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Thread: SWR Workingman's 15- Voltages- DC on Speaker

  1. #106
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    Bstring, you could probably get away with it if you follow the directions and never exceed a couple volts on the output.... but if you get any higher than that, even for a couple seconds, i think you can and will damage the output.
    Jcon.
    And make sure you run it with the heat sinks!

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    Last edited by log1982; 01-30-2020 at 10:07 PM.

  2. #107
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    Quote Originally Posted by BStringThumper View Post
    Hi g1, thanks. After R39A was replaced with the correct 22 ohm (not 220 ohm) the amp would power up on the limiter even with a load. I did read that some amps will not be able to do this because of the soft start not stabilizing first. This has been quite the learning curve.
    It's not about the amps that have a soft-start. Those will just not start. Some amps powered up with load on limiter will come up with big DC offset. In your case, maybe not with 8 ohm load but maybe with 2 ohm load. Now you have the amp trying to deliver current through the load even without signal applied (because of the DC on output).
    What is the wattage of the bulb you are using?
    My thinking is that if the bulb wattage isn't too high (100W bulb or less), you should have been ok driving a 2 ohm load while on the bulb limiter, as the bulb would prevent excess current. Other than on the bulb, the amp is not designed for 2 ohms so you should not do that.
    So it's possible that the amp came up with DC on the output when powered up on bulb with 2 ohm load. If so, the problem is always avoided by powering up on the bulb first, then connecting the load while unit is powered up.
    Still another possibility is that some driver or other part was still marginal and would have failed regardless of the load impedance or being powered up loaded on the bulb.
    We can't really know definitively, just surmise.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  3. #108
    Supporting Member loudthud's Avatar
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    This amp has no short circuit or over current protection other than the fuse on the output. Amps like this typically don't have an extension speaker jack. It's designed to run the internal speaker only.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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  4. #109
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    Good morning,

    Thanks for everyone's thoughts on this. I'm afraid to try the 2 ohm load bias procedure again. Thinking back maybe I turned the volume up too fast or high while I was trying to reach the 2V RMS on the output??? The amp is equipped with an external speaker jack. It even says 8 ohm or higher by the speaker jack. If it wasn't for the factory service manual saying to connect a 2 ohm load I never would have done it. So question, please. If I connect an 8 ohm load, 120mV 1Khz input signal, scope on output, turn volume up to...… 1/4 rotation, adjust bias to just remove crossover notch, will that be a reasonable approach?? The amp was working (again) before I tried the second 2 ohm bias procedure. Thanks!!

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  5. #110
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    You should be injecting the 120mV 1Khz signal at the FX return. Bring master up from zero till you have 2VRMS at output.
    You should not be on the limiter lamp.
    Should be no problem doing it at 8ohm rather than 2ohm load.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  6. #111
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    Quote Originally Posted by g1 View Post
    You should be injecting the 120mV 1Khz signal at the FX return. Bring master up from zero till you have 2VRMS at output.
    You should not be on the limiter lamp.
    Should be no problem doing it at 8ohm rather than 2ohm load.
    Hi g1, great! Looks good to me. We'll see how it goes. Thanks!

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  7. #112
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    Quote Originally Posted by g1 View Post
    You should be injecting the 120mV 1Khz signal at the FX return. Bring master up from zero till you have 2VRMS at output.
    You should not be on the limiter lamp.
    Should be no problem doing it at 8ohm rather than 2ohm load.
    Not that he needs it, but I second g1. Jcon.

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  8. #113
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    UPDATE: AMP IS FIXED !!*************************

    Hi Everyone!

    I replaced all the components we had discussed previously...… again. Powered up on limiter and checked for DC voltage at speaker jack, no load. 52mV at jack. Connected 8 ohm speaker, while powered (thanks g1), no change in bulb limiter. Powered down and connect sig gen and scope. Followed bias instructions, substituting the 2 ohm load for a 8 ohm dummy load. Knob to full wet, master volume brought up to 2V RMS on scope. It had a big crossover notch in the middle. I turned the bias pot with a wood stick until the notch just barely disappeared. I let it heat up a little more and then moved the bias pot to check the notch again. It stayed at the same spot, adjust again until it was just gone. Powered down and tested with factory speaker and cabinet. Sounds great!!!

    Thanks again to everyone!! I would have never fixed this thing without the help! I hope this thread can help others.

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