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Tape heads, how do they work

Discussion in 'Electronic Basics' started by Uriah, Aug 21, 2006.

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  1. Uriah

    Uriah Guest

    I have a tape head with three wires coming out of it. One goes to the
    metal shell around the tape head and the other two come out of the
    thing. If I use a storage scope and ran some tape over the head would
    I be able to see the signal? How would I hook it up to the scope? One
    wire to the ground and one to one of the other lead? I have tried a few
    things but don't get much of a response. Could someone give me some
    details on how to see what the tape head sees on the scope?

    Thanks
    Uriah
     
  2. With 3 wires from the tape head and one known to be a "shield" to
    ground, connect that one and either of the other two to your scope ground.
    Expect likely as low as mere millivolts of audio from the remaining lead
    with respect to the first two. Many scopes do not display well signals of
    just a few millivolts, at least not in my experience!

    - Don Klipstein ()
     
  3. John Riley

    John Riley Guest



    I know little about this, or anything electronic, but is the fact that
    pickup heads are biased alter your problem?
     
  4. You would need a preamp to lift the signal.
    Cheers ........... Rheilly
     
  5. Eeyore

    Eeyore Guest

    Tape replay heads require no bias.

    Graham
     
  6. Don Bruder

    Don Bruder Guest

    Unless you've got a super-sensitive 'scope, you're probably going to
    need to run it through an amp before you'll have much luck seeing
    anything - The "raw" output from a head is tiny. I mean *REALLY* *TINY*.
     
  7. There would have to be something recorded on the part of the tape
    you're using. The signal output will be very low, and it will be
    proportional to how fast you pull the tape across the head. The
    frequency of what is played back would also be proportional to the
    speed. If the tape isn't kept exactly perpendicular to the gap in the
    head, the output will be drastically reduced. That might be hard to do,
    holding things by hand.
     
  8. John Riley

    John Riley Guest

    So what are the different biases specified for different magnetic
    materials on the tape?
     
  9. The bias (an AC signal above the audio range) in a tape deck is to get good
    performance on the tape;it's there to arrange the magnetic stream on the tape as you record (or
    something like that). You can tape without bias, though the results
    were never good. And the bias is only used for recording (it would
    erase the tape at least partially, so if it's left on during playback,
    it would affect the contents of the tape).

    I'm not surprised by three leads. The case is grounded to shield it
    from unwanted signals like AC floating aroud the room. The signal
    out of the tape is weak, and the tape head itself is just an inductor so
    it would naturally pick up things via inductive coupling if it wasn't
    shielded. You only want the signals from the tape, not the rest of
    the junk.

    The other two wires are for the actual signal off the tape head.

    Michael
     
  10. You mean like for Metal and CrO2? The difference is in the recording,
    and I seem to recall the preemphasis (boosting of highs to make things
    less noisy) is different to make use of the improved range from the
    different type of tape, and anytime there is preemphasis, one needs
    demphasis (cutting the highs to bring things back to normal) on the
    playback end in order to put the sound back the way it should. If there
    was no demphasis to correspond to the preemphasis, you'd end up with a
    sound from the tape deck that hat too much high frequency content, ie too
    shrill.

    So the switches on tape decks surve two purposes. Set the bias
    and premphasis properly on record, and set demphasis properly on
    playback. The switches have to be on playback only decks so you
    can play back the different types of tapes, but there it only sets
    the demphasis.

    Again, bias on record is just to improve things, it is not a necessity
    for recording on tape (though you'd be disappointed with the results
    if you didn't have it.
     
  11. Eeyore

    Eeyore Guest

    Recording.

    Graham
     
  12. Eeyore

    Eeyore Guest

    'Disappointed with the results' would be putting it mildly !

    Graham
     
  13. Come to think of it, I was thinking of that cheap early sixties battery
    operated reel to reel tape recorder we had. It didn't have an erase head,
    just a magnet. And without giving it thought as I posted, I was thinking
    that it meant there was no bias oscillator, which isn't necesarily the
    case.

    So in the end, I haven't a clue what it would be like without bias,
    so I'll upgrade that "disappointed with the results" to more like
    "you wouldn't make a tape recorder without a bias oscillator".

    Michael
     
  14. I had one of those, and it not only didn't have a bias oscillator, it
    didn't have a capstan or pinch roller- it depended on the pickup reel
    tension to move the tape. It had a real lot of flutter and wow.
    Without bias, the distortion would be greater. There is a hysterisis in
    the transfer function of magnetic force vs. magnetization for the
    material on the tape. You have to get above a certain signal voltage to
    magnetize the tape at all. If you don't have bias, you get crossover
    distortion, which is worse for low level signals. The bias is a
    supersonic signal applied to the head when recording. The audio signal
    is added to that, so even low level audio signals result in a change
    in magnetization, and a more linear magnetization pattern results.
     
  15. Eeyore

    Eeyore Guest

    DC bias ! It's not very good though.

    Graham
     
  16. Eeyore

    Eeyore Guest

    These machines used DC bias.

    Graham
     

  17. That would make the frequency response even worse.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  18. Eeyore

    Eeyore Guest

    It was miserable anyway.

    Graham
     

  19. It would saturate the head and raise the noise floor, not to mention,
    the tape head would become permanently magnetized, and erase the tape
    after a few plays.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  20. Jamie

    Jamie Guest

    i have a nice set of active Fet probes over here that works real good
    for that..
     
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