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I've got a Colisuem-300 that had some issues, including a shorted output device. Now when I run signal through it, the bottom half of the waveform clips early. All the semiconductors measure OK in circuit and the resistors seem to measure fine also. How do I best go about chasing this down?
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A schematic might get you more replys - you might also try searching on Sunn Coliseum. My guess is that you may still have a problem with some of the active devices and your DMM diode check method isn't finding them. Is the 300 similar to the Bass/Lead version? -
Sorry, here's the link:
http://www.ampix.org/albums/userpics...00_11_x_17.pdf
The problem is in the power amp section, page 8 of the file above. It's not the rails, they are pretty solid.Comment
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The lore is if it messes up without a load, look at the drivers. If it's good without a load but fails loaded, check the outputs.Comment
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Unsolder and lift one end of CR2 and the other one, CR6 (?), whatever that diode is by Q15 and C14. Those are limiters, and could be involved.
Top of the wave will go all the way to just under the rail voltage, even under load? The bottom side clips early. At what voltage does it clip? And does it have full excursion with all load removed?Education is what you're left with after you have forgotten what you have learned.Comment
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Thanks guys, I got it figured out. It was clipping early even without a load, and the top of the wave was fine. When I lifted CR2 and CR8, as Enzo suggested, it worked fine into a load (Good call Enzo!). I went to replace Q15 and discovered that it was a NPN transistor (MPSA42), which was incorrect. Putting the right part in there solved the problem!
It was obvious from solder flux remains on that board, that someone had shotgunned all of the transistors on the board. They had to sub a few parts, and put the same transistor in for Q6 and Q15, which should be complementary parts. Ever since that repair, the amp has been clipping early on the bottom. Now it's putting out over 300W RMS.
It's weird though, I can't get the front end to clip ever, even with my signal generator at maximum output. I checked the preamp out using the test point data on the preamp layout and it matches that data exactly. The preamp puts out 18V RMS cleanly with my generator maxed out. A bass guitar will never make the overload light go on. Is that normal for this amp?
Thanks again for your help guys!Comment
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