Looking for Sony TC-645 Service Manual

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I have a Sony TC-645 3-motor reel-to-reel tape recorder that I would like to refurbish. I am looking for a service manual for this job, could anybody help me? I heard that these Sonys are much in common internally.

I also want to give a small bias voltage to the left AC reel motor so that it gives some tension to the tape. The simple friction tension is not fine enough. The takeup motor gets a smaller voltage during playback and full voltage during fast winding. I want to do something similar with the other motor. Any ideas how can I do this?

Laszlo
 
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