My MicroKorg Synthesizer works fine with batteries, but won't work with a 9V DC adapter. If I take the batteries out and just plug it in and turn the power on, it does nothing.

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Check the mic and connections all through its input into the unit. I would check the cartridge of the mic for continuity for the reason that (I think but not 100% sure) on this model, I think there has to be sound at the mic or it does go silent in vocode mode. This is how it digitizes the mic and keys together. The played keys cannot put onto pitch the sound it is NOT picking up at the mic. I think there is a problem with the mic.

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Mine has the same problem with different keys! I don't know what to do o.O

Sometimes, with BOSS pedals, the battery must be disconnected before using another power source. I ran into the same problem with my DS-1 distortion. If it's not that, then check the power rating of the power adapter. It should read, for the output, exactly 9V DC. If it doesn't, then you have to get a power adapter that has that same rating. Boss pedals will not use anything other than 9V DC (I know, I've tried).

Hope that helps!

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/cameron_463100ffd54b591d

Sorry, the SR16 requires a 9 VAC, NOT DC power.

The AC inout generates BOTH + and - voltages to power the unit. Your DC adapter will NOT work.

I would inspect the unit for corrosion on battery terminals AND the power connector. The connector has a switch when a cord is plugged in, and if this switch doesn't work (they get bent) the unit will not work on batteries.
Best source of power adapters is often your local thrift store.
MATCH the voltage and whether it is AC or DC required AND the polarity if the pin is + or -.
Get one with enough current.

I can tell you what it is from first-hand experience. The audio in LED's coming on and going out means that a power supply putting out more than 9 volts DC was used...or the wrong polarity. Korg sells a repair manual for about $12 but they ultimately need a whole new circuit board. If the wrong polarity supply was used, then maybe only a diode was damaged. This would be a cheap fix. But more than likely it knocked out one of the large surface mount IC's that us techs don't even dare to replace. Korg charges around $280 for a new circuit board. A used working microKorg sells for this much.

Honestly, I would stay away from Korg products and many others. Many companies are now using surface mount components which are cheaper and harder to fix. Moog synths are expensive but they are completely fixable(and are built to last). If Korg would install a $0.25 voltage regulator on the input of the DC jacks, the unit would be saved. I think this pretty much sums up the build quality. This is probably not what you wanted to hear but is how it is unfortunately.

There is a possibility that the receptacle for the adapter plug has one or more broken solder joints on the circuit board.

You might try to clean it. You can buy some tv tuner cleaner at radio shack. If that fails, then the pot. will have to be replaced. If you can solder and de-solder components its not hard to change out. When you open the unit up, the volume control or POT has a ohm value. Like 200, 250 ect…. It may have a K after it, that’s x 1000. Get the right size ohm value and replace it. Hope this is helpful.