Fixing Blowing Fuses Problem in Altec Lansing ATP5 Powered Speakers

Altec Lansing ATP5 Amplified Multimedia Speakers, Front View

Love these amplified speakers. But after a decade, they started blowing their internal 1.6A slow-blow fuse. And, given how sweet these units sound, we couldn’t bear throwing them out. Not without at least taking a stab at fixing our blowing fuses problem in these Altec Lansing ATP5 powered speakers.

Fixing Blowing Fuses Problem in Altec Lansing ATP5 Powered Speakers

Thus, during disassembly of the subwoofer (big) unit, we saw the open power line fuse in the bottom tray. We replaced it. Then, the new one blew again, right away.

Glass Tube Fuses, 1.6 Amp, we replaced the blown one with in the Altec ATP5 speakers.

Tracking Down the Short

So, started reading around the circuits with a volt ohm meter. This revealed a short circuit across one of the main power supply bridge rectifier diodes; a KBU603 four-diode package.

Checked the Power Supply Diodes

Then, to further narrow down where the short was, and to rule out the bridge, we unsoldered the KBU603. But the meter still read a short across the minus terminal and one side of the AC input. Plus, out of circuit, all four diodes in the bridge read good. Nope, not the bridge.

Looked for Burn Marks on Circuit Board to Continue with Fixing Blowing Fuses Problem in Altec Lansing ATP5 Powered Speakers

Then, we inspected the circuit board with a magnifying glass. That showed a blackened “snubber” bypass capacitor. They have four in there, one directly across each diode in the KBU603. We’d show you a picture. But this very small capacitor crumbled apart in our fingers.

Found a Burned Disc Capacitor

Unusual to see a bypass cap in a low-voltage (less than fifty volt) circuit, short out like this. Indeed, this is the first time I’ve ever come across one. So, I was not at first, looking for this type of failure. But the board inspection with the magnifying glass showed this to be the case. Perhaps a power surge or nearby lightning strike caused this burning failure. Fortunately though, these capacitors were easy to spot and access.

Removed this Capacitor

Anyway, with the leads of that charred disc capacitor snipped, the power circuits no longer read a short. Then once we resoldered the diodes and installed a fresh fuse, the speakers played normally. No further fuse blowing. So don’t be afraid to use that multimeter to trace down shorts. Also, carefully look over those circuit boards visually. Doing this inspection can often uncover an obvious problem.

Did Not Replace It

Our research on snubbing bypass capacitors reveals that they reduce diode switching noise. This can be necessary because this noise can occur during normal operation, and might appear in the audio output. So, without the right capacitor in stock, just removed the old capacitors and replaced them with nothing. Yet we hear little increased humming and buzzing in the speaker output. Nor do we hear any loss in sound quality. So, we’ll leave the place for that capacitor blank.

Maybe these Altec Lansing powered speakers will last another fifteen years before requiring further fixing. Now, the final task is to figure out where one extra screw goes that we found on the workbench.