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Sunn coliseum dilemma

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  • Sunn coliseum dilemma

    A few months ago I was getting complaints from my bassist about low-end cut out on his Sunn Coliseum head (solid state by the way). After I took all necessesay voltage measurements across the actual PCB's, jacks, and other possible trouble components, everything checked out (no drops or peaks according to the math).
    The other night we had a gig and he was able to power it on but there was no output to the speakers... period. They're not blown because he borrowed another bassist's Ampeg head, and the speakers were clear & concise.

    Anyhow, today i went there to run checks on the power supply (x-former, filter capacitors, rectifier) and it turns out the secondary side of the x-former is only putting out 36vAC, as opposed to it's prior 93vAC.

    This x-former is obviously fried!! I know because the voltage @ the power switch matches the wall outlet voltage. I've been rigorously searching for a reasonably matching X-former & it seems no company makes a suitable replacement. IT DRIVES ME NUTS! If somehow it helps, the part # they stamped on these models of x-formers is: "28-1419 1052 7325" . I know I'll need new filter capacitors and a rectifier to match the new work, but first I need an x-former ANY help would be appreciated.
    Last edited by ROBDUDE; 05-20-2011, 01:43 AM.

  • #2
    What voltage is marked on the power supply caps?
    Here is a copy of the Sunn Coliseum power supply.
    I am seeing +50 & -50 Vdc on the schematic.
    The transformer is center tapped to give two identical Vac into a full bridge rectifier.
    If you read 36Vac that would be bumped up to +-50Vdc after the rectifier.
    So maybe the transformer is not bad.
    Attached Files

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    • #3
      The transformer is fine.
      What DC voltages do you measure across each capacitor's legs?
      Juan Manuel Fahey

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      • #4
        That's the thing. Now nothing reads across either capacitor & no measurement on the DC side of the rectifier. I've never experienced something so strange before.

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        • #5
          Thanks. i gotta go back to my buddy's house & run the tests again, but like I told J M fahey i'm still getting no DC reading across the legs on either capacitor. So I tested the rectifier and even there got no reading on the DC side.

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          • #6
            O/K then start over.
            Measure the resistance of the primary right at the AC plug.
            This test will show the fuse is good & the priamary is o/k.
            Then, still with the unit power off, measure resistance of the two secondaries with reference to chassis ground.
            If both are the same (it Will be a low reading) power up the amp & measure each secondary Vac to chassis ground. Should both read about 36Vac.
            If this all proves true you most likely have a faulty rectifier.

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            • #7
              Are you sure you're switching your DVM between AC and DC voltage modes?

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              • #8
                THANK YOU VERY MUCH!!!!!!! I APPRECIATE you helping me. Once I went back and took the new measurements low-and-behold it was the rectifier. I went down to the electronics store (not idiot shack) & picked up a new one. Also somehow, at some point a 5A fuse was in one of the 2 10A clips & it kept blowing.

                All squared away now!

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