This is the Oberheim Matrix-12, my personal favorite polyphonic synthesizer.
The Matrix-12 came out around 1984-85.
I was having some power supply issues with mine.
Sometimes it would not boot. Other times the system would lock up during use.
And I was getting the dreaded Voice 1 Processor Malfunction message on the display.
That's just no bueno
Even with today's sophisticated access to all kinds of information,
there are abolutely NO documented power supply repair tips. Anywhere.
That is, until now.
In this video I am going to show you what I did to resolve my issues.
I will also offer some helpul insight into how this unique power supply works
and why it is different from most other synthesizers I have encountered.
Some of these tips may help you with the Oberheim Xpander as they are similar.
Make sure you feel confident in doing these kinds of electronics repairs.
There's no shame in asking a knowlegable friend or a reputable synthesizer tech for help.
First thing, make sure to unplug the power cord.
You won't need it for these repairs.
Failure to unplug the cord could result in electrocution of yourself and your beloved Matrix-12.
Next, you will want to download the power supply schematics from the internet.
Make sure you have the appropriate tools for doing this kind of work.
I use a Hakko 808.
But it's discontinued, so you may find a similar alternative.
Failure to use the right tools could result in overheating and damage to your circuit boards.
Just get the right stuff, it's worth the investment. Trust me.
On the Matrix-12 we are dealing with an SMPS.
This means Switch Mode Power Supply.
Typically, most other instruments use a linear power supply and looks thus:
Alternating Current mains power is stepped down with a transformer.
Then the AC is rectified to Direct Current using diodes
DC is smoothed using capacitors
Eventually, Voltage Regulators will output the power needed by your synth.
On the Matrix-12 The first three steps remain the same
Then a transistor is used to chop the current into a pulse wave.
The amount of power being supplied to the system varies with the pulse width.
The pulse width is controlled by a transducer chip.
This chip gets its input from a feedback circuit.
Here, another transformer isolates any remaining noise in the power supply
from the sensitive digital components of the synthesizer.
As you can guess, another round of rectifier diodes and smoothing capacitors would needed.
because you have a pulse wave
And lastly the voltage regulators.
Why in the world would we want to do this?
#1: To decrease the heat dissipation without heavy/bulky heat sinks.
#2: As you know, heat = energy loss.
So using a switch mode power supply is more energy efficient.
See, Tom was "thinking green" all the way back in the 80s.
Why I Didn't Troubleshoot My Power Supply
Normally when I start a project like this, I would disconnect the power outputs
and use a volt meter to verify each one.
And normally I would use a scope to look for ripple in the power lines.
However, this is not possible with switch mode power supply.
The supply senses the current being drawn by the load,
and continuously compensates by adjusting the switching pulse width.
It's a Catch 22!
I want to check the power by taking away the load.
But if I take away the load, it won't power up.
So what's to be done about it?
I decided to take a routine maintenance approach.
The synth is over 30 years old, and needs certain preventive measures performed anyway.
Maybe by performing this preventive maintenance would have an effect on my issues.
Power Supply Restoration
Again, be sure to unplug your synth! Don't blow yourself up.
Now that you're a little bit more familiar with the Matrix-12, let's look at what I did to resolve my power supply issues
The electrolytic capacitors are over 30 years old.
So I stared out by replacing them with new ones.
I used high quality Nichicon capacitors to the same specs as the originals.
Be sure to inspect your circuit board while doing so.
I noticed on mine there was a darkened area from excessive heat.
It doesn't automatically mean it's toast,
but to check the components in the area.
On mine I went ahead and replaced a resistor and some diodes in that area.
After doing these things, I noticed some improvement. But the problems were still showing up.
It was still not booting sometimes
and other times locking up
On closer inspection, I noticed my shunt resistors had hairline cracks in them.
I replaced them with metal film resistors.
Lastly, I replaced the power entry module.
The power entry module is where the cord plugs into the synth.
It actually contains more filtering capacitors
I have put the Mouser order numbers for the shunt resistors and power entry module
in the description for this video.
I am happy to report that I have been using my synth all week without any issues whatsoever.
No boot problems. No voice processor malfunction BS.
It just works which is wonderful
Hopefully these preventive measures will help you with your problems with the Matrix-12 and Xpander.
If you found this video helpful go ahead and click the like button
thanks for watching.
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