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Audio aplifier with faulty 12VDC Power supply

Discussion in 'Troubleshooting and Repair' started by turrikasd, Jun 27, 2014.

  1. turrikasd

    turrikasd

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    Jun 27, 2014
    Hey. I'm repairing a valuable car audio amplifier that is in almost perfect condition except for the power supply.

    It's a switching power supply that is supposed to take 12VDC as input and output 25V & -25V DC. However the output is 1.5V and -1.5V.
    The four PSU FETs output measure about half of the voltage they should. The highest voltage FET's gate resistor has 10% lower resistance than the rest, however impossible to tell if it is by design or not since the really really small SMDs are unreadable.

    Visibly everything is in perfect condition, especially all capacitors.

    [​IMG]

    Any ideas what gives? My repair skills cut off when there is no visibly damaged components or obvious shorts.
     
    Last edited: Jun 27, 2014
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Got any clear photo?
    cheers Adam
     
  3. turrikasd

    turrikasd

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    Jun 27, 2014
    Sure. Added photo
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Nice one
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    No crispy components then :) Have you tried to get a service manual? Whats the make of the AMP? Could be the power mosfets.
     
  6. turrikasd

    turrikasd

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    Jun 27, 2014
    It's labelled "VDO Dayton" but I believe they went out of business or changed their niche so no luck with manual or any support.
     
  7. turrikasd

    turrikasd

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    Jun 27, 2014
    For replacement FETs, what values matter and what don't? I don't think I can find the original ones.
     
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I'll have a quick look. Does it have a model number or do you think VDO dayton is it?
     
  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    VDO is still going by the looks of it. They might not do audio amps anymore.
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Looks like they might be BJT devices.
     
  11. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Found some origional transistors. But using tablet and cant work out how to copy link. I will post tomorrow
     
  12. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
  13. turrikasd

    turrikasd

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    Jun 27, 2014

    The output transistors are fine. The problem is in the power supply, which isn't producing the correct voltages.
    Thanks.
     
  14. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Ok cool. I am guessing it has some sort of transistor/s to switch the coil of the SMPS. Could that be the issue?
     
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