SMPS Blowing MOSFET - Nad Audio Amplifier C388

[Nerox]

Hi Everyone,

I'm trying to fix a Switched mode power supply from a audio power amplifier - Nad C388 [Service manual pdf schematics is below  Nad C388 Blowing mosfet.pdf (108.61 kB - downloaded 201 times.)]

I bought it used, as a "to be repaired" project. To get back to Electronics  .
- It had the main fuse (the one in the IEC inlet) blown
- The second one FUSE101 too,
- the inrush NTC current limiter, TH101, physically destroyed (But no sign of overheating, though)
- mosfet Q116, Q117 shorted
-  R161,162,199 where blown - The resistors literally exploded.  These are load resistor (1W each)


I .. replaced Q116,117 and the resistors, thinking it could.. be a fix.

Q116&117 blown/fume when  connected back to the circuit. (they are now shorted) .  Amp powered "fine" for a few second, stating that " DC Ok, AC Error" ,which is almost a good sign

My question is :
What could make these Power Mosfet Blew? What should I look after?

[Algoma]

Those are supposed to turn on alternately, Pushing and Pulling arrangement with the signal driving them.

if they both get turned on at the same time. the smoke is released with almost a short to ground through the pair, look at what is driving them.

Especially those diodes, if one has failed short and is letting current flow when its not supposed to.. Use a diode check on a multimeter

That whole area of the circuit, if any one part fails, that whole thing goes bang, and takes the rest with it. Only that NTC was protecting it all.


Looks like you could leave Q116 out for testing and checking signals.

[JayArr]

For the price of components compared to labour we always replace everything. All of those diodes and the two driver transistors for sure. When all of the semis are out of the PCB we will check the resistors in place.

Then we reassemble everything but the power Fets and we find a way to turn it on to send signals to the Fets. We look on the scope and make sure we have nice clean square signals with no DC bias and when that checks out we put the power Fets back in and 99/100 work.

[Nerox]

Hi,

First of all, thanks a lot for the tips I've replaced
-D108,D109,D110 D111
- Q110 Q111
- Desoldered the Caps on the primary to check any  for short circuit/Open -> Nothing
- Desoldered Transformers to Check shorts ( T102 & T105) - Nothing

I Wasn't (yet?) able to desolder Tr101 because it has a lot of pins. So far i didn't see any short circuit.

Yet, i  don't have any  I still don't have any voltage at the outputs V+ & V- . Which are supposed to be +/-42 V

I've read that it's normal for a SMPS to have no voltage without any load? [Buuut.. i'm using only a multimeter atm..)

I've done my "basic troubleshooting" with an incandescent bulb in-line with the AC Mains, and the  lamp only glow when i plug the SMPS in, which is normal. There is no short-circuit otherwise it would glow constantly (from what i've learned)

For reference the error in the amp was that

What could trigger this " AC error"?
I think it might be related to  the IC 800 which is responsible for the "soft start" feature if I understand it correctly.

[Pete66]

You probably need to see whats coming out of the mosfet gate driver IC - Out A and Out B.  You will need to scope it however, make sure you use an isolation transformer.  Do not attempt it without an isolation transformer.

The best bet is to just replace the driver IC it its probably the issue.