Is this an easy one?

Discussion in 'General Electronics Chat' started by gibsoc, Sep 12, 2015.

  1. gibsoc

    Thread Starter New Member

    Sep 12, 2015
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    Hello,

    I hope this might be an easy one. This is on a guitar amplifier and the two pairs of blue leds on the right in the diagram are not coming on.

    Any insight would be truly appreciated.


    upload_2015-9-12_19-24-21.png
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    • +/- 15V aren't present
    • One or more LEDs in each string are burned out
    • The LEDs are disconnected
    • AC power is not reaching the rectifier
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    When the LEDs are out, does the amp still work? If not, fuse. If so, strange coincidence of multiple independent failures of parts.

    ak
     
  4. gibsoc

    Thread Starter New Member

    Sep 12, 2015
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    Thanks for the quick responses. I realize I'm in over my head here.
    There is another LED (toward the upper left in the diagram) that does come on. I assume that runs on the same +15v?
    The LEDs in pairs connect separately directly to the board.
    I assume AC power is reaching the transformer/rectifier because everything else is working properly.
    The non functioning LEDs did go out at the same time.
    Apologies for the lack of knowledge. And poor diagram quality.
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    Measure voltage at the two LED connectors.
     
  6. gibsoc

    Thread Starter New Member

    Sep 12, 2015
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    H&K LEDs.jpg
    I measured the voltage at the board and the LEDs. Here is generally what I got or something like this.
    Both pairs were the same. I assume one LED in each pair is bad?
     
  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    It appears that the LED with 15V across it is open-circuit. It may just be a bad solder joint so check this first.
     
  8. gibsoc

    Thread Starter New Member

    Sep 12, 2015
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    Thanks for your replies. The solder joints on the boards and LEDs "look good". They appear consistent. Is it likely to have two bad soldier joints and separate components?

    As AK stated above, strange coincidence that two "separate" circuit LED's would go out at around the same
    time (not sure if at the same time or around the same time).

    I thought LEDs were long lasting and these, relatively, do not have a lot of hours on them.

    Still not sure. If these are powered by 15v, can I just hook in a 9v battery to light them up?
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Dodgy soldering was usually easy to spot when solder had lead in it. RoHS solder looks a bit iffy even when its good, so the demarcation point is a bit more vague. RoHS solder can develop a hairline crack around the fillet, that's very hard to see. Sometimes badly prepared component leads have a thin oxide layer - the solder joint seems to work OK for a while, its not a bad idea to remove all the solder with a solder sucker or desolder braid and remake the joint from scratch. Talking of scratching - its a good opportunity to scrape the component lead to clear any oxide.
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
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    This is generally true, but a smallish number will fail due to infant mortality and some will fail due to improper handling during assembly/soldering.
    You need to limit the current if you do it. 200 to 560 ohms will give you 5-15mA.
     
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