Sony DVCAM DSR-70A Operating Instructions Manual page 39

To enable re-recording
Set the cassette's REC/SAVE switch to REC.
If you insert a cassette into the unit when this switch is set to SAVE, the
unit will not record when you press the PLAY button while holding down
the REC/SEQ button.
Checking the tape for slack
Using a paper clip or a similar object, turn the reel gently in the direction
shown by the arrow. If the reel does not move, there is no slack. Insert the
cassette into the cassette compartment, and after about 10 seconds take it
out.
Paper clip, etc.
Reel
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Chapter 3 Recording and Playback