Love these Altec Lansing ATP5 amplified speakers. But after a decade, they started blowing their internal 1.6A slow-blow fuse when plugged into mains power. And, given how sweet these units sound, we couldn’t bear throwing them out without at least taking a stab at fixing this blowing fuse problem in this ATP5 Altec Lansing speaker set.
Fixing Blowing Fuse in ATP5 Altec Lansing Speakers
Thus, during disassembly of the subwoofer (big) unit in the above picture, we spotted the open power line fuse in the bottom tray, and replaced it. Then, the new one blew again, right away, upon power up.
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 itself as the current-sucking culprit, we unsoldered the KBU603, suspecting them to be bad. But the meter still read a short across the minus terminal and one side of the AC input, even with the bridge diodes disconnected. Plus, out of circuit, all four diodes in the bridge read good diode action. Nope, not the bridge.
Looked for Burn Marks on Circuit Board to Continue with Fixing Blowing Fuse in ATP5 Altec Lansing Speakers
Then, inspected the circuit board with a magnifying glass, and found 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 when cut out.
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 the internal insulator materials in the capacitor to break down, to arc, and subsequently, short out and burn. Fortunately in this case however, these capacitors were located for easy view ability and accessibility.
Removed this Capacitor
Anyway, with the leads of that charred disc capacitor snipped, the power circuits no longer read a short, and once we soldered the diodes back into place and installed a fresh fuse, the speakers once again played as they should, no longer blowing the line fuse. So don’t be afraid to use that multimeter to trace down shorts, and also, carefully look over those circuit boards visually; this can often reveal an obvious problem.
Did Not Replace It
Our research on the function of snubbing bypass capacitors in power supply circuits. revealed that they reduce diode switching noises appearing in the audio output, that occur during normal operation. So, without the right capacitor in stock, we opted not to replace it. Yet we heard little increased humming and buzzing in the speaker output. Nor did 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 after reassembly.
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- 2019-05-07: Added key word targeting for ‘Fixing Blowing Fuse in ATP5’, removed ad code scripts, and added tags.
- 2016-05-15: First posted.