I've had an cheap pair of CA-2012 speakers for years, and yes, they do sound like garbage. Their quality was then affected even more when the speakers began suddenly emitting a loud, buzzing hum. The sound is similar to a 200Hz tone, but much more distorted. I remember (I was much younger back then) putting the speakers away and hooking up my computer to the AUX jack of a boombox instead. At that time, it seemed much simpler to put the problem aside and get a temporary fix.
Years later, I found the speakers lying with some other old hardware, including a CRT monitor and Windows 98 desktop. They had been gathering dust for years. I was becoming more experienced in electronics then, having built a PupDAC and currently working on an O2 amp. Somehow, I thought that it would be worth a try getting those things fixed and repurposed as a set of cans for an older machine.
In case you're wondering, the CA-2012s are a set of powered computer speakers, receiving 5VDC from USB to push the amp. The actual sound comes in through a 3.5mm jack (line level). A picture is below (Figure 1).
Like a doctor, you need to do some basic tests before trying to fix the problem. Here's what I found:
- I tried plugging the speakers into a variety of different USB ports, including those on my computers and AC adapters. The hum persisted on all of them, making me believe that the noise isn't caused by the actual current going into the device. (a.k.a. Ground Noise)
- When I turned the volume pot up and down, the hum did not get louder or softer. This makes me think that the speakers, not the source, are causing the problem. If the intensity of the noise does change, then the problem might be with your soundcard. Try plugging the speakers into an iPod or similar to see if they still hum.
With this in mind, let's start tearing it apart!
Every speaker is different. There might be screws, tabs, or other fasteners that prevent you from opening the device. You might need to remove the speaker grille to see the screws underneath, or as a last resort pry at the sides with a screwdriver. Once it's opened, remove the PCB (Figure 2) and stretch it out as far as the cords can go.
I generally try to think out how a device works after I take it apart. We don't need to reverse-engineer the whole schematic, but knowing the basic layout of things might help if you plan to start swapping parts.
The 3.5mm audio signal enters a potentiometer, which is then sent directly into the inputs of the opamp. Power from the USB cord goes through a switch, powers an LED, and then enters the opamp to boost the line level signal. The amplified sound is then distributed to the left and right speakers.
To easily work on my PCB, I have to get rid of those wires. However, it's suicide to start cutting or desoldering them without thought, as you could potentially make a big mess and not be able to rebuild it later. Grab a marker and draw a line on the cord you'll remove, then draw another line on the solder pad or hole the wire leads to. If you've got a bunch of them, use different colours (Figure 4).
It might also be a good idea to label where the cord actually leads to (USB, LINE IN, etc.) to reduce confusion later (Figure 5). Ready? Grab your soldering iron and get those wires off.
Inspection & Cleaning
Look at the image above. For some reason, the PCB is covered in some strange sludge that might or might not be affecting performance. Sometimes, a wipe is all you need to make the board work again. Personally, I use a cloth soaked in alcohol (medical, not drinking!) and wipe at the grime in order to get it off.
Now's also a good time to look at obvious problems, such as blown capicators or broken wires. A busted cap might have a bulging or leaking top (Figure 6) and must be replaced immediately.
|Figure 6 (Credit: Wikipedia, CC BY-SA)|
Resistors have their own problems too, and excessive voltage or pressure can cause visible damage like cracks and burns (Figure 7). If you have a multimeter, remove them and compare their actual resistance with the one marked on the resistor itself. If it's wildly off, replace it.
|Figure 7 (Credit: Wikipedia, CC BY-SA)|
Replacing parts is beyond the scope of this article, but remember that we've got the Internet.
Fixing Cold/Bad Joints
Look closely at your board again, with a magnifying glass if required. Do you see any globs of solder that appear to be rough, dull or discoloured (Figure 8)? If so, you've found a "cold" joint, where the solder was not melted properly and therefore forms a loose connection.
All you have to do is touch the tip of your iron onto the solder until it melts and turns shiny. You rarely even have to add extra solder. There's a bunch of other soldering problems too, including stuff I've never heard of like "solder starved" and "overheating joints". Take your time and clean everything up, keeping in mind that most of these low-quality electronics were made in factories with little regard for QA. A simple touch of the iron or the addition of some new solder is often enough to get the job done.
Wire up your patched-up board and plug it in (Figure 9). To my surprise, the noise had suddenly been reduced to almost nothing. I don't know if it was the cleaning or the cold joints, but it now works. That's all that matters.