Akasa 400W atx 12v - magic smoke escape

[Rogue Pony]

Hi all, and apologies for any faux pas I make in this thread.

I've an Akasa branded PSU (AK-P040FG7, 400W ATX 12v with PFC) I'd like to revive, but it's currently deader than a dead thing.

Was installed in an old system I was given, about 2006 vintage, which had been stored in a stable for a while (without horse). On switch on, video display was showing digital noise, and then was interesting spark from rear of PSU and system shut down. Usual checks proved PSU had died (trial with replacement PSU showed that the system was ok). There was a lot of dust on the video card surface, so I wonder if slight dampness was causing shorting and a) caused the video artefacts and b) shorted/loaded the card power circuit and thus the PSU.

Digging into the dead PSU found an obvious burn by a ceramic (fuse?) resistor (R95, I think) and a not so obvious burned out SMS resistor (R97) on the track side. No 5vsb, though fans on a connected mobo will twitch when power button is first pressed. No obvious bad caps, but I only eyeballed so far.

I have found that these Akasa supplies were badged "Enhance" units, but not much more than that. I hope that replacing the resistor and SMS resistor will bring some sort of life back so I can check outputs. What I need are values for them - the ceramic res should be easy for anyone who had normal colour vision (guess who hasn't :P ), but the sms device is a bit trickier. Suggestions please people

Pics attached, some overall views and then closeups of the ex-resistors.

Thanks for any input on this, even if caps are not yet involved ;P

[Behemot]

Re: Akasa 400W atx 12v - magic smoke escape

The small IC is NCP1200D60R2 SB PWM controller http://pdf1.alldatasheet.com/datashe...1200D60R2.html and the resistor R97 is fusible for the HV input which burns deliberately if something goes wrong, it did not go bad just on its own. Dtto the R95, seems liek fusible MOX for the nearby transistor? Something went bad, probably the whole chip or something around it, definitelly do NOT connect it to power just like it is even when you'll think you've repaired it - use lightbulb in series as protection load.

Check that TO-220F SB MOSFET and also the ZD7 and D11.

[Rogue Pony]

Re: Akasa 400W atx 12v - magic smoke escape

Thanks Behemot
I'll have a go at checking the MOSFET and other items as you suggest. The datasheet for the PWM chip shows an example circuit that matches the PSU layout, so I just need to work some math to come up with a value for R97. R95 does indeed feed the MOSFET, a ST branded P3NK90ZFP. While checking this, I've noted another casualty, R96, from the junction of R95 to the MOSFET gate, which looks to feed into a resistor network tied up with pin 3, current sense, of the PWM device. I'll check this section also and feedback what (I think) goes on there.

[Behemot]

Re: Akasa 400W atx 12v - magic smoke escape

It is quite often manufacturers just follow reference designs for many different controllers and pack it together as one PSU. No engineering really in the mainstream. Good for us to repair than

[Rogue Pony]

Re: Akasa 400W atx 12v - magic smoke escape

Well, I pulled the FET and following a basic test it looks to be shorted across all pins, which would have put the full voltage from the source pin into the drive circuit on the gate pin, explaining why the SM resistor in that line looks to have been overheated. That the resistor that was limiting the driver chip's power current is blown suggests that the chip has fried and shorted, followed by the main resistor in the FET's source circuit blowing before things got really toasty...

Though I can see a way forward to repair this, I'm going to admit that it probably isn't really viable/economic

Still, it will give me some parts for the spares box, and has taught me a little about SMPS basics

Thanks for the help, Behemot