TU58 problems

As I understand it the TU58 is not NRZI. According to the manual (and I see this sometimes) a 'bit cell' starts with a +ve magnetic flux transition (OK, that is meaningless, it really means there is a +ve edge at the output of the read comparator at the start of every bit cell). There is a -ve edge somewhere in the bit cell. If it is before the halfway time it's one state of bit (I forget which), if it's after then it's the other. They are recorded at the 1/4 and 3/4 times respectively to allow for some errors. So an exact half way transition is meaningless. When I say I see it 'sometimes' I can't know exactly which bit of the tape I am reading each time I sample the signals. So it's very possible the read circuits are behaving consistently and the tape is the problem. -tony