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Bias problem on SWR SM-400S

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  • Bias problem on SWR SM-400S

    I have a SWR SM-400S here for repair and it is giving me a bit of a issue. As long as there is no load on the output when it is turned on the bias is OK. I put on a load and can set the bias and it stays stable, the output waveform is great with a signal generator hooked up to the effects input and output voltage adjusted to 2V RMS by setting my signal generator. If I turn it off and wait for the rail voltage to settle below 30VDC the bias goes to rail voltage when I turn it back on if there's a load but is OK without a load or if I plug in the load after turning it on or if I turn it back on before the rail voltage drops below 30VDC. All of the output transistors turn on hard and pull the rail voltage down to 13VDC when running through a current limiting 100W light bulb. I have never seen an amplifier act like this and I'm looking for assistance.
    Attached Files

  • #2
    You can not power it up on the bulb with load connected. You must power it up then connect load while turned on if using bulb.
    But you can't adjust the bias correctly on the bulb anyway. If you have no DC on the output unloaded, power up without bulb.
    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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    • #3
      I worked on large SS amps in a busy shop for 25 years and never used a light bulb limiter even once. Some of the old TV guys did.. they were the only ones allowed to. I used a variac with a current meter. As factory service we were inspected yearly by certain manufactures. Yamaha in particular would pull your authorization if they even saw a lightbulb limiter. But yeah.. start up timing, protection circuits, any manner of things can get totally disrupted using a lightbulb limiter on a sophisticated amp. Power it up without the LB limiter (with a variac if you have one) and no load. No DC offset? Power up with a load. Follow the manufactures instructions for bias. Absent that use a scope and look at the crossover notch.

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      • #4
        Sorry, didn't read closely enough, what you have found is normal for this model and many others.
        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
          I fixed it. Thanks g1 and olddawg for letting me know this amp will not run properly on a current limiter. The bias is set correctly now.

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