need help troubleshooting an amp

[emremeydan]

Hi,

I've been using a set of M-audio Bx5a monitors, and today -suddenly- one of them started "buzzing". It's not very loud, but it doesn't need to be loud in order to be disturbing It still seems to be working - there's nothing wrong with the sound, but with the added buzz.

The buzzing is not related to the signal source (it stays there even when i unplug the cable), and it's at constant volume (it's not affected by the volume knob).

Now, keep in mind that, each of these speakers have its own amp circuit inside (actually two circuits in each - these are bi-amplified: separate amps for the tweeter and the woofer) , so by comparing the broken speaker to the other one, here are a few more differences in behavior:

- when i turn off the working speaker, its power led keeps glowing for a few more seconds before it goes off. But on the broken one, the led turns off instantly as i turn the power off.

-when i turn off the broken speaker, there's a very loud popping sound, but not with the other speaker.

What do you think could be wrong with the circuitry? I'm thinking it might be a capacitor failure (maybe in the power supply?) causing all this, but i'm not sure. If it's something as simple as that, i can just replace the broken component instead of sending it away for repair (which might take months).

Thanks in advance.

...
emre

[teemuk]

With a 99% chance it is indeed a supply capacitor related problem. Would explain the buzzing and the quick dimming of the led (not enough capacitance to keep it powered). Check out the capacitors and their solder joints.

[Leolabs]

Agree with teemuk,check the caps first.

[emremeydan]

Thanks teemuk and Leolabs

I opened it up, and took this photo. As you can see, there seems to be something wrong with the cap on the right; it looks as if it's about to explode, not to mention it somehow managed to free itself from the glue. Am i safe to assume that this is the cap i should be replacing?

...
emre

[emremeydan]

If you can't see the attachment, here's the photo:



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e

[Bobken]

Hi,

Seeing this pic there is certainly something wrong with that cap.
The top is bulging due to pressure inside, and the sooner you replace it the better. This will very likely cure the present problem, but I would have a careful look at all other similar caps in the circuit while you are doing some work on this.

After maybe 10 or so years (depending on use and cap quality), these electrolytic caps do deteriorate somewhat, and if I was doing this job I would replace any similar caps as a matter of course, because they will probably go the same way soon.

However, if the unit is only a year or two old, then maybe this was just a rogue cap, and the others should be OK for a few more years.

Regards,

[emremeydan]

Thanks Bobken.

The thing is, i bought these speakers only 4 months ago! This particular model was first introduced at the 2005 NAMM, so they can't be older than 2,5 years old. I'm guessing it was just a defected cap.

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e

[Bobken]

Originally posted by emremeydan
Thanks Bobken.

The thing is, i bought these speakers only 4 months ago! This particular model was first introduced at the 2005 NAMM, so they can't be older than 2,5 years old. I'm guessing it was just a defected cap.

.
e

HI,

Be vary careful now that you have told me this, and at all costs, avoid switching the unit on whilst there is no cover on it.

With 'old' electrolytics, they tend to dry out over the years, so if and when they explode, there is usually less of a problem.

However, with newer caps, they will often spew out caustic electrolyte from within the caps when they fail like this, and it will make a real mess inside the unit. The greatest danger is if you are working over the unit when this happens and if your eyes are not covered at that time, you could end up with severe eye-damage!

Unless you purchased this second-hand, I would get back to the seller and get it properly sorted out, if it is as new you say. If you cannot do this for some reason and you need to do the repair yourself, assuming that the other other cap is potentially similar (probably from the same batch in manufacture) I would replace this at the same time, out of caution. Such caps shouldn't be too costly, and if one does later explode, it will certainly be a big mess and a lot more work to put this right.
Good luck.

Regards,

[emremeydan]

Thanks for the warning Bob. I know they can explode, i'm usually pretty careful around electrolytics. There's only two of these big caps (6800 uf, 25v) per speaker, so it shouldn't be too hard to replace them all.

By the way, they can't explode with the power off, can they?

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[Bobken]

Originally posted by emremeydan
Thanks for the warning Bob. I know they can explode, i'm usually pretty careful around electrolytics. There's only two of these big caps (6800 uf, 25v) per speaker, so it shouldn't be too hard to replace them all.

By the way, they can't explode with the power off, can they?

.
e

Hi,

I'm glad to know that you will be careful here as I have seen some rather nasty sights when this happens.

No, I doubt that any explosions could occur when not being powered up, but I would still be cautious when working on these, and especially I would wait a few hours after the last time power was applied, as gasses could still be building up for a while.

Good luck, and do take care.

Regards,