Thanks to Jeff Travasos for the donation of two amps to the Bose Amp Repair Free Information Project.
Note: This is just a working draft, and is by no means complete. If I can locate a schematic for these amps, or reverse-engineer a partial one, it'll be much easier to troubleshoot problems. My first impression is:
Why? Because it sounds really good? No, that can't be it. To save money? Not in the long run. Because you want to keep it stock? Okay, maybe. What breaks on these? Everything. The amps self-destruct and the speakers fade and die in the sun.
Think about this. These critters are really complicated. The circuitry is a lot smaller than the cluster. Do you really want to do this, when you can send the entire enclosure to Bose and get it completely rebuilt for $75? That's a great price. If your amp is old enough to be shot, the speaker probably is, too, and there's no chance of fixing that one yourself.
If the answer is yes, think about what you'll be left with when it works again. A crappy 50W one channel amplifier connected to a 4" speaker in a box that takes up waaay too much space. 6x9's fit in this space perfectly. I have a set of 6" 2-way components in mine and couldn't be happier. You'll need to buy an amp that'll run them, but small 100W amps (50Wx2) that interface with the Bose wiring and that fit neatly under the storage compartment or front seat or even in the enclosure of one of the rear speakers cost about $50.
Still want to fix your Bose amp? Take it out of the enclosure and smell it. Burned semiconductor junction has a smell like no other. If your amp smells burned, don't bother - you can't get all the ICs that you need to fix this problem.
Next look at the back of the board. If the enclosure leaked water and corroded the copper on the back of the board, don't bother trying to fix it. Instead save it for parts or better yet, mail it to me to include in the project.
Next, look at the electrolytic capacitors. They're the ones that look like a small aluminum can with a green or blue plastic overwrap. See any residue (other than the hot-melt glue all over these things) at the base of these? If so, replace 'em. You can find parts at Digi-Key. Chances are that whatever killed these also killed your driver FETs, but who knows, maybe you're one of the lucky ones. Plug it in and try it. If it's still not working, proceed to the next step:
Look at all those little flattened yellow cylinders at the IC end of the board (C151,C162,C163,C141,C140,C131,C122,Z121,C121). They're cheap-ass polypropylene capacitors. Replace them all with components you find at Digi-Key. Use 50V versions of each. This simple repair solves the most common problems with these amps - most of the cracking, popping, hissing noises are due to these things leaking (shorting internally). If you had a Digital MultiMeter you could remove them and check to see which had the problem, but if one's bad the others probably aren't long for this world. Digi-Key gives great quantity discounts, so you might as well order enough to do all 4 amps.
While the amp is out, you might think about resoldering all the high-current connections on the back of the board. These corrode over time and cause problems of their own that can be very similar in nature to the capacitor problem, so while you're soldering away, have a go at these. Resolder the two inductors and transformer (yellow things with red wire around them), all four FETs (the two three-legged components on either side of the inductors), all 6 wire connections to the outside world, and anything along the larger copper traces on the back side of the board.
Here's some info from readers:
Brian - I did find out the # for the power fets they are irf z44n or ir 902. Thanks, firstname.lastname@example.org.
From Bob Golden:
Bose Radio Compliant—The speakers cut off and on at times. This may require amplifier repairs or replacement. However before you spend all you dollars for amplifiers try cleaning the tone controls. The Bose audio is a unique audio system. It requires total control over any pre-amp parameter. If the tone controls are scratchy, chances are they are causing the channels to drop out. Use a cleaner that contain a lubricant and spray a small amount in each control. Work them up and down a few times to clean the oxidation off. Again make sure you use a cleaner with a lubricant. If the controls are void of lubricant they will become very difficult to operate.