Background: I purchased a home theater system back in the 90s. The first subwoofer was a Definative Technology unit. It lasted about 18 months before failing. It was repaired under factory warentee. 18 months later, it failed again. It was just out of warentee, but Definative Technology was nice enough to repair it again. Sure enough, 18 months later it failed again. I made a quick attempt to repair, but when I didn’t see any obvious failure, gave up on it, and gave it away.
It was replaced was a slightly larger Outlaw Audio subwoofer, a few years back. A couple of weeks ago, the amp on this replacement failed with a blown fuse. Initially, I replaced the fuse, power came on momentarily, but the fuse soon blew, again. I decided to see what the Amp looked like, to see if there was any obvious component replacement that would fix it. Disassembly was easy, and I found the Amp was made up of three boards. There were no obvious component failures on any of the boards.
Since it was blowing fuses, I guessed that the problem was on the first board, which was a switch mode power supply, with Indigo and Bash logos. All components were through hole, with no custom parts, so I figured it would be repairable. Testing components with a diode tester and ohm meter revealed only possible issues with one of the two power FETs. I decided to order some replacements.
After the new FETs arrived, I replaced the suspect one and put everything back together. To my dismay, but not surprise, the fuse blew again. Clearly some other problem caused the FET to blow. I now devised a primitive ESR tester, and checked the caps, but they all tested as good. Continuing my investigation, I took a control daughterboard off the main board and tested each component, some by pulling, so I could test out of circuit. In particular there is a part type that I never heard of, before. This is a DIAC. I pulled it, found the specs, and devised a test fixture and it also looked good. Finally the lone transistor on the daughter card broke off from it’s leads. This was very strange as the daughter card did not experience any especially rough handling. Could it have been already broken from the stress of operation in the subwoofer environment, and my light handling just finished the job?
Since I could find no other problems, I figured I’d replace it and the two power FETs and see what happened. After replacement and reassembly, so far the subwoofer is performing like new, so I think that the problem was vibration induced failure of the MPSa92 transistor on the power supply daughter card.
Since my experience with subwoofer amps is that they have very poor reliability, I am very happy to have found a way to fix this one, at least this one time.