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SUNN Beta Bass Power Amp Making Me Pull Out My Hair

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  • #1

    SUNN Beta Bass Power Amp Making Me Pull Out My Hair

    i've got a beta bass that runs fine without a load but when i plug in a dummy load my lightbulb limiter lights up like a streetlamp. all electro caps have been changed, all transistors and diodes test as good on my multimeter diode test (i have changed a few that didn't test good, had to substitute 2n5401 or 2n5551 for a couple of the less common parts, will update when it's in front of me) , high voltage and low voltage (+/- 15v) supplies are right around where they should be. it's got me pulling out what little hair i have left. i have c

    here are some voltage readings (i thought i took all of em but apparently didn't) while on no load and through my lightbulb limiter

    IC1
    1 -11.8v
    2 6.4v
    3 6.4v
    4 -13.3v
    5 0v
    6 0v
    7 200mv
    8 14v

    Q2
    e -700mv
    b 56mv
    c 6.8v

    Q3
    e 8.5v
    b -31mv
    c 6.8v

    Q4
    e 8.5v
    b 9.1v
    c 39.7v

    Q5
    e 40v
    b 39.7v
    c 1.3v

    Q6
    e 1.1v
    b 240mv
    c 0v

    Q8, Q9, Q10
    all collectors measure 40v

    Q12
    e 5.2v
    b 4.6v
    c -40v

    Q13
    e 1.3v
    b 650mv
    c -1.3v

    Q14
    e -40v
    b -40v
    c -1.3v

    Q15
    e 0v
    b -240mv
    c -1.1v

    Q17, Q18, Q19
    all collectors measure -40v



    i appreciate any help/direction. i'm much more of a tube guy (they make a lot more sense to me) so i am lost!
    Tags: None
  • #2
    Can you supply a schematic?

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    • #3
      Originally posted by Jazz P Bass View Post
      Can you supply a schematic?
      yes of course, this should be the whole beta series service manual: https://drive.google.com/file/d/0B01...ew?usp=sharing

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      • #4
        No DC on output when running with no load?
        If not, have you tried connecting load while amp is powered up? Some SS amps won't power up properly on bulb limiter if load is connected, so you have to connect the load while it is already up and running.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

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        • #5
          Where's page 13 (power amp schematic) of the manual?
          - Own Opinions Only -

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          • #6
            Originally posted by Helmholtz View Post
            Where's page 13 (power amp schematic) of the manual?
            Probably this one (attached)
            Attached Files
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            ​
            • Likes 1

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            • #7
              Q3 voltages don't make sense. I don't see how the emitter could have positive 8.5V. Did you forget a minus sign? And, if it were indeed negative voltage, it would indicate an open B-E junction. Check the transistor, and recheck voltages.
              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
              • Likes 1

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              • #8
                These are the manuals from Fender, the Alpha series uses the same power amp but the manual is clearer then the Beta Manuals
                Alpha Series.pdf Beta Series Service Manual.pdf Beta Bass.pdf Beta Lead.PDF

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