I turn on the power and get just a buzzing noise. The keyboard does not work at all. The power light is on but no sound from the keyboard other than a buzzing sound.

Have you ever changed the internal Lithium Battery. Try the reset here, although it is for a different model. The Keyboard was actually made by Casio and sold with the Radio Shack name on it.

REPLACING THE MEMORY BATTERY

Your keyboard uses one lithium battery to protect some types of data when it is turned off. This battery is installed at the factory,
and lasts up to 5 years. If the keyboard's memory is erased when you turn the keyboard off, take the keyboard to your local Radio Shack store to replace the lithium battery.

RESETTING THE KEYBOARD

Note: Resetting the keyboard clears all information you stored in the keyboard's memory, including the parameter settings, song
sequencer, pattern sequencer, and registration memory. Resetting the keyboard's parameter settings clears only those settings from
the keyboard's memory. To reset the keyboard, turn the power off.
Then hold down ENTER. Reset? appears. Press YES.

https://www.google.com/search?q=Radio+Shack+Midi+Keyboard+MD-4072+reset

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Electronic Repair Services for the Entire Music Industry MITA International

Sounds like a classic ground-loop issue. Sound-on-Sound had a great article some time back that may help you.

For a start , check headphones out.


• if it don't work , check power supply of the preamps.


• If it is OK, could probably the main amp have fail.

But check the main volume .

First thing: Make sure ALL and I do mean ALL of your interconnected equipment is POWERED from the SAME receptacle. Powering different parts from different receptacles in a building will surely get ground bounce buzz and ALSO subject the equipment to hazards of circuit damage.
The audio connecting cables should be BALANCED lines where possible... either XLR or TRS cables. Problems like you are having are common when CONSUMER grade equipment is mixed with PROFESSIONAL equipment. Anyway, get all the stuff powered from the same receptacle and get back to me if you still have the problem. Note you may have to run three wire extension cords if devices are separated a bit... the pro's have to do that to avoid noise. All devices should have three wire (grounding) cords and/or grounding wires connecting chassis...

so its a buzzing noise. are your turntables grounded? some dont have to be. the buzzing could be several things feedback from something around you or the mixer is buzzing cause of to much power. just mess around w stuff your not gonna break it. and what i mean about that is more wires check wires. dont preform open heart surgery on your equptment. sometimes the buzzing means the faders are going i own 3 vestax mixers and on my pmc 17a right before the faders went i had massive buzzing sound.

The amp inside probably has failed on the one side. The buzzing might be the ripple of the power supply driving into the speaker. DO NOT use it till repaired or speaker MAY be damaged.

Unplug for several minutes and try again.

If that doesn't work, look up the "factory reset" procedure for your particular model via google and follow that procedure. It usually involves holding certain buttons down before and during power up.

PLUG YOUR LEAD INTO AMP TURN UP JUST A LITTLE SEE IF IT BUZZES WHEN YOU TOUCH THE END OF YOUR CABLE.IF IT BUZZES THEN THE FAULT IS IN YOUR GUITAR MOST LIKELY THE ACTIVE WIRE HAS COME OFF THE INTERNAL GUITAR PLUG.

If it runs on a wall wart transformer, the transformer rectifier in it may have failed. Try a new one of the same rating. It is also possible that the input filter cap in the keyboard has failed resulting in un-pure DC voltage to the keyboard.