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L2 wmp12h 300watt floor Monitor

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  • L2 wmp12h 300watt floor Monitor

    Good afternoon to you
    Now I might be being a dick here. But this amp is causing me pain

    It came in to me with a intermittent noise issue.
    I found the fault ir9 was soldered in but leg cut short it wasn't really soldered. Replaced it.
    Out the job went ok
    1 month later it came back distortion after a while.
    I found by heating and freezing the area around iq5 it would go away. In the end I replaced all the transistors in that area iq 1to 7 and 14. As all very close together.
    All changed like for like.
    Tested the amp heat and freezer it all seemed OK.

    Then it went to about 1/2 volume without touching it I wasn't heating or freezing at that time was on for 20 mins before it went
    When I tested it I can see its working perfectly without distortion but low output

    On checking using scope and signal gen I can see the signal in and out of pre amp is OK and higher level leaving the pre amp than the input.
    I'm I being a dick question
    But the signal entering the power amp is at same level as the output.!!!!
    Is this possible?
    Check all voltages 54'v +/- and 15v +/-
    Checked all the transistor /diodes
    Made sure the transistors I changed are good and correct ones
    At output have around 3v ac with gain /vol max
    I'm not an experienced tech. Just play for fun
    Any assistance would be grateful thanks in advance
    Arbutt
    Schematic to follow

  • #2
    Please bare with me struggling to get the map up loaded. I only use a phone. It says I can't upload a pdf format
    Click image for larger version

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    Sorry it's the best I can do at present
    Click image for larger version

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    Last edited by Arbutt; 02-20-2020, 03:05 PM.

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    • #3
      So set it up again so you have 3VAC going in to the power amp.
      Measure the AC voltage at the following points and post your results:
      IQ1 base and collector
      IQ6 b & c
      IQ10 b & c
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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      • #4
        Thank g1.
        OK as follows using dmm on ac range. Between chassis and transistors
        Iq1 base 3v collector 0v emitter 3v
        Iq6 base 0v collector 3v
        Iq10 base 3 .5v collector 0v emitter 3v

        I hope this helps. Many thanks for the help.
        I feared no one would help

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        • #5
          Hello again
          Is there anyone that can help or shed some light on this.
          Too me it makes no senses.
          But believe me with a scope probes on input and output both on same setting per div etc.

          Arbutt

          Comment


          • #6
            Do you have both rails- +VCC and -VCC?
            "Yeah, well, you know, that's just, like, your opinion, man."

            Comment


            • #7
              Hi. Yes I did. 54v +/— it's OK I have sorted it now.
              Resoldered the area again. And cleaned it.
              Worked for 4 days 6 hours a day. Been collected to sit about in storage �� till this shit is over


              Mr a

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