Repairing M-Audio BX5a Studio Monitors

I've had a pair of M-Audio BX5a speakers for use with my computer for a few years and over the last year or so, the gain in one channel, and then the other, has steadily decreased.  They're pretty decent computer speakers and I didn't want to throw them away and get new ones, so I searched the web to find service information but came up with nothing.

These speakers are biamplified and the specs claim 40W for the bass driver and 30 W for the tweeter.  The transformer that powers the speakers is rated at 2 x 16V 1.5A (marked on the transformer).  There is a pretty big heatsink on the two, probably class-AB, amplifier chips, but it's mounted inside the enclosure and the only thermal communication with the outside environment is via the steel back panel of the speakers (that is warm all the time when the speakers are powered) that the heatsink is mounted on and a little air that might make it to into the box via the bass vent. The result is that the speakers run warm.

Heat and electronics is never a good combo and leads to failure of electronic components.  The parts that are most affected are semiconductors that tend to fail catastrophically, and electrolytic capacitors that tend to degrade over time.  The speakers still worked, but gain was dropping, suggesting that the semiconductors were still functioning, and that signal coupling capacitors or power supply electrolytic and bypass capacitors may be failing.

I decided to repair the speakers using a brute-force approach that I used to use when repairing old vacuum tube radios- start by replacing all the electrolytic capacitors.  It's a reasonable approach given the low cost of capacitors and their relatively high probability of failure.  Also, if one has failed, others operating in the same environment may fail soon, too, so replacing all of them is the best way to ensure that the speakers will work for a few more years before they need any more attention.

Disassembly

Whenever I'm going to do something like this, I take lots of photos as I go so that if there's any doubt about what goes where, I can review the pictures and get things back together the right way.  I suggest you do the same.  As I take screws out, I put them in a tray so they don't get lost.  When there are different types/lengths of screws, I will sometimes turn them into their holes so that I will know which ones go where.

The first thing I had to do was get the speakers apart.  I have some M-Audio AV-40 computer speakers which suffered from failed capacitors a while ago.  They were a nightmare to work on because of the way the speakers were built, so I expected the same in the BX5a speakers.  I was pleasantly surprised to find them pretty easy to take apart.

There were only a couple tricky things to deal with.  Once the back panel was open, I had to reach into the speaker and clip a zip tie off the wires that connect to the drivers so that I could get my hand in far enough to disconnect the wires from the drivers.  There's an LED on the front panel and the wires from it go to a connector on the amplifier circuit board.  For some reason they put glue on the connector which made it a PITA to separate.  The wires from the power transformer went to a connector that was also glued.

Clip the zip-tie circled in green to release the LED and speaker wires, then reach in and pull the speaker leads off the drivers.  The LED attaches to the amplifier board with a connector so it gets released there.  There's a drop of glue holding the connector on the PCB for some reason.

Unscrew the two ground wires circled in green, then cut the zip-tie circled in pink to remove the ground plate to gain access to the amplifier PCB.  Unplug the transformer from the amplifier board - the connector is glued.  The final step is to unscrew the input connectors and volume control, then the three screws that hold the heatsink on the rear panel of the speaker.

This is the amplifier PCB and heatsink.  The nonpolar electrolytic caps are all green. There are two on the small PCB behind the volume control pot and one on the main board. The other electrolytic caps are all black.  All are rated for 105C.

After getting those items sorted out it was pretty easy to extract the amplifier assembly from the box and identify all the capacitors.  I drew a picture of the amplifier board and added major landmarks, then marked the values of the capacitors and their approximate locations.  The PCB is marked with component numbers so I made a list of all the numbers and values.  I also measured the diameter of all the caps so that when I ordered replacements I could be sure they'd fit in the same space.  The board is actually pretty generous with space around most of the parts, so matching the component sizes wasn't entirely necessary.

Capacitors and their locations on the PCB.  I just started using the Pilot white board markers and they are fantastic.  The colors are very bright and they erase cleanly and easily.  Highly recommended!

The 1 uF and 0.47 uF non polarized electrolytics were hard to replace.  Non polarized electrolytics are often used for coupling audio which means they may be exactly the caps that need to be replaced to get my speakers working like new again.  I was unable to find electrolytic replacements, so I used film capacitors instead.  Film caps will probably perform better and last longer, so it isn't a problem.  I found suitable parts in my junkbox, and picked out replacements from the DigiKey catalog.  The film caps have wider lead spacing than the original parts and are physically larger, but they fit into the board just fine.

I made a spreadsheet that has all the original capacitor values and designations and a list of DigiKey part number replacements for them.  All the replacements are rated for at least 105C operation, and in a few cases I was able to select parts that were rated for >1000 hrs operation at that temperature. It costs about $11 per speaker for all the capacitors.

Replacing the Caps

The parts are all through-hole type which makes replacing them very easy.  Be sure to pay attention to the polarity of the caps as you remove and install them.  I removed and replaced the old caps one by one.  I grabbed the cap that was coming out with a pair of pliers and heated its leads on the underside of the PCB until the part was loose enough to pull out.  Then I cleaned up the PCB as needed with some desoldering braid, and installed the new cap, matching the polarity of the old cap.  I kept a printout of the spreadsheet on my work table and checked off each capacitor as it was replaced to ensure I wouldn't miss any of them.   It took me about an hour per speaker to replace the old caps.

Reassembly

Reassembling the speakers is the exact opposite sequence of disassembly.  When you put the speakers back together, you have to be careful to reconnect the driver leads the right way.  In the first photo, above, the black lead for the bass driver connects to the terminal nearest the bottom of the photo, and the red lead goes to the other terminal.  The tweeter's black lead goes to the connection nearest the top of the photo and the white lead goe to the other terminal.  Be sure to reattach the ground wires and the metal ground plate that goes under the main PCB.

Results

The speakers sound like new again.  I used to leave them powered up all the time which probably led to the short lifespan of the original capacitors.  In the future I'll power the speakers off when I'm not using them so they don't sit and cook.  Maybe they'll last longer this time...