I have a Harman Kardon SUBTS15 subwoofer with problems. I would like to try fixing it myself. I have basic troubleshooting experience (including soldering skills), but I don’t know what I should next (besides tossing it).
Here’s what it does, and what doesn’t.
When I power it up, the front led lights amber, and after a couple of seconds turns blue (and stays blue). At that moment you can hear a loud thump (couple of thumps actually). While on, there is no sound coming from the speaker. If you feed a signal (at the line level, with test tone from receiver), you can hear it extremely faint (the volume does not modify that). When powered down, another thump.
What I checked: the relay works both when turning on and off (both times coincide with the thumps). The +/- 15 V are both OK. At the visual inspection, there is some discoloration (darkened color) of resistor R146 and 149 (especially 146 that looks a bit burned) but both test correctly 330 ohms. The PCB around them also looks a bit darkened.
Any idea where to look for? I doubt that the issue comes from the “darkened” area. The troubleshooting flow chart does not help, as the led does not turn red or green (only blue, and stays blue).