Lawrence,

Great. I'm glad it worked out!
April 27th 2014 15:05 EDT
david
hi, it seems like you know what you are doing^^. so i am asking you: when i turn on my newly purchased omnichord om84( i am from germany and it was quite hard to get my hands on ^^) i am hearing a buzzing sound when increasing the vibrato voice of the strum plate when using on adapter. there is no ... Read More
May 2nd 2014 15:20 EDT
Keith
Hi David. I'm guessing that you might not have the original power adapter that came with the OM-84. The original one has a label saying 'Suzuki' model 'OCA-1', and has filters built-in for ripple and noise. There is very little inside the actual Omnichord to 'clean up' a ... Read More
May 3rd 2014 20:15 EDT
phil williams
A friend of mine bought an om-27 from the states.(he lives in the uk) stupidly he just bought a plug adapter and thought that was enough. On plugging it in the light came on then went out. I guess this could be due to the power adapter having 240 volts through it instead of the 120v it was ... Read More
May 9th 2014 06:57 EDT
Keith
Hi Phil. If the OM-27 works correctly with batteries, but only gives a loud hum with your adapter, then the adapter output voltage type may be wrong. The output of the adapter should be 12 volts DC (not AC). The OM-27 doesn't have a rectifier, so you need to feed it DC. Also, you need to ensure ... Read More
May 10th 2014 19:52 EDT
James
My OM-27 has a weird issue, the manual chord setting makes the bass note jump an octave up after approx. 1 second when a chord button is depressed. It does it on every note, If the memory button is activated it only happens once and stays an octave up. Do you have an explanation or a fix for this ... Read More
May 18th 2014 12:50 EDT
1 2 3 4 5 > [last]
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