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Plastic ring gear failure

Discussion in 'Electronic Repair' started by N_Cook, Sep 13, 2013.

  1. N_Cook

    N_Cook Guest

    Reminds me of a notorious generic fault with Philips car cassette
    players of years gone by. A steel disc set in a hard plastic periphery,
    differential expansion with cold or heat (never discovered which with
    the Philips). In that case periphery was of pulley form taking a drive
    belt, this time cog form mating with a drive cog.
    Part of the take-up spool structure of Sony EV S700 video8 VCR from
    1985 , used in 6 channel audio mode, so again a functioning m/c required
    to salvage audio from a tape archive (like the DAT m/c in recent thread
    here}.
    I placed the broken away quarter of the ring gear back with the
    remaining 3/4 ring and there is a stress-relieved gap of about half a
    tooth. Any advice?
    At the moment I intend grinding back a sliver from the steel, along its
    exposed quarter, to allow the bits of plastic gear to align again, and
    allow non-jamming tooth engagement. There is a bit of a ledge to the
    plastic cross-section to allow for some glue along the length as well as
    the broken ends. This spool drive assembly contains a magnet that
    "engages" with the steel disc , as a slip clutch
     
  2. I have no repair sugestions. This same crappy plastic gear molded onto
    sheet metal was a problem with the sony professional walkman lines.

    I finally trashed my last one after replacement parts were not accessible.
    Even if some are in a warehouse somehwhere, in yellowed plastic bags
    they're certainly cracked as well.

    The other problem is they used some sort of that bright white and very
    slippery plastic. Nothing seems to adhere to it, which is why I gave up.

    I really liked the WM-DC2 otherwise.
     
  3. N_Cook

    N_Cook Guest

    this m/c has been stored in the studio over the years, so no extreme hot
    or cold. So as no sign of rust , either the plastic shrinks with age or
    there is stress set into the plastic when formed around the steel and
    then the plastic weakens with age
     
  4. Probably both. As you're seen the crack starts at the bottom of the teeth
    where the stress is highest and it seems impossible to squeeze the gear
    back into shape, so it's not just a plain old crack, plus there's just not
    enough power on portable tape deck to split gears apart in the first
    place.

    On one forum I came across there is talk that the gear in the walkmen may
    have been polyethylene, matching the issue where you can't glue it.
    another person says delrin, which explains shrinkage over time.

    I even considered drilling holes into the area around the cracks and
    wiring them together to bridge the gap, but there just wasn't enough
    space, material and give to work with.

    Too bad "3d printers" can't make truly useful parts yet.
     
  5. They can, and you can get entire working setups for less than $500 these
    days. They are the bare printer, no fancy looking box or case that looks
    like an HP inkjet.

    They work so well that someone in Texas, where it is legal, made the
    lower receiver of an AR-15 assault rifle with one. His first attempts
    shattered from the forces of firing one round, he has YouTube videos
    of making one that lasted over 100 rounds.

    Sounds pretty "useful" to me.

    What somebody needs to build now is a 3d scanner, which will take an object,
    such as a cracked gear, and produce a file of commands to the 3d printer
    to get it to make a new one. It would need some fairly sophisticated
    software, so you could make multiple scans (such as one lying on the top and
    one lying on the the bottom) and combine and correct them.

    Geoff.
     
  6. Den 15-09-2013 10:39, Geoffrey S. Mendelson skrev:
    Something like this:



    At work we have a 3D-scanner that can use reference dots that are placed
    on the object for combining multiple scans of the same object into one
    3D model.
     
  7. N_Cook

    N_Cook Guest

    Shaving back the steel allowed resetting the ends, a ring of heatshrink
    around and carefully heated the heatshrink , only, with soldering iron
    and glued around whole disc/plastic join. Looks a goood , non-jamming
    and non-oval repair, will see tomorrow, get it back in the deck

    I thought of one use for a 3D printer. Making false teeth. Taking dental
    cast, multi-scan and stitch to get a 3D computer model, patch in via
    copying sections for missing teeth, take negative mathematical casts
    wherever necessary. Break such a set of false teeth then easy to
    generate a replacement from the mathematecal model. Unfortunately a 3D
    printer cannot do what is so easy with a 2D printer, colour up the
    resulting false teeth to colour match.
     
  8. N_Cook

    N_Cook Guest

    As a 3D printer is about the cost of a set of false teeth, I can see
    walk-in High Street agencies producing false teeth almost while you
    wait, perhaps in the next 5 years. Perhaps a secondary melt-mixing stage
    to add pc controlled dyestuffs to the melt to colour up the teeth/plate
    at the generation stage
     
  9. N_Cook

    N_Cook Guest

    Back to the original problem. Laces and unlaces and plays , FF and REW
    ok. Looks as though the gear broke over time in storage.
    The PCM legend flickers randomly and no sound out, no 6ch VU bargraph
    display , the original problem. Not tried playing a video tape but I
    expect no picture either. Other than cleaning all tape running surfaces
    , anything to check for? does not like PLAY+REW drops out. Reminds me of
    Beta video failure mode, usually slip clutch problems ISTR
     
  10. N_Cook

    N_Cook Guest

    I decided to inspect under the deck, easier than first seemed. Now with
    a better viewing angle for the heads, one of the 3 is broken.
     
  11. You've obviously never seen these machines in person. The outputted parts
    are pretty horrible by an measure.

    No $500 piece of junk can make the usable gears for the OP's problem or a
    walkman.
    this stuff exists, they had some at the MD&M show in Chicago last week.
    Still, no printer anybody could afford will produce usable intricate
    parts, and they won't be of the type that dribbles a plastic ribbon on a
    crooked warped table.
     
  12. I don't want to send you on a wild goose chase but have you double checked
    the power supply voltages as the deck is running and under load?

    I have one of those Pioneer D70's I mentioned in the earlier post, never
    really used the deck for video as much as that 6 channel PCM function, that
    allowed you to record up to 24 hours of continuous audio, in the LP mode.
    Was handy for talk show marathons and other crap worth recording back when
    the 10 foot satellite dishes were considered home entertainment.

    A few years ago being those decks are as rare as hens teeth with that
    function (there was some RIAA legal threat, later models kept the PCM but
    dropped the 6 track feature), I decided to dump the 50 or so cassettes to
    digital and make mp3's out of them.

    Since it wasn't used in years, it sort of worked but died after a few days
    with similar symptoms, some of the display wasn't lit, other parts were and
    trying the either forward or reverse scan kicked it out of the play mode.

    For some reason I started to look at the STK module, which I think was
    bolted to the back of the unit and discovered it was working but "sagged"
    under load. Almost like a capacitor problem, would put out the +12v until
    you hit play, then it sort of dropped to 9v and somewhat came back up to the
    12v. Same for the 5v out, but not as bad.

    I was able to cobble together some discrete parts that replaced the bad
    functions of the STK (not available from what I remember) and was able to
    finish up the tapes, but it did include a lot of finger crossing every time
    a new tape was put in.

    I'm just recommending to monitor the voltages before tossing in the towel,
    it's easy enough because I think that whole power supply module can pop out
    and rotate around and the output voltages are clearly marked on the board.

    -bruce
     
  13. N_Cook

    N_Cook Guest

    I watched a utube of them making false teeth the traditional way, no
    wonder with so many intermediary stages , they are so expensive.
    With such a mark-up I can see 3D printing come in there with a wallop,
    like the 2 bucks reading glasses that started coming in 10 or 15 years
    ago, wheras previously there was only expensive opticians scripped
    reading glasses
     
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