Love these Altec Lansing ATP5 amplified speakers. But the other day, 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 ATTEMPTING repairs.
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 immediately upon power up.
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 bridges; a KBU603 four-diode package.
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.
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.
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.
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.
Our research on the function of snubbing bypass capacitors in power supply circuits. revealed that they are deployed to reduce potential diode switching noises appearing in the audio output, that occur during normal rectifier operation. So, without the correct capacitor in stock, we opted not to replace the burned up part. However, we observed no increased humming or buzzing in the speaker output. Nor was there any perceptible decrease in speaker sound quality or overall performance. So, we’ll leave the place for that capacitor blank.
Maybe they’ll 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.