I need help writing and reading off magnetic tape

Discussion in 'General Electronics Chat' started by MarFene, Mar 27, 2016.

  1. MarFene

    Thread Starter Member

    Feb 27, 2014
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    i have created a small device which i wish to use. the device consists of a rotating drum covered with VHS magnetic tape , and 2 tape heads from an old radio , which i know work.

    i would like to write and audio signal with one of the heads and read it again simultaneously with the other head while the drum is rotating. this is to see what kinds of distortion magnetic tape creates.

    one of the heads has a resistance of 200 ohms and the other head has a resistance of 4 ohms.

    i believe that i am missing something crucial. tried several signals on the "input" but got nothing out of amplifier circuits on the outputs.


    Help? i know some of you veterans know tape like the palm of your hands.
     
  2. bertus

    Administrator

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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The electronic circuits necessary to record and play back audio with low distortion are not simple circuits, but they are well known and understood. A much bigger problem is the way the heads touch the tape. Without touching, there is not enough signal to recover. But unless the contact is perfectly smooth, the heads will grind down the tape. This is why wrapping the tape tightly around a drum almost certainly will not work. Notice that on tape recorders, the tape is pulled across the heads rather than pushed up against them.

    It is possible to splice the tape into a loop, and use a motor and wheel to pull the loop around the heads. This is the basis behind 8-track and other cartridge tape systems.

    ak
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    If these are heads from a video tape recorder, please be aware that their gaps will be very narrow and so you will probably have to lay down a lot of cycles per centimeter on the tape to get any sensitivity -and as I recall the old Catridgevision heads need to run between 10 and 16 MHz with the tape whizzing by at about 20 meters per second.

    Video tape recorders used Hellicl Scan so the tape can run slower. Helical Scan
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    @DickCappels , it looks the heads are from some cassette tapedeck.
    The hooks on the sides of the heads are usually for guiding the tape.

    @MarFene , watch out for the guiding hooks to touch the tape you use.
    Better use a tape that fits in between the hooks.
    Also do not use a hard drum.

    Bertus
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    As per AK's comments, these are used heads so will be worn in the centre and will not contact the tape if it is mounted on a flat surface. Take a look at the old Watkins Copicat echo units for ideas on how a simple but more successful design could be built and perhaps use the record and playback electronics from a couple of junk cassette players.

    You will also need an erase head.
     
    Last edited: Mar 27, 2016
  7. MrChips

    Moderator

    Oct 2, 2009
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    Magnetic tape recording requires an AC bias in the 40 to 150kHz range.
    That is, if you are attempting to record audio signals in the audio frequency range, you have to modulate a carrier signal in the 40-150kHz range.
     
  8. MarFene

    Thread Starter Member

    Feb 27, 2014
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    i think i will re create the device better. also i did not like the idea of only having 4 cm of tape loop.

    for now i will face a problem at a time since i beleive that the reading part is a bigger problem. i will use a walkman to get the output for now


    do you think that the drum being brass and directly in contact with the tape is a problem?
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Did you have a look at the provided PDF?
    In there the magnetic head operation is explained:

    Magnetic_head_operation.png

    I do not know how much influence your brass drum will have.

    Bertus
     
  10. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    The "problem", or at least one of the main ones, has already been identified in earlier posts.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    The "problem" has been explained to you.
    The tape needs to be in intimate contact with the head and you can't do that with a drum.
    Note that the head has a slight curvature and the tape has to follow that curve for proper head contact and signal recovery.
     
  12. bertus

    Administrator

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  13. MarFene

    Thread Starter Member

    Feb 27, 2014
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    is this looking better till now? 20160327_215115[1].jpg
     
  14. MarFene

    Thread Starter Member

    Feb 27, 2014
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    im letting the epoxy on the motor and the tape to dry before applying tension. i will find a suitable place for the head. does the little sponge behind the tape where the head presses in a cassette necessary?

    20160327_223131[1].jpg
     
  15. MarFene

    Thread Starter Member

    Feb 27, 2014
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    time to circuit!
     
  16. MarFene

    Thread Starter Member

    Feb 27, 2014
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    20160328_003636[1].jpg
     
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