Boston Acoustics CR400 Subwoofer / Fuse Blower

May 25, 2009 6 Replies

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pc_whocares 17 years ago

A friend of mine brought me a BA CR400 subwoofer that's blowing fuses. I told him I'd have a look because that sort of failure will usually produce brown or cooked components on a PC board, making it easy to fix.

After cracking the case, there is nothing obviously wrong with any of the components.

The unit has some interesting features where it will automatically turn on and off under certain circumstances, so, even when the power switch is OFF, when it is plugged in the speaker hums a nice 60 Hz sound for a second or two, and the 1 amp slo blow fuse goes.

It's about ten years old. I'm thinking power supply capacitor -- there are two. I'll try BA on Tuesday to see if they have any inputs.

In the interim.. Suggestions?

tanks

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Dave M 17 years ago

Chances are that one or more of the output transistors are shorted, and if the unit has a relay that switches power to the power amp, its contacts are likely welded shut. I've repaired a fair number of subwoofer amps, and this is the most common problem that I've seen. You'll be ahead of the game if you unsolder the output transistors, check them for shorts and go from there. If you find that one of the transistors is shorted, be sure to replace them in pairs. A subwoofer usually doesn't need matched pairs, but some models might. Also, if you find shorted or open transistors in the output, be sure to check the transistors that drive the outputs. They could easily be damaged as well.

Cheers

Dave M masondg44 at comcast dot net One good thing about Alzheimer\'s; you get to meet new people every day.

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CJT 17 years ago

If it hums like that, I would suspect the power supply rectifier(s).

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Mark Zacharias 17 years ago

Hum caused by the power supply cap(s) would not blow the fuse. A cap would just be open circuit and the resulting ripple would cause the noise, but as I say, the fuse would not blow. There's a short somewhere (main amp, likely), but I think if a main cap were shorted, you'd never hear the hum before the fuse blew.

Mark Z.

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Geo 17 years ago

I agree - if it /is/ 60Hz then half a bridge has disappeared...

Geo

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CJT 17 years ago

... or shorted.

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pc_whocares 17 years ago

Good advice from all of you.

I would expect that if a cap were bad but not shorted, it would not necesessarily blow the fuse right away.. but who knows?

Rectifiers are easy to check, and that certainly makes sense.

I'll keep 'yall posted.

tanksAbunch

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