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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
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
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