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Power and control of Mitsubishi LT-70 Linear Turntable

Discussion in 'Electronic Repair' started by Julian Bunn, Oct 27, 2013.

  1. Julian Bunn

    Julian Bunn Guest

    This turntable was part of an integrated system: I only have the turntable and am trying to figure out how to make it work standalone.

    There is an 8-pin DIN plug on the back that once connected the turntable tothe main unit. This plug is marked "TO CACEIVER" J106 on the following schematic for the turntable:

    http://farm4.staticflickr.com/3814/10505248315_6b2891424d_o.png

    The same plug is marked "PL CONT" on the schematic for the main unit:

    http://farm8.staticflickr.com/7353/10505238786_f966d7d075_o.png

    Working from the schematics, I saw that the turntable uses a -12 0 +12 power supply, which I've duly connected to pins 1-6-4 on the plug (29,28,27 on the turntable schematic). Sure enough, the unit powers up, the track indicator 7seg LED lights up, and I can operate the tray using the "Open" button on it, the fwd/rev buttons that move the cartridge left/right, and the various programming buttons on the tray (track select, program, etc.)

    However, the turntable never spins, the "Start" button has no effect. I think this is because I am not providing the correct signals on the pins marked "SYNC", "AF" and "STP" (shown on the main unit schematic).

    Two of these three pins (24 & 25) are connected to IC151, a BA612 quad driver, on the turntable circuit board: 24 seems to be an input signal to the turntable (since it goes to a BA612 driver input, pin 2). The other, 25, looks like an output signal from the turntable to the main unit. Finally, pin 26 also looks like an output, some sort of current source from Q221?

    I'm at the limit of my expertise with this - can anyone suggest how to connect pins 24,25,26? Any ideas about what "SYNC", "AF", and "STP" might mean in this context?

    Thanks!
     
  2. Guest

    Pretty sure that pin 26 is just digital; IC152 (CPU?) outputs a signal,
    IC151 inverts it, and Q221 inverts it again.
    My guess: Let 25 and 26 float.

    Ground pin 24 to power supply ground in an easily-disconnectible way,
    and press the "Start" button on the turntable. If nothing happens right
    away, disconnect pin 24 from ground.

    If grounding pin 24 didn't do anything, connect it to +12 V in an
    easily-disconnectible way, and press the "Start" button on the
    turntable. If nothing happens right away, disconnect pin 24 from
    +12 V.
    I bet "STP" means "Stop". On the receiver schematic, it seems to be a
    diode-OR of three things, one of which is labeled "TUNER" on IC702.
    The receiver may have used this to stop the turntable when some other
    source (tuner, tape) was selected. My guess is that this would have
    shown up on pin 24 of J106 at the turntable.

    "SYNC" may be a way for the turntable to tell the receiver that it was
    ready to go, so the receiver could unmute, start a tape deck, or
    something like that. On the receiver schematic, the "SYNC" line goes
    (among other places) into the "muting" block, through diode D7B3. My
    guess is that this is either pin 25 or 26 on the turntable.

    "AF" I'm not sure of. On the receiver, it goes to "A in PLAYER" on
    IC702, and also seems to be an input to a diode-OR. It may tell the
    receiver that the turntable is present and on, whether or not a record
    is currently playing. My guess is that this is either pin 26 or 25 on
    the turntable - whichever one SYNC isn't.

    Matt Roberds
     
  3. Julian Bunn

    Julian Bunn Guest

    Hi Matt,

    Thanks for the analysis - very helpful.

    Pin 26 measures +12V, so looks like it is an output signal, probably the "SYNC" as you say.

    The other two, pins 24 and 25, look to be inputs.

    I tried grounding pin 24, putting it at -12V and at +12V, with no luck - nothing changes.

    Pin 25 - same story. (In the DIN plug this is pin 3, and is shielded by the braid ground. The wire gauge is also thinner.)

    It does seem that pin 24, the "STP" pin, should be the one that signals the turntable to play or not. But connecting it to any rail of the PSU doesn't do the trick.

    Perhaps this is more subtle.

    Julian
     
  4. Guest

    Good catch. I didn't think of trying to measure the voltages when
    nothing is hooked up.
    I would advise against connecting it to -12 V unless you have pretty
    good documentation that negative voltages are OK on that input. The
    digital logic levels in most consumer devices are 0 volts and some
    positive voltage; +3.3, +5, and +12 are popular but it can be almost
    anything.
    From the turntable schematic, I would expect pin 25 to be at about +9
    volts, if the turntable is in the same configuration as it was when the
    schematic voltage readings were taken. There should be a note somewhere
    that describes the conditions: for the turntable, record loaded/not
    loaded, playing/not playing, etc; for the receiver, tuner/turntable/
    tape/aux selected, volume muted/low/high, etc. This note might be in
    the part of the manual that you don't have, though.
    It might be a sequencing issue; maybe STP has to be in a particular
    state when the power comes up in order for the turntable to recognize
    a change in state of STP later. Or, worse, maybe it expects the
    receiver to flick STP off and on once at powerup... something like
    that.

