


Old 2.1 speaker system: replaced the main filter capacitor, still getting humming
I'm looking for some advice on an old VS2421 2.1 speaker system. It's a very cheap multimedia speaker set, but it has sentimental value so I'm trying to repair it instead of replacing it.
The issue is a constant low-frequency hum (I'd estimate around 100 Hz) that starts as soon as the system is powered on, even with no audio cable connected.
Based on previous advice, I replaced the main filter capacitor (Samxon 4700µF 35V, ~20 years old) with a new Nichicon PW 4700µF 50V 105°C(the big green top right of photo 1).
Unfortunately, the hum is still present. It may be slightly lower than before, but it's definitely still audible.
Here are a few observations:
- The hum is present in both satellite speakers and the subwoofer.
- The master volume control has no effect on the hum.
- The bass control affects the subwoofer. Increasing the knob increases the hum from the subwoofer.
- If I turn the bass control all the way down, the subwoofer becomes silent, but the satellites still have the hum.
- The subwoofer cone can be felt vibrating slightly from the hum.
I'm attaching a picture of the PCB.
Based on these symptoms, what would you investigate next?
Would you suspect:
- other electrolytic capacitors around the amplifier,
- the power supply,
- the bridge rectifier,
- or simply the design of this very cheap amplifier?
Any advice would be greatly appreciated. Thanks :-)