Audio power supply repair

[tennisballg]

Alright, my buddy brought me an amp of his that wouldn't work. I opened it up, and one of the mosfets had exploded, and ALL of the gate resistors were charred. Couldn't quite see anything else wrong with it, so I replaced these parts, as well as the ocillator chip, checked the waveforms, and tested it for a few hours on my bench supply, which is 12V from a PC supply. Everything worked great.

I gave it back to him, and almost immediately the same thing happens. It's not as bad this time, but 2 of the 4 gate resistors are fried, and I haven't torn into it, but even though the damage isnt so bad, I suspect the problem is the same. This is really bugging me! Any ideas as to what could cause this? By the way, the ocillator chip looks fine, not fried at all, haven't checked to see if it works though.

[tennisballg]

Well, the ocillator/3525 works fine. three of the four mosfets are shorted from gate to drain. I increased the resistance on the gates from 47ohm to 55 ohm, which is probably why the damage isn't so bad this time well, at least things aren't looking as charred, and the mosfets didn't actually explode.

Could this be a problem with his car power system? Everything else in his car works fine, and the amp works fine for 10-20 min. There was some whirring alternator noise, but we filtered the RCA line input and it was fine after that.

If the gate/drain connection is blowing, is that an issue with the gate pulses or the drain to source power? In my mind it could be either, I suppose. I just don't understand why everything looks beautiful on the bench, then I hook it up in his car and it blows.

[Grucho]

Hi.

Well few years ago my friend have similar problems.

Every time he turn volume to high mosfets and sometimes sg3525 got fried.
After couple of mosfets and sg3525 we figure it that power supply cable to amp was too thiny, after replacing it with bigger cable everything was OK.
I dont know why mosfets and chip got fried with thiny cable ???
(it was some cheap amp)


BTW: what kinda filter you used to remove noise ?

G.

[maylar]

Seems like the FET's are shorting out under load. You need to monitor the switching voltage across them with a 'scope while running the amp. Maybe there's a snubber network that's burned out. It could also be heat related... is the amp cooled sufficiently in it's location?

[tennisballg]

yes, the mosfets don't heat up significantly in his car, at least not while I'm there! i did find that the inductor that filters the +12V was shorted out, so it was basically ´just a jumper. Would this have such a bad effect though, its only 100 uH or so.

[tennisballg]

Umm...what if the tabs on the mosfets got shorted to the chassis? I would think that this would blow the inline fuse, not just fry all the gate resistors.

[tennisballg]

Well, I repaired it again, made sure the tabs weren't shorting to the chassis, it worked great on the bench again, and it blew after about half an hour in the car again. could it be an issue of using too low of a gate resistor? the first ones were charred beyond recognition, so I threw in some 47 ohm resistors.

The funny thing is that the amp doesn't blow the fuse, and the mosfets don't even get warm on the bench at 10A, so I don't think it's a heat issue.

[misterresistor]

Throw it away. Or fix it then immediately sell it. The new Alpine amplifiers are just horrible to repair. I can't figure out why..?

[Immo_G]

I would have given up by this point

Anyway, sounds like something is borderline in the amp, and the cars voltage is pushing it over the top. You could run it connected in the car, but open casing (depending on rail voltages) and make measurements with it running carefully.

[tennisballg]

Well, he wanted to sell it, so it would be nice to make sure it works first. i am wondering if there isn't something causing it in his car power system...maybe power ripple, or some random spikes caused by a corroded connection somewhere. I actually didn't look under the hood.

I was wondering if it would be a good test to place a 15V zener diode in parallel with the amp. Then if the voltage in his system is doing what it should, nothing should happen to the zener. If something funny IS happening, the zener will regulate, and the worst case would be that the zener fries, but at least then we would know that it was his charging system and not the amp. Does anyone think this would be a good idea to try?