CurtPalme.com Home Theater Forum

**Curt Palme**

Hey techs, I'm spending the day working on assorted PC boards, and I'm now working on some SMPS, Marquees and NECs. I have a bunch that have various MOSFETS blown that I've been putting aside, about 5 tops and bottoms of the MArquee SMPS, and have found when repairing car stereo amps that if the SMPS MOSFETS blow, there's a good chance that there's a problem with the driver chip, and putting in new MOSFETS will simply blow them again.

In the case of the Marquees or NECs having blown MOSFETs, should I automatically trace a few steps back in the driver circuits, or should I just change the MOSFETS, and risk blowing $20 of semiconductors? What's your experience?

Right now I have two boards with shorted IRF450s, and three top boards with a blown MOSFET in the standby supply circuit. I've also got other boards that blow the 3 amp fuse, and yet again another MOSFET is blown that the fuse supplies.

I've still got 10+ working LVPS here, but either want to repair my dead ones, or throw them out. 10 LVPS might last me less time than I think. PLus, I like the challenge of repairing these. Thumbs Up

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

I never assume a mosfet just blows. Or any other component for that matter. When I identify a bad component the first thing I look at is the solder joint on it. A ringed joint will definitely take it out. I also check the heat sink mount and also for excessive compound. I will then trace back each legs trace, checking both solder joints and for component failure to a point where I'm satisfied for cause.

Of course after all this I still have a little sweat on my brow and that slight adrenaline rush when powered up and have normal function.

Thats why I enjoy electronics so much. That adrenaline rush never goes away. Its there almost every time.

Of course its short lived when you hit the power and nothing but sparks and smoke occur...... Laughing

But thats fun sometimes too as long as it involves cheap parts.... Someone posted recently about tripping the breaker for the house......HA!!!!! if I had a dime for every time thats happened..... well you know what I mean..... Laughing

Nashou66 · Joined: 12 Jan 2007 Posts: 16171 Location: West Seneca NY

Curt look at this thread, Steve also listed the bunch of parts he replaced on it . i need to do the same to one of mine. all three mosfetts blew on mine too when i shorted the 390 VDC line Shocked

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https://www.curtpalme.com/forum_archived/viewtopic.php@t=18297.html

Athanasios

mp20748 · Posted: Sun Oct 25, 2009 9:00 pm Post subject:

OK, I'll have to run over to the shop later today and pick up a few parts. While there I'll get you what you need to make that problem go away.

I should have plenty of the parts for this and if so, I'll send you the parts you'll need along with the other thing I'm sending you this week.

I'll get back to you later today.

**Curt Palme** · Posted: Sun Oct 25, 2009 10:51 pm Post subject:

Thanks guys!

I just worked on an Energy Microstar amp that had an SMT electrolytic blown off the board. THese caps can do that, so I checked stuff around the cap, all but one fusible resistor checked OK. SO I powered up.

I got my +/- 15 volts where that cap was blown, so I was all excited, but got no speaker output. I flipped the 'active' switch to 'on' from 'off', and BANG! got a nice loud BANG and smokeshow about a foot away from my hand.

Talk about adrenaline! Very Happy

It looks like there was an arc between the MOSFETS and the heatsink, it probably had a pinhole in the insulator between the two, and now a whole bunch of SMT stuff is blown. I'm writing it off, as a new one from Energy is only $250.

mp20748 · Posted: Sun Oct 25, 2009 10:57 pm Post subject:

You got PM.

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

**Curt Palme** · Posted: Sun Oct 25, 2009 11:12 pm Post subject:

Thanks guys! This time I checked all the components around the blown cap and saw nothing. POS!

A whole bunch of this audio stuff is disposable. Give me 'proper' sized parts any day. Enough of this SMT crap! Smile

tse · Joined: 03 May 2006 Posts: 1014 Location: Sweatbucket, Fl.

To lower the parts count and drama you can usually insert the rail for the regulator chip from an external supply and get it running without the high power rail. Then breakout your variac and bring the main converter up a little to check for proper operation. Use an isolation transformer with the variac so you can connect scope probes in the hot side of the circuit.

Scott