M-Audio BX5 speakers

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billybob

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I'm about to be in possession of these speakers from a friend. I know one of the speakers has a problem though and I need to know where to start to maybe fix them. After 20 min or so the one starts popping randomly and crackleing a little. I think its temperature related but I dont know where to start to fix this (I've been trying to find the schematic for the speaker)
 
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they are powered, they have their own amp inside. I forgot to mention their the D3 model
 
first check and make sure you don't have any cracked solder connections on the amplifier board. one obvious sign of cracked solder is a dark ring in the solder connection. this usually happens on through-hole components that heat up like transistors, resistors of over a watt, power output chips. as the component warms up it's through hole wires expand and can actually put stress on the solder. or there could be a semiconductor (transistor or IC) that has a thermal failure internally... you would likely need a schematic to trace such a problem, however, if you can get the amp to fail with the speaker box open, you can spray components one at a time with freeze spray. when the problem stops, you have found the bad part. i try to "drip" the spray to minimize the chance of oversprayingand hitting a bunch of parts. if you can't find freeze spray, you can get "canned air" at an office supply or computer store. turning the can upside down makes the propellent come out as a liquid and it's the same chemical as freeze spray.
 
Thanks a million I will do that! You probably just saved me $80. Do you have any tips on checking components with a multimeter or a link I can check out. I find it hard to check things like capacitors since sometimes they can be in parallel with another capacitor or situations like that.
 
You can do it with a multimeter (ohmmeter).
Bad: If you have for example 3 capacitors in parallel and the multimeter measures almost 0 ohms constantly, then one or more capacitors are broken/shorted.
Good: If you measure them and switch the measure tips polarity it should beep for a quarter of a second and show 0 ohm then goes up to x megaohm immediately (which tells you that they are charging/loading which is good). But the real problem is when they "looks" to be fine and have a high ESR resistance. They would work for a while but may collapse anytime or not work properly from the beginning. You could see that probably with an ohmmeter also, high ESR resistance would cause the capacitor to load slower than other exact same good capacitors or wouldn't even start to load up (to make that measurement you need to desolder the capacitors). Or you need an ESR meter (https://en.wikipedia.org/wiki/ESR_meter). I personally use the ELV KT200 it is a component tester & ESR meter. Good luck

PS: I wouldn't buy that ESR meter for just one device, it would be cheaper to renew all capacitors to be honest. But if you repair often electronic devices, then it would be helpful to have such an ESR meter.
 
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Here is an update to the situation, I finally got the speakers and opened the problem one up. There are to chips connected to a heatsink, but are a little hard to get at. One of the Ics on the heatsink heats up even when the speakers are disconnected and nothing is being played. The model number is TDA7294
I don't know if I should change this out or if there is a deeper source to this problem. The speaker was choppy when I first plugged it in and tried it out. Then it poped a few times and went out and all I could hear was 60hz humming.
 
Those amp ICs are readily available, I'd get a replacement from a reputable supplier - Mouser, RS or whoever, not ebay.
 
Alright, I also checked the caps they seem fine. The two larger ones are in good condition. One reads around 5,000 uF when it's rated 4,700 uF is this an issue?
 
billybob said:
Alright, I also checked the caps they seem fine. The two larger ones are in good condition. One reads around 5,000 uF when it's rated 4,700 uF is this an issue?
Click to expand...

No, the tolerance of electrolytics is very wide - and the larger the better in this case.
 
check the speaker too make sure it's not shorted or open circuit. with a shorted chip, a bad speaker is also likely
 
I'm such an idiot, yes the speaker is not the problem, because as I mentioned earlier, the ic got hot even without the speakers connected. I'm going to go ahead and get the replacement ic. I'm just not to excited to solder it in in such a small space.
 
Did you check the TDA7294 for any internal short? Just measure the pins 13 and 15 those are the Vs- and Vs+ (if short then your ohmmeter should show ~0 Ohms)
You can also measure the 13th and 15th pins on the pcb while the TDA7294 is not soldered to check if the short is coming from that IC or if it's still available.
Or might be that the IC burnt something else there also
 
Alright, will do.
 
Ok, so I measured pins 13 and 15 while still on the pcb and my multimeter couldn't decide that resistance, but it was somewhere in the high kilo to low Mega ohm range, but I figured that might be expected when connected to the pcb. I guess I'll have to try and unsolder this this thing. Not to happy about that.
 
Your original post was Dec 2nd - you should have changed the chip on Dec 3rd

As a professional service engineer the first thing you would do with something like this is replace the chip - the odds are high that that's what's wrong, and it saves time and money to replace it first thing.
 
I know, I guess I was holding on to the hope that it wouldn't be the ic. Also with school and such time has been a little issue. Also I did not actually get them until Jan 6.
 
Ok, so I removed the ic got the new one in hand, but then i noticed the ic seemed to have been soldered on both sides is this a double sided board? If it is I dont know how Im going to be able to solder the new one it was tricky enough removing the bad one. I can send a picture later, but what is the chances of a double sided pcb on this amplifier circuit?
 

Very high, pretty well any modern board is at least double sided.

However, as long as you've not damaged the board it makes no difference - the two sides are joined together internally (through hole plated), so you only need to solder from the bottom side anyway.
 
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