Repairing a jbl 1201.1II... class D

[caraudiobum]

(moved from from the solid state amplifier forum) I have a JBL 1201.1II class D monoblock car subwoofer amplifier I bought it off of ebay 'like new' but it came fried. They refunded my money (yay) and now I'd like to try my hand at fixing it. I am a complete novice, so I might need some baby talk... Symptoms: when hooked up to power, either 1. The 'power' led is off, and the 'protect' led is solid on, or 2. The 'power' led is solid on and the 'protect' led blinks fast (so fast it never appears to reach full brightness). Some components become very hot and start to smoke. No sound comes out at all. There are no apparent burn marks, though one side of the board has been apparently wiped clean with alcohol or something (looks new). The other side has a whitish residue all over it. Is this from blown electrolytic capacitors (mosfet), perhaps? The fuses are intact (though they could have been replaced). The plastic around where the fuses attach has been melted somewhat. From this information, does it sound like it is repairable? I have the service manual (with complete schematics)- pm me with an email if you want it. It is also available online with a quick google search. Where do I begin? Thanks

[Perry Babin]

It sounds like there are shorted output transistors in the amp.

Email me if you want the service manual.
babin_perry@yahoo.com

[caraudiobum]

*googles output transistors for a while* I have the manual, but I don't know enough to tell which transistors are outputs. any obvious ways to tell that they are bad, or must they all be tested (and, apparently, removed)? How can I tell which transistors are the output transistors? I found a pic of a class A/B amp, and the output resistors were the two rows heatsinked to the case on either side of the pcb, on the side opposite the transformers. Here is a pic of my amp. the following is the class A/B pic with output transistors marked. are mine in the same positions? (large pic) (click for pic, because it is large)

[Perry Babin]

The photo is a class D amp.

The output transistors are the IRF640s and the IRF9640s.

[caraudiobum]

Thanks for the info! I checked them (in circuit...) and found one had an anomaly- a short. It is one of the IRF640N's (Q319 to be precise). I can order one and replace it... but I'm still curious. Does fixing the shorted transistor typically fix the issue (ie, protect circuit usually prevents further damage), or is there often another problem? Anything else common I should check out?

[Perry Babin]

Cut the shorted one out, clamp ALL heatsink mounted components tightly to the heatsink and power the amp up with a single 15 or 20 amp fuse. If the amp plays normally, at low levels and doesn't blow the fuse (20A max), you may only have to replace the output FETs in that group. At the very least, you need to replace all of the 640s in parallel with the one that failed but I'd suggest that you replace all in the output group (640s and 9640s) with the one that failed.

[caraudiobum]

OK, I cut out the transistor, and hooked up all the heatsinks like they are during normal use. Instead of hooking it up with a small fuse, I hooked it up to a variable dc power supply (12v @ about 2 amps). The protect light is off, and the power light became solid on. I hooked up audio input and sound comes out clear, and the gains and everything else seem to work perfectly (and did not overamp the power supply). I will order replacement transistors soon. Is that enough to tell that it is working fine, or should I hook it up in a vehicle and try it at 10 amps or so? Thank you very much for your help. Yesterday I didn't even have a clue, and today I have an amp!

[Perry Babin]

You can drive it a bit harder but not near the normal, full power output.

Also check the gate resistor for the shorted output transistor.

[caraudiobum]

ok. I tried it again under about 10-15 amps, and everything worked smoothly for a few minutes, so I'm confident now that everything will be peachy. I checked that gate resistor (and the others for good measure) and they all check out.

One more question, though, to be sure I'm understanding you. Does "output group" mean basically everything under that same heatsink clamp thing? Or do I need to replace all of the 640's and 9640's, including the ones on the opposite side (a total of 18 transistors)?

[Perry Babin]

They would likely be under the same clamp. They are the ones in the attached section of the diagram. 9 total.