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QSC Power Amp gets hot

Discussion in 'Power Electronics' started by gerryt, Feb 5, 2021.

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  1. gerryt

    gerryt

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    Feb 5, 2021
    I have a QSC Series One Model 1100 Power amp. It is working normally, however on channel two the output transistors and heat sink get very hot. Wondering if this is a problem with too much current getting to them or if the transistors are failing? Channel 1 runs cool.
     
  2. WHONOES

    WHONOES

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    May 20, 2017
    Do both channels work?
     
  3. gerryt

    gerryt

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    Feb 5, 2021
    Yes, both channels are working normally, channel two gets really hot, channel one does not
     
  4. Martaine2005

    Martaine2005

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    May 12, 2015
    Perhaps the heatsink compound has dried or is non existent?.

    Martin
     
  5. gerryt

    gerryt

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    Feb 5, 2021
    A bit more information that may or may not be relevant. I tested the DC supply to both channel one and channel two output transistors. The reading going to channel one output transistor was 1.18 and reading to channel two output transistor was 2.77.
     
  6. Audioguru

    Audioguru

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    Sep 24, 2016
    Your numbers 1.18 and 2.77 cannot be the supply voltage of the stereo amplifiers that produce 50W into 8 ohms. Their supply will be at least 60V or more than plus 30V and minus 30V.
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009

    Indeed,
    @gerryt , I think you had better tell us how and where you did your tests ?
     
  8. gerryt

    gerryt

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    Feb 5, 2021
    What I tested was the two wires (blue and white see the attached photo) that go directly to the output transistor on the heat sink. I disconnected the wires from the output transistor and then turned the amp on and connected each wire to my multimeter on the auto DC voltage setting. Channel one, which does not get hot, had the reading of 1.18, channel two, which gets hot, had the reading of 2.77.
     

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  9. davenn

    davenn Moderator

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    Sep 5, 2009

    Yeah, we cannot see from the photo where the 2 blue wires come from or go to. So at this point, those 2 voltage reading dont mean much :)

    Have you looked online for a service manual or at minimum a schematic ?
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    Martaine2005 likes this.
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    OK, the PSU section first
    you need to check that those + and - 37V DC rails ( on the right of the diagram) are there and correct for each of the channels

    upload_2021-2-6_19-34-25.png
     
  12. davenn

    davenn Moderator

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    Sep 5, 2009
    Here's the main amplifier section ......

    upload_2021-2-6_19-40-38.png


    The voltages shown should be referenced to the 0V rail ... that should be the ground shown as a downward pointing arrow

    Where your 2 blue wires fit into there I dont know
     
  13. WHONOES

    WHONOES

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    May 20, 2017
    Can you check the Base - Emitter voltage on each of the output transistors in its quiescent state i.e. with no amplification taking place.
    Also, check the d.c voltage on the output of each amplifier across their output terminals under the same conditions.
     
  14. gerryt

    gerryt

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    Feb 5, 2021
    Thank you for all of your input, I will do some more testing. I have also discovered that Q2 in the schematic also gets hot. Thinking I might need to replace Q1 and Q2 as a matched set. Or, wondering if there is a bias problem causing Q4 to do most of the work.
     
  15. WHONOES

    WHONOES

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    May 20, 2017
    Can you measure voltages as requested.
     
  16. gerryt

    gerryt

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    Feb 5, 2021
    By each amplifier do you mean channel and channel two?
     
  17. davenn

    davenn Moderator

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    Sep 5, 2009

    as per the schematic in post #11 :)
    it shows one channel, do it for both
     
  18. gerryt

    gerryt

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    Feb 5, 2021
    Please bear with me, I am just learning about how to do this. I have found the ground rails on my circuit board, but I do not know where to put the leads from my voltage meter.
     
  19. gerryt

    gerryt

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    Feb 5, 2021
    Ok, figured how to test the + and -37V DC rails. They are both running at -35 and +35 DCV
     
  20. gerryt

    gerryt

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    Feb 5, 2021
    Voltage for power transistors in quiessent state:
    Channel one -34.00 and +34.00 DCV
    Channel two -33.00 and +32.00 DCV (this is the channel that gets very hot)

    DC Voltage for amplifier output in quiessent state:
    Channel 1 -1.2 DCmV
    Channel 2 1.0 DCmV
     
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