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Sunn Col 300 one thing after another...

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  • Sunn Col 300 one thing after another...

    Thank you guys for getting me to this point, with all the good information on here. I just started to try and fix some old amplifiers. I'm not in the industry, and have only some basic knowledge of electronics, so it has been a challenge. Nevertheless, I'm learning a lot!

    So, I have a Sunn Coliseum 300 solid state amp, that I was using as a PA for karaoke, by plugging my mixer into the power amp input. It worked great for a long time, then it made a loud hum and I learned from this site that I should check the output caps. I replaced them, put them in backwards, popped a couple fuses, de-soldered and put them in right, and the hum is gone.

    But, now no sound at all with a mic at the input. Checked with the mixer into the power amp input, and that was working perfect. So I checked all the cables out of the preamp, looked over the entire preamp section, and found a loose cap. I re-soldered the cap and put it back together.

    Now, with the mic in the input, I hear a faint sound from the mic that is distorted like it was run through a fuzz-box. I ran the preamp-out to another power amp and it sounds the same. I didn't see anything else loose, burnt or otherwise out of place. What should be my next step?

    I have one other PA amp (Crate PA-4) I'm trying to fix. It had a similar hum that was not effected by volume/gain, so I replaced the output caps (correctly this time) but it made no diferrence. So I tried to test the transistors (2 big ones & 2 little ones on the heat sink) with my DVOM and although there were no direct shorts or opens, I am not satisfied I did it right. Each of the 2 pairs gave similar readings to each other, but I was getting variations in the readings each time I checked. I have a button that turns on or off a diode symbol, and I guess I just don't know how to use it right. Should I just replace all 4 transistors and hope for the best? Or, is it worth spending more time trying to get a better diagnosis first?
    **forgot to mention .8 to .9 vdc at the speaker** How much DC voltage indicates blown out output transisters?

    Thanks
    Last edited by rcrmck; 11-23-2009, 11:03 PM. Reason: Forgot important thing

  • #2
    I'm such a blockhead, no wonder I got no replies...

    I checked the ribbon cables for continuity and to be sure they were not crossed, since the cable was delaminating. But, I didn't look at the solder joint until last night. There it is, 3 of the wires from the input section ribbon cable wiggling around with broken solder.

    I'll test it tonite, but I got a good feeling!

    Still no luck on the crate PA-4 though.

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    • #3
      Check this out for help testing the transistors:
      Bipolar Junction Transistor Testing Basics - AudioKarma.org Home Audio Stereo Discussion Forums
      ST in Phoenix

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      • #4
        Originally posted by Phostenix View Post
        I think I can follow that. I'm going to pull the transistors first though, and try it. The PA belongs to my BIL and he used it for karaoke. It's not worth the cost of repair, except I'm gonna try to do it myself for free, just for the experience.

        Thanks

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        • #5
          Originally posted by rcrmck View Post
          I checked the ribbon cables for continuity and to be sure they were not crossed, since the cable was delaminating. But, I didn't look at the solder joint until last night. There it is, 3 of the wires from the input section ribbon cable wiggling around with broken solder.

          I'll test it tonite, but I got a good feeling!
          Woo-hoo!! That did it! My first successful amplifier repair. We busted it out last night with a little get-together and that thing is smooth as silk! Boom-boom-boom!
          To recap:
          Loud oscillating hum with nothing plugged in and not effected by controls was fixed by replacing the two big caps in the power section.
          The no preamp output was fixed by re-soldering a loose cap on the preamp board.
          The ugly buzzing low preamp output (after the cap fix) was fixed by re-soldering the input section to preamp ribbon cable, and re-laminating the ribbon cable using clear rtv silicone.

          I'm very proud of myself

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          • #6
            There you go, early Christmas present to yourself.
            Education is what you're left with after you have forgotten what you have learned.

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