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Service manual or schematic for Concord 350 reel to reel recorder

My parents have an old Concord 350 5" reel to reel tape recorder that distorts on recording. It monitors free of distortion. I would like to fix it for them because they want to record our senior relatives voices and stories of old times while they are still around. Can someone help me find a service manual. Or maybe someone can make a suggestion where I can start to solve this problem.THANK YOU