Posted October 10th, 2009, in: 1| Computer Problems and Fixes

Copy of a forum post I posted at M-Audio’s user Forum.

I love my BX5’s. One of them died. Here’s what’s going on. I would really appreciate any advice on further troubleshooting/diagnosis. 

• One of my BX5’s stopped outputting sound. It may been have been sitting turned on a few days before I noticed that it wasn’t working.
• Upon putting my ear to it, I realized that it was making a steady hum/buzz from both drivers. This was similar to a 60-cycle hum but a little raspy-er if that makes sense, and if I’m not out of my mind. Also it wasn’t a very loud hum. Only about as loud as you’d get from a bad cable or something.
• I tested for a bad input connection and tried using XLR instead of the 1/4 inch input. The problem was clearly inside the unit.
• I opened it up carefully, to see if there was anything obviously burnt or shorted or broken inside.
• After finding nothing that was obvious to me, I googled around and found a few posts on the m-audio forum talking about problems caused by worn-out capacitors. See: http://forums.m-audio.com/showthread.php?t=3871&page=1 and http://forums.m-audio.com/showthread.php?t=11926 and many others.
• One of my two main power-supply caps was indeed bloated. And I noticed some dark-brown crustiness on top of the other which I concluded might be leakage of “electrolyte,” whatever that is.
• I de-soldered and removed the old caps and set out to find replacements.
• I accidentally ordered 16V 6800mfd instead of 25V 6800mfd, which I didn’t notice until I had already soldered them in.
• I tried them anyway because in theory, since all the caps are supposed to do is smooth the supply current, too-low of a voltage rating on the cap would just mean that the caps will wear out sooner. The constant hum/buzz was gone with the new caps in. Instead what I got was a pop a few seconds after the unit was powered on. The pop is new. My functioning unit does not pop.
• I went and got the correct 25V 6800µF Capacitors and put them in but there was no change in the above behavior. Incidentally, I had to mount one on the bottom because the ones I got were more than twice as wide and a bit taller than the originals.
• I’ve double and triple-checked the soldering. I even opened up my functioning BX5 to make sure that the caps are in the right polarity.
• I tested for continuity between the capacitor leads and the destination/source on the printed circuit (for instance one path goes to what I believe is a Rectifier IC so I checked to makes sure that path was solid all the way from that component’s lead to the cap’s lead to rule out a bad solder joint… I did this for all the paths in the printed circuit)
• I’ve quadruple-checked for any visible shorts.
• I believe that my capacitor-replacement surgery was a success, so now I am beginning to doubt that bad caps were the real problem. It’s entirely possible that the buzz/hum I was hearing was there for some time since it wasn’t loud enough to hear until I put my ear right up to the unit. Maybe I have two problems, one of which I just fixed.
• I can now hear some faint white noise and hum from both drivers. The original, louder buzz/hum sound is gone. Instead what I hear is about like what you’d expect from an audio amplifier, but that is not the case with the fully functional unit I have… The functioning unit is very clean. The volume knob makes no difference in the slight noise/hum. And it’s so faint that if I didn’t have the other unit to compare with, I would probably think what I’m hearing is the normal hum of the amp.
• The only other sound I can get from the dead unit is when I change the “Low Cutoff” switch, I hear slight fuzzy, static-y sound during the switching, but none of the other switches make any noise.

What should I do next? Perhaps the real problem here is in the audio signal path? A pre-amp problem? How can I rule that out?

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