...I unfortunately had to learn this the very hard way, when I was working on a deck that didn't seem to play any sounds, though all seemed OK. It turns out the erase head was in full erase mode when ever the deck was running, whether the record protect tabs were in or not :-(.....so I was rewarded with nice, 30 second to 2 minute gaps of silence in 3 different calibration tapes, two of them very expensive Nakamichi tapes..