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Panasonic SA-HT70 not working! :-(

Discussion in 'Electronic Repair' started by aarpay, Aug 13, 2007.

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  1. aarpay

    aarpay Guest

    Hi all,

    I was recently repairing my SA-HT70 dvd player because the subwoofer kept
    cutting out. I fixed the problem. Then, I was about to remove a little piece
    of plastic that had fallen onto the circuit board with my tweezers. I
    accidently dropped the tweezers onto the circuit board - this caused a big
    spark with some smoke. Now, when plugged in, the unit displays the red stand-
    by light, but when I press the power button, it goes green for about 2
    seconds then back into standby (red), whilst the relay near the power input
    clicks once or twice. Does anybody know what that means and if I can fix it?
  2. Arfa Daily

    Arfa Daily Guest

    Check for +28v on pin 14 of IC502. If missing, safety resistor FP549 is
    open. Failing that, you've probably fried IC502. I've got the relevant part
    of the schematic as a pdf if you need it. Contact me off group with a valid
    e-mail if you do.

  3. I'm sorry, but I'm a bit of a novice....

    Should I be testing the voltage difference between pins 1 and 14? What is the
    reference? Thanks

  4. Arfa Daily

    Arfa Daily Guest

    The reference is ground ie any piece of metalwork on the unit such as the
    antenna socket outer, which will definitely be ground. However, I must say
    that if your level of expertise with this equipment is actually such that
    you are not aware that ground is the standard reference point unless
    otherwise stated, then it is unlikely that you are going to be able to
    troubleshoot the unit to the point of being able to fix it. Also, you should
    be aware of the safety angles of working on a piece of live equipment.
    Whilst the 'HT70 is better than some for safety in that it uses a linear
    rather than switch mode supply, there is still exposed line voltage in
    places around the power supply, so do take care ...

  5. Also, I would like that schematic. How do I contact you off group?
  6. Thanks for your reply:
    I figured the frame was reference, but just wanted to make sure. Pin 14 is
    NOT reading +28V. In fact, it has no voltage. Does this mean I need to
    replace the FP549? If so, how much resistance (in ohms) does the FP549 have?

  7. Arfa Daily

    Arfa Daily Guest

    Right click on one of my posts' headers, then left click on properties,
    where you will find my address.

  8. Arfa Daily

    Arfa Daily Guest

    First you should locate FP549, and check the voltage ( with respect to
    ground ) on either side of it. If there is +28v on one side, and nothing on
    the other, then it is open circuit, and will need to be replaced.
    Officially, it is a 500mA fuse, but is actually shown on the schematic as a
    fusible resistor. It is quite common for such wire-ended circuit protectors
    or 'fuses' to actually be special low value resistors which are designed to
    go rapidly open circuit once their nominal power rating is exceeded. I would
    expect this one to be around 1 ohm at 0.5watt - but a fusible type, not a
    standard resistor. If you can obtain an official wire-ended 500mA circuit
    protector, then fit that. If not, the fusible resistor. If you're lucky,
    there will be no other damage. But make the checks first, before proceeding
    to replacing it.

    BTW, it is common convention to bottom post on newsgroups ( without wishing
    to start another furious debate on the subject |:-\ ) as it makes the
    thread a lot easier to follow

  9. Arfa:

    I rechecked the voltages on the unit.

    With the unit in standby mode, pin 14 on IC502 reads about +2.0 V. FP549 also
    reads about +2.0 V on either side.. The voltage on both of these gradually
    drops the longer the unit has been plugged in. Also, If I unplug the unit, I
    get the same readings. I suppose charge is stored in the capacitors somewhere.
    If I press the power button, the voltage reading (for both IC502 and both
    ends of FP549) jumps to about +12.0 V, then the unit goes into standby mode
    after 1 second, and the readings go back to about +2.0 V (dropping gradually).
    Any suggestions? Thanks

    By the way, I can't figure out the right place to right-click. I'm not seeing
    an email address.
  10. Arfa Daily

    Arfa Daily Guest

    OK. Well that basically tells us that the problem is not the protector, or
    probably its feed either, so it will be necessary to start checking the
    output voltages of IC502, which is where it gets complicated, because as it
    only stays on a second or two, before dropping back to standby, you don't
    have long to evaluate what is appearing on the chip's outputs. As it is also
    possible that the input voltages never have time to reach their normal
    running level, it takes a degree of experience to determine if output pins
    that are reading low in that second or two, are *heading* for the right
    level ...

    For an e-mail direct to me, use

  11. Ok. What do you recommend as my next step? Should I try and measure the
    voltages on all 14 pins and post the results?

  12. Arfa Daily

    Arfa Daily Guest

    You should have the schematic that covers this part of the machine now, and
    all of the voltages are printed at the pins of IC502. However, as I said,
    it's probably going to be difficult for you to interpret what you're seeing
    in the brief time it remains out of standby, but you can give it a try, and
    I'll see if anything leaps out at me.

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