Today’s Project: JBL PowerBass PB12 Troubleshooting

All about that bass (sorry, but I had to)

The problem

The PB12 Subwoofer
The PB12 Subwoofer

I got this JBL PowerBass subwoofer free from an old client, and it sounds real nice. I’m not an audiophile, or a serious home theater person, but I like well-rounded sound. This thing fills in just the right amount of kick when listening to music, and that’s with the level control set around 10-20%. I’ve never had it above 50% for fear of what might happen to these old cracked plaster walls. All and all, it’s a very nice addition to an almost-decent set of speakers.

Well, it was, anyway. A few months after I got it, it started randomly dropping out; clicking in and out of standby mode. My first thought is that there’s just a loose connection in there that needs re-soldering. Let’s find out together, shall we?

Disassembly

So many screws...
So many screws…

Cracking into this thing isn’t all that hard, but there sure are a lot of screws. All the obvious screws around the edge of the back panel need to come out. There are also two sneaky ones near the corners, circled in blue in the picture. Once those are out, the entire amplifier unit pulls right out. We just have to be careful not to rip the wires off; there’s the two wires for the speaker itself, and two more little wires for the power LEDs on the front.

The amplifier unit
The amplifier unit

Fortunately, they were nice enough to use removable connectors for those wires. It’s almost as if they wanted these things to be repairable.

The push-on connectors for the speaker were amazingly tight and needed a little persuasion to remove. They must have designed this thing to stand up to a bit of vibration, for some reason…

Once we disconnect the wires, we’ve got the amplifier in hand and we can give it a look.

Diagnosis

Well, there's yer problem
Well, there’s yer problem

Sometimes, it’s easy. Not as easy as I was hoping, but still pretty easy. Instead of the bad connection I thought I’d find, I spotted this blown electrolytic capacitor pretty quickly. This is a pretty common problem with electronics, and an easy one to fix. Some other problem might have caused this cap to explode, but it’s most likely just capacitor plague. This subwoofer was made during the early 2000’s, when several factories were making capacitors that failed prematurely.

So, we spend a few pennies on a new capacitor, swap it out, and life is good, yeah? No:

Well, [expletive].
Well, [expletive].
The image above is the back side of the circuit board, directly underneath that blown electrolytic capacitor. This is a .1 microfarad ceramic capacitor which is in parallel with the larger exploded one. When the big one popped, this poor little SMD found itself trying to pass way more current than it was ever meant for.

These SMT parts are hard enough to identify even when they’re not half-vaporized. Fortunately a quick Google search was able to find a service manual for this subwoofer, courtesy of diagramasde.com

It’s hard to tell from the half-burned label under that tiny capacitor, but it’s C71. The parts list in the manual tells us that it’s 0.1 microfarad capacitor, rated for 50 volts. This is a very common part, sprinkled throughout just about any reasonably modern piece of electronics.

Repairing it – eventually

Unfortunately, I don’t have the parts or a good enough soldering iron to fix this now, but at least we know what’s wrong with it. As soon as I can afford some proper tools, I’ll get the bad parts out, clean up the burned board, and (hopefully) bring it back to life.

Keep an eye out for my follow-up post on this. I have a feeling we’ll be learning about cleaning and repairing burned up solder pads and circuit board traces, whether we want to or not.

-Steve

 

 

2 thoughts on “Today’s Project: JBL PowerBass PB12 Troubleshooting”

    1. Thanks. I actually brought the amp in there last night to use one of the good irons, but I wasn’t lucky enough to find the parts. But they’re on order from Digikey!

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