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PA System Problem

Discussion in 'General Electronics Discussion' started by dm99, Dec 24, 2012.

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  1. dm99

    dm99

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    Oct 29, 2012
    I run a small restaurant and we have a PA system, mostly for piped in music. The PA system seems quite old; it was here when I bought the place a few years ago.

    The system started seeming a little off a few months ago. I noticed needing to turn up the master volume and that the music sounded increasingly mushy. Then last week it started making a strange popping sound periodically. Today the thing let out this horrific shriek, then a loud bang, and is totally dead.

    Where do I start?
     
  2. dm99

    dm99

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    Oct 29, 2012
    Does anyone have any idea how I'd start troubleshooting this? There's a pre-amp and an amplifier. I assume that the problem is in either the pre-amp or the amp. Can I plug an the output from the tuner right into the amplifier bypassing the pre-amp?
     
  3. davenn

    davenn Moderator

    13,671
    1,892
    Sep 5, 2009
    unless you have circuit info its probably a waste of time
    would end up cheaper just to replace it :)

    but if you insist ;)
    check for blown fuses, if more that one fuse in the unit ( often there's a DC and an AC fuse) seeing which one has blown will give an indication as to where the problem lies

    look for blow resistors, capacitors, transistors
    take some sharp well lit pix and post them on here .... keep the size to 800x600 and less than 100kb

    Dave
     
    Last edited: Dec 24, 2012
  4. dm99

    dm99

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    0
    Oct 29, 2012
    It's an older one and it uses vacuum tubes. If one of those has blown would that be something I could troubleshoot and repair myself or do those only blow at the end of life of the entire unit?
     
  5. davenn

    davenn Moderator

    13,671
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    Sep 5, 2009
    so did you find anything that had visibly blown ?
    the loud bang you said happened should be an indication that there would be obvious damage

    Dave
     
  6. dm99

    dm99

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    Oct 29, 2012
    I don't know a lot about electronics, but I took both the amp and the pre-amp apart. Nothing looks burned or anything in either of them and all the vacuum tubes still light up the way they used to. Also, the indicator lights come on like before and the fuses appear intact.

    Would I do damage to the amplifier if I disconnected the pre-amp and tried connecting the tuner or CD player directly into the main input on the amplifier?
     
  7. davenn

    davenn Moderator

    13,671
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    Sep 5, 2009
    you need to find out what went BANG before you even consider putting power back into the unit
    You have admitted to not knowing too much about electronics
    BE CAREFUL old valve gear has HIGH voltages you could easily zap yourself if not
    careful. The most likely thing that would have gone bang would be one of the smoothing capacitors with a value of probably ~ 8uF and 250V give or take a bit

    Dave
     
  8. davenn

    davenn Moderator

    13,671
    1,892
    Sep 5, 2009
    Seriously tho....
    with your admitted lack of skills in electronics, you really shouldnt be playing with it,
    junk it and buy something more modern :)

    I say this for your safety !!

    Dave
     
  9. dm99

    dm99

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    0
    Oct 29, 2012
    The loud shriek and bang came out of the speakers not out of the equipment.

    Would there be any value in an old public address amplifier or should I just junk it?

    There is an old repair shop sticker on it. If they are in business still I will see if they can fix this or if they'd help me with the new install.
     
  10. davenn

    davenn Moderator

    13,671
    1,892
    Sep 5, 2009
    info like that is helpful at the right time ;)

    none whatsoever

    worth a try, get a quote first ... I suspect you will find it may be cheaper to buy a decent newer second hand one that resurecting the old one ( unless its a really basic fault)

    Dave
     
  11. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    I would think that there may be some value in the amp. The valves could be worth something and if the output valves are SP25s you could go on holiday!

    I would try an auction website, you have nothing to lose.

    I have an amp here with a couple of EL34s in the output. You could have this at a reasonable cost if you popped in to collect it.
     
  12. dm99

    dm99

    26
    0
    Oct 29, 2012
    I had someone come out and take a look at it for me. The repair shop was still in business and the tech told me he'd fully rebuilt this amplifier in the mid 90s. He replaced all the tubes and told me that if I ever start noticing changes in the performance of the amplifier I should call him in right away.
     
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