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wierd optical sled behavior on Alpine car audio 6-disk CD changer

Discussion in 'Electronic Repair' started by [email protected], Apr 12, 2007.

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  1. Guest

    I have an Alpine CHM-S630 6-disk CD changer that is giving me grief.

    The unit selects each CD properly from the magazine, but as soon as it
    loads it into place, the optical sled slams out to the full extent of
    the limits of the track, out to the extreme edge of the CD diameter.
    It never appears to go to the center of the CD, and it never takes a
    read from the disk.

    After the sled hits the track limit, then for about 2 seconds, the
    worm & roller mechanism keeps trying to push the sled out through the
    side of the case, resulting in a horrible grinding noise as the
    plastic block on the sled strips itself against the "worm" screw,
    since the sled can't move any farther.

    After about 2 seconds elapse, the unit gives up and stops trying to
    push the sled past the limit of the track. Another 2 seconds after
    that, the unit gives up on spinning the CD, and the CD comes to a

    This happens every time the unit loads a CD.

    I've tried cleaning the lens on the sled, but it doesn't seem to be an
    optical problem. It doesn't seem to be a mechanical problem, either.
    The worm & roller for sled movement works just fine - the problem is
    that for some reason, the unit is telling the sled to slam out past
    the limits of its track. I'm stumped.

    Can anyone point me in the right direction for a fix?

  2. It sounds like the "home" switch or sensor is stuck or damaged, making
    the unit think the sled is stuck at the center of the disc and making it
    constantly try to move the laser away from the center.
    I would look for a broken switch or the tab that activates it.
    Good luck.
  3. Guest

    Thanks for the idea.

    I tested the "home" switch for the sled with my multimeter, dialed in
    to the 200K ohm setting. It's just an ordinary (tiny) spring-loaded
    two-position pin switch.

    With the switch pressed in (ie. sled is at center of CD hub), I get no
    resistance (00.0 on the multimeter display). But with the switch
    released (ie. sled is anywhere *except* right at the center), I get a
    reading of 47.8 on the multimeter display. These results are both
    using the 200K scale setting on the ohmmeter.

    I would have expected infinite resistance, not 47.8, with the switch
    released. So that's a bit surprising. The switch isn't perfectly
    open when the pin is released, although there's obviously much more
    resistance than when the pin is pressed in.

    Do you think there is inadequate resistance with the switch released,
    or would 47.8 a normal result for this kind of switch?

    I'd love the problem to be a bad switch, but Murphy's Law says I can't
    get off this easily.

  4. With the switch pressed in (ie. sled is at center of CD hub), I get no
    Did you disconnect the switch when you tested it? If not, then you
    are reading whatever it is connected to when it is open...
  5. Guest

    You're right, I did not disconnect the switch before the test. So
    there's the reason that I don't get infinite resistance when it's
    open. Thanks.

    So the implication is that the switch is working properly, I guess. I
    suppose I could desolder the switch and double check, but since I get
    0 resistance when it's depressed, and the resistance increases when
    it's released, it appears to be working properly.

    Anything else worth checking? I hate seeing this thing turn into an
    expensive paperweight.

  6. Tim Schwartz

    Tim Schwartz Guest


    While I don't know the Alpine car units at all, I've seen some Sony
    single disc home players do this when one of the clocks (oscillators)
    fails. If left there making that awful noise, it will damage the rack
    gear on the optical pick up, by grinding the teeth off it.

    Seeing this will require powering the unit on a work bench with an
    oscilloscope, and probably a service manual. I don't work on car audio
    stuff myself.

    Tim Schwartz
    Bristol Electronics
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