My home theater had a small powered subwoofer, an Insignia NS-RSW211 Rocketboost 6-1/2″ 70W Wired/Wireless-Ready Subwoofer. After several years of use, it started exhibiting some strange effects and I disconnected it. Since I’m not a huge home theater buff and it was a modest unit to begin with, I didn’t really miss its absence. It sat forgotten in a corner until I saw Monoprice held a sale on their item #8248, a similar-sized powered subwoofer that would be a great replacement. Before I hit “Buy” on the Monoprice item, though, I thought I should make an effort to fix the one I have.
The failing symptoms indicate an intermittent connection somewhere in the system. When I turn on the subwoofer, it is fine for the first few minutes. After that initial period, sound would start cutting in and out at irregular periods. Every time it cuts out, the low bass sounds disappear. When it cuts back in, a deep “thump” announces return of low frequencies. This would start out tolerably infrequent, like hearing a distant firework show. Interruptions then become increasingly frequent. Eventually it will sound like automatic weapons fire in the background even when we’re not watching an action film, at which point I would turn it off. After a few hours of rest, I could repeat the cycle. Intermittent issues are always annoying to diagnose (part of why I’ve been putting it off) but I should at least take a look. On to the workbench it goes!
There are a lot of fasteners visible on this back plate. This is not a huge surprise: a subwoofer’s job is to push those low frequency thumps. Each thump will rattle anything not securely fastened, and every thump will be trying to loosen every fastener. In fact, the large numbers of fasteners are quite welcome: if it had been glued together, opening it up would be a destructive act making a successful repair unlikely.
But it wasn’t glued, so I could get to work. Removing the outermost eight fasteners allowed me to remove the rear module. I was a little surprised to see all electronics were sealed inside an airtight box. This might be good for acoustics but bad for air cooling circulation. The only thing poking into the acoustic chamber are the pair of speaker wires going to the driver itself. They used commodity spade connectors and were easy to disconnect so I could focus on the electronics box.
Removing the next outermost set of six fasteners allowed me to open up the electronics box. I was greeted with the thick stench of fried electronics. Something definitely died in here and, if it smelled this strong, I should be able to see it.
Yep, there it is. Capacitor C28 is toast. Finding this dead capacitor is good news, much easier than diagnosing an intermittent issue. The bad news is I’m not familiar enough with power supply theory of operation to explain why this absolutely and completely dead capacitor would cause an intermittent failure.
One end has completely blackened and appears to have broken open as well.
The yellow circuit board appears to be the power supply subsystem. 120V AC power cable (black & white wires) goes to the power switch, then into one corner of this yellow board near the dead capacitor. Diagonally opposite them is this connector delivering +24V to the rest of the subwoofer.
Unplugging AC input and DC output wires, then removing four screws, allowed removal of this power supply board so I could unsolder the dead capacitor easily. It came off in two separate pieces, very dead.
Reading markings on the charred capacitor carcass was a challenge. After playing with lighting, camera settings, and photo editing, I could make it out as:
I’m not familiar with this type of capacitor and didn’t know how to interpret those numbers. Looking around online, I found this page which said “105” meant 10 * 105 pF = 1000000 pF = 1000 nF = 1 uF and the “K” meant +/- 10% tolerance. The voltage rating portion didn’t line up with anything on that page, though. I’m inferring that “250KC” means something that can handle up to at least 250V, as this device can take up to 230V AC input.
Looking around my various assortment trays of capacitors, I didn’t find anything +/- 10% of 1uF. I then looked through my pile of teardown remnants for capacitors to salvage. The closest candidate was a 0.68uF 450V capacitor from the Antec power supply that caught on fire.
It even had the same footprint as the original toasted capacitor, making for an easy fit in the available space. However, 0.68uF is still short of the capacitance of the original so I continued looking.
I found a 0.22uF 250V capacitor inside the surprisingly complex evaporator fan. There was a clear conformal coating over everything that made removal a bit of a pain (and the result looking messy.) But they gave me a theoretical 0.68uF + 0.22uF = 0.90uF and my multimeter says they’re actually a tiny bit above rated value. Bringing me within 10% of 1uF, good enough for a test run.
Since the original capacitor slot was already occupied by the 0.68uF capacitor, the second parallel capacitor had to sit on the back.
I buttoned everything back up and preliminary test looks promising. After playing through a two-hour movie, I have yet to hear the thumping “fireworks” to “gunfire” failure sequence. Still unknown: what killed the original capacitor, and whether the same will happen to these replacements. Time will tell. In the meantime, I’ve managed to keep something out of landfill and resisted the temptation to buy a Monoprice powered subwoofer on sale. I’m thankful the design & engineering team built this device in a repairable way.