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Sony TC-640 reel to reel service manual

Hi does anybody have a service manual for a sony TC-640 reel to reel player. i got the unit working after cleaning it and relubricating and it plays well and sounds nice but there is one problem. when you are in play mode and hit the stop button there is just enough slak to cause the tape to sag and keep the unit from being played again with out winding the reel alittle by hand to tighten it back up. i have the tape threaded right so thats not the problem,and it seems to be stopping right, any help or advise about this problem would be appreciated. thanks