    It's possible that STP is some kind of serial data output from the
    receiver, but the fact that it participates in that diode-OR seems to
    argue against that, for me.

    If you can get at pin 13 of IC151 in the turntable, or R152, you might
    measure that point with a voltmeter to see if it tracks changes in the
    state of pin 24 on J106. With pin 24 at 0 V, the IC pin should be
    about 7.3 V; with pin 24 at some higher voltage (probably approaching
    +12 V), the IC pin should probably go to around 0 to 0.3 V. If this
    doesn't happen, IC151 may be screwing up.

    Matt Roberds
     
  5. Julian Bunn

    Julian Bunn Guest

    I made a discovery by accident: if I push the plinth/platter on its left hand side, everything starts working!

    It suggests either that there is a dirty switch somewhere, which isn't making properly unless the plinth is pushed, or that the plinth isn't engaging into its fully closed position.

    On the plinth, at the left hand side, are two buttons: "Power" and "Open". With the unit closed, when the Power button is pushed, the plinth slides out. Oddly, with the plinth out, pressing the Power button doesn't make the plinth slide back in - should it?

    So, I've been gently pushing the plinth back in. It seems to latch when it's fully in, but clearly it's not making the switch (whose location I can't see) close. Also, in this closed position, the plinth can be moved from side to side somewhat: this doesn't seem right, as surely the plinth should be rigidly fixed while a record in being played?

    I guess I really need the full manual to understand what is going on. But Iam now quite optimistic.

    Julian



    Good catch. I didn't think of trying to measure the voltages when

    nothing is hooked up.




    I would advise against connecting it to -12 V unless you have pretty

    good documentation that negative voltages are OK on that input. The

    digital logic levels in most consumer devices are 0 volts and some

    positive voltage; +3.3, +5, and +12 are popular but it can be almost

    anything.




    From the turntable schematic, I would expect pin 25 to be at about +9

    volts, if the turntable is in the same configuration as it was when the

    schematic voltage readings were taken. There should be a note somewhere

    that describes the conditions: for the turntable, record loaded/not

    loaded, playing/not playing, etc; for the receiver, tuner/turntable/

    tape/aux selected, volume muted/low/high, etc. This note might be in

    the part of the manual that you don't have, though.




    It might be a sequencing issue; maybe STP has to be in a particular

    state when the power comes up in order for the turntable to recognize

    a change in state of STP later. Or, worse, maybe it expects the

    receiver to flick STP off and on once at powerup... something like

    that.



    It's possible that STP is some kind of serial data output from the

    receiver, but the fact that it participates in that diode-OR seems to

    argue against that, for me.



    If you can get at pin 13 of IC151 in the turntable, or R152, you might

    measure that point with a voltmeter to see if it tracks changes in the

    state of pin 24 on J106. With pin 24 at 0 V, the IC pin should be

    about 7.3 V; with pin 24 at some higher voltage (probably approaching

    +12 V), the IC pin should probably go to around 0 to 0.3 V. If this

    doesn't happen, IC151 may be screwing up.



    Matt Roberds[/QUOTE]
     
  6. Guest

    Sounds reasonable. It might be a mechanical switch; sometimes the metal
    leaf of the switch is bent, and sometimes the thing that pushes on the
    switch is bent or missing. Or, it might be an optical switch; these are
    usually a U-shaped piece of plastic with an LED in one side and a
    phototransistor in the other; a tab or blade slides into the slot of the
    U to interrupt the light. (The LED is often infrared, so it may be
    working fine, even if it doesn't look like it's lit.) Another variation
    is an LED and a phototransistor in the same flat package; the light
    reflects off a shiny spot on something that moves over the package.
    Also possible. Are there any rubber belts in the drivetrain for the
    plinth? If so, they may be stretched/floppy and not quite driving it
    as far as they should.

    If it's all gear teeth, it's possible that one side or the other has
    slipped a tooth, which means the plinth isn't being driven as far as it
    should be. The service manual may have directions on how the gear
    teeth should be aligned at assembly; if you decide to change it on your
    own, mark or take a photo of how it was before you start, so you can put
    it back that way.
    I don't know. I have never owned or used a turntable where the playing
    surface slid out like that, so I don't know what the usual sequence is.
    Depending on exactly what's moving, this might be OK. If the plinth
    and tone arm / cartridge support move together, then this is just the
    suspension of the turntable. If the plinth can move relative to the
    tone arm, then that may require attention... but I'd suggest getting
    the open/close/latch operation working first.

    Does the schematic show a solenoid or motor that would be involved
    with sliding the plinth and maybe latching it?

    Matt Roberds
     
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