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Cassette tape speed adjustment

Discussion in 'Electronic Repair' started by N_Cook, Dec 8, 2009.

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  1. N_Cook

    N_Cook Guest

    Assuming my test tape is stretched (I doubt it , as it would have to be even
    stretch the whole length).
    I normally test speed monaurally so the beats are unmistakable. That is 1KHz
    test tape output and floating accurate 1KHz sine signal both fed into one
    headphone cup. Easily gives 0.1 percent resolution (assuming test tape is
    ok).

    If I record accurate 1KHz tone onto a blank tape for exactly ( to human
    reaction time accuracy) 100 seconds. Then replay for beats of 1Hz or so and
    output lasts for 100 seconds +/- reaction time then is the speed correct? I
    suspect that repeating this process with the speed regulator changed , say 5
    percent, would also give "correct" speed, that time also


    Does anyone know how much effect types and thickness (play duration C30 to
    C90) have on replay speed perhaps in conjunction with too soft or too hard
    pinch wheel rubber ?
     
  2. N_Cook

    N_Cook Guest


    Strobing of what rotational part against what reference?
    How do you know if the tape is stretched?
     
  3. N_Cook

    N_Cook Guest


    Whatever touches the capstan spindle could wear and if the reference source
    for stobing is a mains lamp, that is only set for (UK) 50x60x60 cycles per
    hour (for synchronous clocks) and short term only +/-0.5 percent targetted
    frequency.

    I have the related problem of someone who says he has perfect pitch.
     
  4. N_Cook

    N_Cook Guest


    Much easier to say the test tape is stretched.
    Perhaps perfect pitch testing would only work by playing the likes of JSB
    Tocatta and Fugue in D major, not pure tones

    How come I cannot google the rpm of an audio cassette capstan? A calibrated
    strobe and tipex mark on the capstan periphery should give a definite answer
    (assuming the spindle is clean and not worn).
    It should be determinable from tape speed of 1 7/8 in per sec and spindle
    diameter, that is about 1.9mm, but what should it be to 0.1 percent
    accuracy?
     
  5. Ian Jackson

    Ian Jackson Guest

    That's exactly the way I have (occasionally) adjusted the speed of a
    tape deck.

    Record a CD track on a known (or hopefully) good deck, and play it back
    on the suspect deck, while simultaneously re-playing the CD. A bit of
    skilful nudging of the tape deck 'pause' and 'fast-forward' buttons will
    be needed to get the audio adequately synchronised.

    As you tweak the tape speed control, you will may get the two tracks in
    almost perfect synchronism (producing a pleasant echo effect!). However,
    this condition will probably hold for maybe only 10 or 20 seconds, after
    which the tape audio will start to creep either 'early' or 'late', and
    may even wander between the two.

    At the end of a typical 3 minute track, the audio may be a a few seconds
    out, and no matter hard you try, you are never going to do better. But
    not even the most ardent audiophile with perfect pitch could ever detect
    that the speed was incorrect.
     
  6. N_Cook

    N_Cook Guest


    http://www.dynamicdemand.co.uk/grid.htm
    Wokingham, Surrey, UK monitored mains frequency,
    is instructive, try it in the middle of a national televised football match
    or similar


    spindle consistency
    Not from my random sample of 8 audio cassette capstan spindles, all were
    1.9mm or a bit lower say 1.89, with no cleaning etc, just as found.
    If 1.9mm then rotational speed of 478.72 rpm and if the specified diameter
    (cannot even find that) is 0.075 inches (bit less than 1.9mm) then speed is
    477.46 rpm for 15/7 ips.
    Googling including "478" or "477" throws up nothing, googling on rev/sec not
    possible in like manner
     
  7. N_Cook

    N_Cook Guest

    I've since found 2 capstan spindles of 2.4mm diameterm so no fixed
    specification for cassette tape dynamics.

    Another possibility as a test tape - record some constant tone, any f, on a
    few minutes of tape, pull out a long length and pass a magnet over two parts
    a measured distance apart. retract, and then time the interval between dips
    in play mode
     
  8. N_Cook

    N_Cook Guest


    Things counldn't be easier with a "known good machine " .
    My 3 test tapes were created on what was supposed to be such a machine in a
    pro AV studio.
    One tape got knackered at one point but rest of tape agrees with the second
    one. The third remains unused while first 2 agree with one another, using
    any old speed consistent machine for cross-comparison, in relative rather
    than absolute terms.
     
  9. N_Cook

    N_Cook Guest


    3 of them made on the same batch of tape and on the same machine but more
    than 15 years ago. As they are all stored in the same environment they could
    theoretically all have chemically degraded/stretched to the same degree. I'm
    trying to find some process that gives an independent verification that does
    not require a know good machine.
    eg quartz controlled (so low tens ppm) f-meter coupled strobe fed to
    rotating capstan if diameter/s of the spindles are manufactured to specific
    dimensions of precisely known tolerances. Perhaps 1.9 +/- 0.002 mm and 2.4
    +/- 0.002mm say, no such data found googling. I can only measure to +/-0.02
    mm and no slip gauges for absolute calibration.
    +/-2 in 200 or so, is not accurate enough
     
  10. bz

    bz Guest

    Suggestion:

    Google for
    guitar tuning meter

    There are meters available for ~20 bux and even some 'on line' meters.

    Record a known good 'string note' frequency and use it for your standard.
    Should get you close enough for any audiophile's ears
     
  11. N_Cook

    N_Cook Guest


    I found a large quantity of salvaged capstans+spindles.
    26 off on my mike (rather than callipers before) measure 1.99 +/-0.01 of
    reading accuracy and 2 nearer 2.00
    5 off measure 1.79mm
    1 at 2.19mm
    9 measure 2.49mm

    So I will give a few to my engineer mate and ask him what they measure and
    the temperature at the time.

    The machine in question uses 2.49.
    Assuming it is actually 2.5mm then doing the maths and strobing with quartz
    f-meter calibrated strobe then my test tapes are 0.4 percent out from
    calculation via 15/8 ips etc.
    spindle rotation speed of 6.05 rev per second with an error of about 0.4
    percent , needs longer gate time or repeating more times to bring that
    accuracy up (rounding errors? so probably longer gate time required than my
    meter has).
    If 2.49 diameter then 0.6 percent out
     
  12. Bob Larter

    Bob Larter Guest

    You should be able to get a very accurate reading of the diameter of the
    capstan with one of those cheap digital verniers.
     
  13. Bob Larter

    Bob Larter Guest

    <nods> 440Hz is the usual reference tone.
     
  14. N_Cook

    N_Cook Guest


    The mechanical structure of a digital is the same as a vernier or DTI one .
    With the same susceptibility for the plate-form arms to deflect from
    unrelieved manufacturing stresses , let alone abuse. Compared to micrometers
    which have a more rigid , less internally stressed, rod like basic
    structure. Just because you can read to greater accuracy does not
    necessarily mean better measurement in absolute terms
     
  15. Bob Larter

    Bob Larter Guest

    Sure, but you can zero out a digital vernier each time you use it, which
    helps. And for your purpose one should be plenty accurate enough.
     
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