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stereo amplifier fault

Discussion in 'Troubleshooting and Repair' started by David Parker, Aug 14, 2010.

  1. David Parker

    David Parker

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    Jul 31, 2010
    Hi. I found an old car stereo power amp (Sony 23W + 23 W) that I haven't used for years. It worked fine previously. I connected the power to a car battery (correct polarity), the input to my ipod and the speaker wires to a pair of 4 ohm car stereo speakers. It all burst into life and I was happy... momentarily. After about 30 seconds one channel cut out, then came back on, then the other cut out, then both cut out. I disconnected the power and tried again with a different set of epeakers (6 ohm) and the same thing happened. My guess is that something in the amplifier is getting overloaded and some form of overload protection is cutting in. Can you give any advice about how to find out what it wrong?
     
    Last edited: Aug 14, 2010
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Are the speakers still working?
     
  3. David Parker

    David Parker

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    Jul 31, 2010
    Yes the speakers still work OK. If I disconnect the power and then try again it works for about 30 secoind then the channels cut out intermittently again. This is the same with two sets of speakers.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Great. That means the problem is not with the speakers, or that the amplifier is destroying the speakers.

    Do you have any experience with electronics?
     
  5. David Parker

    David Parker

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    Jul 31, 2010
    A little. I have played around with breadboards, LEDs, etc. I have read a bit.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That's a start!

    Do you have a multimeter?

    I would start by opening the unit and looking to see if there is any obvious damage.

    Given that the problem affects first one channel, then the other, there is clearly some systematic cause, and something probably NOT in a shared part of the device (like a PSU).

    One possible problem is that the amplifier id overheating and going into shutdown. This *may* be caused by heatsinks choked full of dust. However, considering this device is designed to be used in a car I would think it's unlikely to be happening at such low power levels in what is probably cool ambient conditions.

    A related cause may be that devices have become detached from their heatsinks. This would be due to some mechanical issue (which is also unlikely)

    Another possibility is that the biasing has been adjusted to a far too high level and the amplifiers are operating in something closer to class A than they should be. Again, I think this is unlikely unless someone has been in there before you and played with things. This cause also makes assumptions about the main amplifier that may not have any basis in fact :)

    Is the car battery fully charged?

    Does the amplifier cut out if the volume is very low?

    Does it take longer for it to cut out in this case?
     
  7. David Parker

    David Parker

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    Jul 31, 2010
    Yes I have a multimeter. I will open the unit and have a look for dust or detached bits. I know that noone has adjusted anything in it since it was last uesd (several years ago). I will try it with the volume very low. Thanks for your suggestions.
     
  8. David Parker

    David Parker

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    Jul 31, 2010
    I tried running it on low volume. It seemed to take a bit longer to cut out, and only one channel stopped working. I have opened it up and everything looks clean and tidy. No sign of damage, burn marks of floating pieces.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Did anything feel hot? (be careful where you stick your fingers)

    It may be best to turn off the power and unplug it before you start sticking your fingers on things.

    You could also measure the DC voltage on the speaker terminals (i.e. across the speaker) It should always be very close to zero.
     
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