If it matters, i have a Rockford/Fosgate 800a[sup]2[/sup] car amplifier with about 100,000 miles and around 3000 hours of bass on it. I think it is dead b/c the power LED no longer comes on and the subwoofers show no sign of life. I checked all the fuse, power, ground, and remote connections and everything is just as it’s always been.
Since all those are still good i think it’s a problem inside. I had a neighbor years ago check an old Sony amp for me and he found a FET that was shorted out with a multimeter. What can i check (or what should i check) with my multimeter to see what isn’t working right? Can some of the electronic whizzes here lend some advice please?
Here’s a bunch of useful info about diagnosing amps in general.
Having a diode checker on a DMM can be a big help. I always go to the output transistors first and check all pins in both directions. Then I start checking the nearby diodes.
If it appears completely dead though, you should do a voltage trace thru key parts of the system. Start at where V+ comes in and start verifying that other “hot looking” things are getting some V+ the further away you get from it.
Blackened parts and blown traces are “sometimes helpful to note.”
Strange thing is, there is no fuse in the amp at all. I took it apart yesterday and found no such thing. I had to buy a seperate fuse holder for the 4 gauge power wire, 60 amp fuse btw.
Hmmm. So it expects an inline fuse. I guess this is pretty common w/ automotive electrical hardware.
At any rate, if the LED doesn’t even illuminate, and the amp is “completely dead,” then I would first look at the power supply rails (i.e. the power supply’s DC outputs). Something in the power supply is probably cooked. But here’s the thing: even if you find out what’s wrong with the amp’s power supply, and fix it, there’s a good chance that the amp’s power supply will blow up again. This is because there’s a good chance that something else “took out” the power supply. Like a shorted output FET (as suggested by ftg). So I would also check all the output FETs (and maybe even the intermediate driver transistors) before applying power. You could also disconnect the power supply rails and see if the load is shorted.
I realize that this is completely anecdotal and please understand that I’m neither an electrican nor a mechanic:
I had very similar symptoms to what you’re describing with my car (no power to the amp, no sound). I figured the amp was blown and waited to take it in until I had a few hundred dollars (to cover the replacement costs). I ended up spending $35 : turns out that there was a blown fuse (.5 amp) on a wire labeld Remote Control that came off the head unit that had blown (to be fair, I had replaced it once, but it was improperly grounded and blew again immediately). For $35, the tech grounded the stereo and replaced the fuse (again) and it’s been good ever since (about 3 months).
It’s not a blown fuse at all. I got out my 12vdv tester and get it to light up from B+ and remote. I even directly connected the remote to the B+ and still nothing happened.
ftg, i checked that site and it is way over my head. I was hoping for something very simple that showed a picture of multimeter leads going to a FET and telling me what setting i need to look for. There’s about 32 or 34 of them that i want to check). I took my 12V tester on the board and followed the solder lines around as far as i could and they were powered, i just had to scratch a bit of the coating off to get contact though.
(Am i correct in say that a FET/MOSFET is this little 3/8" black square with 3 leads soldered to the circuit board?
I had the same problem with my Kicker 2.200. No LED, but a test light showed power from the 12v main power wire. I didn’t have a multi-meter so I don’t know how much power it actually was putting out, couldn’t have been much though, the test light was pretty dim. I checked the fuse block, 60amp fuse btw, and even though power was getting through, it wasn’t enough to power the amp. The wire at one of the fuse ends was horribly corroded and rusty. The cable had pretty much done it’s job for well over 100,000 miles in various cars and the more I cut the wire, the more it was corroded. You could practically count the rings on the inside of the wire and count it’s age! Solution: new 20 ft wire, with new fuse block and fuse. System works great now!