Why did my MOSFET blow up?

[ohren]

I had a switching transistor and PWM IC blow up as part of a repair, where a big chunk of each went missing. Now I would like to understand the likely turn of events, and maybe determine if I did something stupid. If nothing else, I should at least extract some lessons from it.

"IC10" can be found on page 169 in the service manual:

https://www.manualslib.com/download/...n-Mas-101.html

The entirety of the repair is basically:

1. Device doesn't power on (IC10 supply voltage unstable)
2. Replaced C18 and C66
3. Device works pretty well for quite some time
4. I discover an intermittent issue (unstable IC10 supply voltage on power on resulting in the device not powering on, like in #1) that appears only after the device has been left unplugged for a long time
5. Replaced bad C60 — better but didn't remove the issue completely
6. I replace C18 again, assuming the cap used last time wasn't good or had degraded (I salvage parts; ordering new is unpoportionally expensive while experimenting and learning and not being confident in where the faults are)
7. IC10 and Q14 blow up on power on, as in a loud bang and a bright flash (and D9, R98, and R28 also fail). Both components have bits blown out of them. Q14 measures shorted, IC10 not shorted.


Would any failure mode of C18 have these results? I can't put my finger on what went wrong. I didn't install the cap backwards.

Any educated guess as to whether IC10 or Q14 failed first? Or maybe something else?

The repair thread can be found at https://www.badcaps.net/forum/showth...=102928&page=2 where I also question why they seem to have designed this thing so that V_LUVP of IC10 always exceeds its absolute max value. I don't know if I just made some error in my calculation (shown in link), but then I must've also made a mistake when measuring the same thing...

Oh and lastly, my level is: hobby amateur teaching myself through repairs

[CapLeaker]

Re: Why did my MOSFET blow up?

I think the Mosfet blew up first dumping full power to the gate drive and the the gate drive IC blowing that to bits and sh*it too. There could be various reasons for this. The link does not work for me, so I can’t look at the circuit you are describing.

[ohren]

Re: Why did my MOSFET blow up?

Pasting did not preserve the link target, sorry:

https://www.manualslib.com/download/...n-Mas-101.html

[harp]

Re: Why did my MOSFET blow up?

I dont catch up whats going on with your repair, but it is always good idea to use some current limiter in testing any new condition on pcb you make, aka light bulb limiter. After repair I always detailed cleaned flux to better see repaired area, and do some general measure with dmm before plug in into the wall.

When packages blown apart, that means the circuit is using much of current in way to not supose to.

[ohren]

Re: Why did my MOSFET blow up?

I think the Mosfet blew up first dumping full power to the gate drive

It shorted drain to source. Does that bear any relevance to what you're talking about?

[CapLeaker]

Re: Why did my MOSFET blow up?

If any source or drain pins were shorted to the gate pin then the mosfet blew first and took the gate drive IC with it.

[ohren]

Re: Why did my MOSFET blow up?

They did not, unless they subsequently failed open.

[CapLeaker]

Re: Why did my MOSFET blow up?

Something is wrong, because D9 at the gate drive did get damaged, but you are saying that the mosfet drain or source pin isn’t shorted to the gate? I still think the Mosfet failed first.

[ohren]

Re: Why did my MOSFET blow up?

Yes, exactly, I've triple checked. Drain and source are not shorted to the gate. It might have been, though — here is a photo. A piece is missing nearest the gate pin.