The group home page says "Other Kurzweil products welcome include [...] EGP, Mark III ?& IV [...]" so I will try to help here.
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Kurzweil's web site distributors list () has Gema Music in Sofia as their Bulgarian distributor.
Gema Music's web site, ,? (per Google Translate, at least) says "We provide warranty and post-warranty service."
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If that's no help, the owner's manual is online:
The owner's manual strikes me as being rather sparse. I came up empty searching for a service manual online.
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Does the power LED come on? If not, the problem could be the AC adapter, an internal fuse, or (less likely) the power switch.
If the power LED lights but no sound comes from the speakers, does the headphone out work?
If the headphone out works, but not the speakers, the problem is likely with the audio amplifier.
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If neither speakers nor headphone out works, my next step would be to open it up and start looking for bulging or leaking capacitors, any area on the circuit boards with corrosion, charring, or melting, or an internal battery that has discharged or leaked.
Take lots of photos as you disassemble! Use small containers to keep groups of screws together.
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I also found an interesting page () by a person who couldn't get the sound generation to work but salvaged the key bed, control wheels, and associated circuitry as a MIDI controller.
