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lazzer408 14th August 2012 05:34 AM

Klipsch Subwoofer issues
I bought this sub at a garage sale for $20 "as-is" (broken) and the story goes like this...

The original owner said it stopped working and he gave it to his friend to fix. His friend said he had it working again but it still had issues. He gave it back and it went out for the garage sale. It's powered by a 200w BASH amplifier.

What it's doing is this... It will sometimes power up and play but when something loud "thumps" it goes into oscillation and sounds like 2-bit digital audio. A squeel that fluctuates with the music signal. The volume control still functions when the problem is happening. It's mostly SMT on a dual-layer PCB and following traces is a pain because they keep switching layers.

What I noticed are two regulators that have been replaced, a 7812 and a 7912 but what's interesting is the supply to pin-1 of each regulator is +28v. I'm placing my dvm's negitive on pin-2 for testing. I would think the 7912 should be on a negitive rail but it's not. The output of the 7815 is +14.95v. The output of the 7912 is +13v and unstable. Interestingly, if you subtract the regulators rated output voltage (15v) from the supply voltage (28v) you get 13v which is what it's putting out on pin-3. Almost like it's working backwards. Maybe because it's on the wrong rail.

I wonder if the "tech" accidently put in a 7915 where a 7815 should have been. Considering the voltages I'm measuring, I'm tempted to drop in a 7815 and try it.

Any reason I shouldn't?

EDIT - nevermind I think thats ok. I forgot the negitive regulators swap pins 1&2 vs. the positive regulators. Positive regulators are 1in 2gnd 3out. Negitive ones are 1ground 2in 3out. In which case the 7915 is outputting 14.93v.

Guess I have to look for other issues.

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