seems to be an electrical problem,cuz buttons work perfect, i dont hear any motor sound when i hit play, question, it could be when i select "tape" may be the button is not making contact, i bought this gf 777 for 40 dollars, on a gas station, they got it in a closet outside, so it was dirty , ive been cleaning my unit and things start working, all speakers work, but one of the subs gots less volume than the other, i will try cleaning spray to see,i will post sum picks of my progress on the restoration, any help, i will apreciate. any manual to fix tape deck i apreciate.

Might need a new motor...or possibly aa resistor that controlls the motor has blown...(actually there not called resistorc) can't remember though...its quite common for one of these to blow on sharps and other radios....good luck

join up with boomboxery... i think you can get a free pdf service manual there

if the decks are completely dead,there is  a couple of transistors that control the power to the motors,cant remember the part number,but i remember having trouble with a GF575 years ago that had the same fault,and thats what it was..

try switching the "tape operation" switch to the right, then hit play....if you don't hear the motor running then perhaps maybe it is the motor that needs replacing...good luck in restoring it...