Looking for help diagnosing problem with powered speaker

[xilefone]

Hi everyone,

I'm hoping to get some help diagnosing a problem I'm having with a powered studio monitor. They are M-Audio model BX5a. As soon as one of them is plugged in, a loud buzz is heard that gets progressively louder until I unplug the power. This occurs even if there is no signal going to the speaker (nothing is plugged into the input). I'm hoping it's something I can replace in the circuitry in the speaker. Any help/advice is appreciated. I can post pics if anyone thinks that would be helpful.

Thanks!

[52 Bill]

As soon as one of them is plugged in, a loud buzz is heard that gets progressively louder until I unplug the power.

If you give the cabinet a good Enzo thump with your fist, can you make the buzz stop or change in any way? That sort of symptom is typical when a power supply filter capacitor becomes unsoldered from the pc board, and thumping the cabinet will sometimes cause the bad connection to temporarily reconnect itself.

[olddawg]

Those small powered monitors are notorious for underrated power supply electrolytic filter capacitors. They are usually glued down and attached to a double sided board which can be a pain if you are not familiar with this type of work. The biggest problem is finding caps that fit. I would try over rate the replacements if possible. The last ones I worked on I put 35v rated caps in place of the original 25v rated caps. I left the leads a little long so I could position the caps and it also made it easy to solder both sides of the board.

[Chuck H]

If they are not plugged into anything and still making a hum then I would suspect the power supply for the internal amps. Otherwise, that is, if the powered speakers do have a signal input that is connected when the problem happens, I might suspect a ground loop. You could try lifting the AC ground for the powered monitors/speakers (provided whatever is plugged into them is grounded) and see if the hum stops.

[km6xz]

How loud? Is is the loudest you have heard from the speaker when using cleaning unclipped input signals? If so, it is probably the speaker getting full power supply DC supply voltage. Don't plug it on again with the internal speaker connected to the amplifier circuit. Open it up and disconnect at the speaker wires. Do you have any test equipment such as access to an oscilloscope? Odds are, if the noise is REALLY loud that the power output amplifier IC is shorted. I do not have a schematic for it but assume it uses a bridging amp topography which can generate relatively high power for a moderate power supply voltage. If it does, it will be a large Intigrated circuit with a number of pins going into the pc board, in a single row or 2 rows. It is fastened to a heat sink for cooling, maybe the rear panel. Measure all the voltages on the pins, when on, and copy down the numbers on the IC and list them all here. This assumes you disconnected the speaker wired inside the unit, if you do not you will ruin the most expensive part of the speaker system needlessly.
If you do not have a meter to measure the voltages, I suggest you take it to a friend or shop that has experience repairing solid-state equipment. Many hobbyists and even a some pro techs only work on tube based equipment because it is so simple and don't know much about electronics in general. If you did not damage the speaker driver by leaving it on too long, it should be a simple cheap repair.

[ArianeWhit]

good work

this is good work !

[olddawg]

Seriously, look at the 2 power supply filter caps. If any are domed up replace them all.

[J M Fahey]

this is good work !

Nice knowing you, spambot.

[Chuck H]

I like Steve's idea to arrange a room at the forum that would actually attract and be exclusively for spambots. Then we could watch how they interact with each other. It'd be a hoot.

[J M Fahey]

Their (cheap)A.I. dialogues will be real funny.
I have met a few quite sophisticated ones, which were triggered by, say, the word "Stratocaster".
They'd post "Stratocaster is great/shit/for girls/etc." and start fierce battles, sometimes over 100 posts long.
Someday I will launch 2 "Eliza" type programs, answering ech other.

[Steve Conner]

Great idea. Now if only we could figure out a way to make the bots stay in that room. Then we could deploy the same technique to kick them off the forum automatically.

On the original topic: If it is DC as Stan suggests, you'll see the woofer cone strain outwards or inwards as far as it can go. That means the voice coil will catch fire any minute, so turn the power off straight away and suspect a blown output stage.

If it just sits there humming and doesn't catch fire, then it's more likely to be a bad filter cap, as others have suggested.

Catching fire is somewhat of an exaggeration as these things are supposed to be UL rated flameproof, but the speaker will smoke and be destroyed.

[R.G.]

If it just sits there humming and doesn't catch fire, then it's more likely to be a bad filter cap, as others have suggested.

... and when the electro cap then completely fails, it shorts or opens, the amp then puts full other-polarity DC on the speaker trying to bring things back. Then the speaker dies that way, too.

[tim]

Just fixed a powered M-Audio speaker this weekend with the same complaint. As stated above, I changed both filter caps...as I remember, they were 4700uf @ 35v. One of them had a bulging top. dead giveaway.

Works as advertised and a happy customer to boot.

[J M Fahey]

*How* can a commercial audio product have "bulging top" PSU capacitors?
Do they feed 42V to 35V caps?
Do they run them at a constant 100ºC by sitting them by a 10W ceramic resistor, going full tilt?
I don't understand it.
I'm as cheap as any manufacturer, usually feed 42/45V to generic Taiwan/China 50V caps, or +63V to 65V ones, and *never ever* had a bulging cap in more than 10000 amps along over 40 years.
The only (very few) ones were when they were inserted backwards (duh!) or a 110V set amp was plugged into 220V ... but never under "normal" use.
Only failure seen was some salts around the (punctured) discharge hole in the rubber cap, in 20 to 30 y.o. amps and even then, still working but with increased hum.
Oh well.

[Steve Conner]

Do they run them at a constant 100ºC by sitting them by a 10W ceramic resistor, going full tilt?

They're stuffed inside an unventilated speaker cabinet, on a board bolted to a toasty heatsink. And probably snuggled into a big wad of acoustic wadding too. So probably not far off it.