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SWR SM-900 Troubles

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  • SWR SM-900 Troubles

    Got an SWR SM-900, fire it up and the two pairs of stacked 1k 6.5w resistors get hot as blazes, there's a noticable hum coming from the chassis, and the power amp and preamp clip indicators stay lit regardless of the settings on the amp. No fuses have blown, but the 1k/6.5w resistors did get hot enough to melt their solder off and burn the traces they were hooked up to. The power transistors aren't even getting warm, and they'd be the first thing I'd imagine would pull enough current to cook the resistors like that. Any thoughts?
    Last edited by DominoTree; 02-18-2011, 02:06 AM.

  • #2
    All four get hot? then your +15 and -15 are either loaded down or shorted together. Seems unlikely they'd both fail at once.

    Might as well check all 6 diodes in the 15v supplies for shorts as well as the two 100uf/16v caps.

    Otherwise, check the wiring, and are there any ICs getting hot? Any ICs with the top blown off?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      For some reason, the only socketed things Fender decided to put in this thing were the ICs (all but one of which being opamps), all the boards and components and transistors are all hardwired, so I pulled them all out and it was doing the same thing. I already rewired most of the boards because whoever built this amp did a terrible job and the wires were coming apart and unshielded.

      I'll check the supply's diodes and the and caps next. I'm starting to think maybe a couple frayed wires might've shorted, and gotten hot enough to solder themselves together and I hadn't noticed.

      Cheers!

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      • #4
        Disconnected the +15v and -15v from both channels and even with them gone the resistors are cooking. Should be simple from here

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