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| Swapping the caps |
From the same pile headed for the heap where I got the amp in the previous post, I also picked up an active subwoofer with a 10” speaker. My dad was throwing it in the trash because it started making an annoying loud noise after being on for a while. The noise was at a steady frequency and unbearably loud. My initial thought was that the rectifier bridge lets the 50 Hz AC from the mains to the amplifier circuit, so I started off by replacing the capacitors in the bridge. This didn't solve the problem, but it was a good idea to swap those caps for new ones preemptively.
I let the subwoofer stand in the corner of my living room and kept pondering about the issue. While trying to figure out what the problem was, I cleaned the potentiometers for the volume and low-pass filter. At some point a thought of building a new amplifier circuit from a kit also crossed my mind.
One potential culprit was the coil in the speaker, so I measured it with a DMM by hooking the probes to each terminal of the voice coil and moving the membrane back and forth. The resistance values kept changing as they were supposed to, so the fault wasn't there. This was good news because a new speaker would have been way too expensive for this project.
At some point I realized the noise started after the subwoofer had been in use for a couple of minutes. From it was easy to deduce that this is a result of some component overheating. I reached behind the sub and felt the heat sink with my hand. It was completely cool. This meant that the heat was somewhere else, so I took the amplifier apart once again and removed the heat sink from the power amp transistors. All the silicone paste that was supposed to be between the transistors and the heat sink had found its way to the sides of the transistors and away from the heat exchange surface. Now instead of conducting heat, the paste acted as an added insulation for all the heat. I removed all the silicone paste and replaced it with silver paste that was left over from some previous project.
The subwoofer had been in daily use ever since for the past three years and is still working without any problems. I didn't really need a sub for my home theater, but this was a fun experience and I learned something new. In the end I got a quality sub for the price of the capacitors which was around 20 euros.
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| Test drive |
