Cerwin Vega Mobile amplifier - blown MOSFETS, please help

[euroticcustoms]

My first post on here, thanks ahead of time for any help. I've got a CVM 500.1 car audio amplifier. 500 watts RMS at 2 ohms. It recently quit working, but wasn't in protect mode, so I took it apart to see if I could find anything wrong. The problem appeared to be a clearance issue between the board and case. There was a cluster of six 1000uF 80V capacitors in the middle of the board. They were wired in parallel I believe? 3 x 2 with the 6 middle leads going to the positive output of the amplifier. Anyway, one of the legs in this circuit was touching the case, burnt the paint off and arc'ed against the case. I called Radio Shack and the biggest capacitor they had in stock was a 1000uF 50V, which I thought about putting 2 in place of this 1, but decided to fix the problem at hand and go from there. I figured either A.) all the capacitors were fried due to shorting to ground, or B.) none of the capacitors were fried as this would be like hooking the a speaker wire to ground, hoping the circuitry of the amp may have prevented it from damaging anything.

Also worth mentioning...as I was taking the amp apart, I was removing the hold-downs for the MOSFETS and one laid over AGAINST THE CASE, not the board, and a static spark arc'ed off of one of the MOSFETS. I thought nothing of it, as I didn't even know what a MOSFET was at that point lol Anyway, I (as a temporary fix) tried lining the center of the amp with electrical tape. Put it all back together - the bottom cover to the amp which nothing connects to by the way. I re-connected all wires to the amp, turned the key on, radio on and heard the subwoofer kick once, then a pop, static through the speakers, then sounded like a sparkler or a book of matches going off in the back. I looked in the trunk and those mosfets were igniting one by one. Fried 5 of 8 of the small ones DEFINITELY. I shut the key off, but the power light stayed on on the amplifier until I quickly removed the ground wire. Still wasn't in protect mode, no blown fuses.

I'm an auto technician, decent with automotive electrical. I've seen the inside of a lot of electronics, but I know enough about them to get myself in trouble. The board APPEARS to be alright, as do all the other components on the board (capacitors, resistors, transistors, etc) none are VISIBLY damaged, no arc'ing.

What are the chances that a static arc onto a mosfet would cause 5 in all to self destruct? I can get the ones I need for cheap enough, just wondering if it would be worth the time/money? Or would there be an underlying issue that would cause the FET to go up in smoke?

I know how to solder and use a multimeter. Learn really fast. A push in the right direction would be great. Thanks again.

 

[Sceadwian]

What are the chances that a static arc onto a mosfet would cause 5 in all to self destruct?

If it hit the gate, it's virtually garunteed. The maximum gate voltage of most fets is only in the 20 volt range, you don't even have to be able to feel the spark for it to be dead, if you saw one, garunteed you fried it. If anything like that glowed and smoked as you described it sounds like it was a cascading failure, where one little thing took out the rest, good chance half the components on that board are now useless.

Cut your losses and scrap it.

 

[bountyhunter]

Never replace any capacitor with one which is rated for lower voltage. BTW, high power amps that run off 12V systems have built in switchers to generate a high bias voltage, hence why your amp needs 80V caps. Also, it they are in the power supply, they may need to be low-ESR switcher caps, not generic Radio Shack junk electrolytic caps.

As stated above, arcing one of the FETs down and blowing a bunch could have done massive damage. Without a schematic, no way to know.

 

[Tann-44]

@euroticcustoms, most car amplifier's work on pwm psu's or dc to dc converters, in short, 12 vdc from the battery in and the psu splits it to +/- dc for the main pr-amp and power amp circuit's.. the mosfet's with the psu act a switching plus there is a protection circuit in case the main amp goes faulty.. the front panel will show ready/protect bi-led that light's red at power on then turns green for ok...

sorting this mess out could cost more money if your not sure what to do and asking questions like...what bit to replace may not get it working...sometimes it's best to replace amp and save time/effort/money.. or you might be lucky with it..

start by checking for shorted fet's with in the psu..there is a pwm ic in there aswel...oh the o/p devices are bog standard npn/pnp transistors and not fet's...these have base...collector..emitter pins ie left(base pin) (collector pin middle)(right emitter pin) plus driver transistors etc...


to aid you get a schematic of a car amp to get and idea...