ext. Power Brick Lenovo PA-1151-11VA pulsating output

[MiataMuc]

Hi,

this is an external power brick for a mini desktop pc.
It is rated 19.5V  / 150w.
PC didn't want to turn on; so I took this power brick home.
The attached image shows the output; it shows the voltage without anything connected to it (pulsating) and then drawing around 30mA; it breaks down and needs quite some time to recover.
I tried changing the output caps, but that didn't change anything; they did look fine.
Any idea?
I can take pics of it - just have to get the cam out of the car; but it is filled with a lot of white gunk..

Thanks,

Flo

[Grapsus]

High res pictures of both sides may help, it's not like we're going to find the schematic online.

The sudden drop and then recovery could be some kind of overcurrent security kicking in or a thermal protection on some component or a lot of other things.

Edit: Out of curiosity, how did you manage to open the brick ? They're often sealed all along the edges, do you have any trick or you just dremelized the thing ?

[MiataMuc]

I hate taking pics.. always looking horrible. Here you are :-)

I had luck.. an side wasn't glued anymore, which gave me same space to bend it.. if there is no other chance, I take a screwdriver and a hammer, but this wasn't necessary.

Edit: forgot to attach the pics.

[radioFlash]

It looks like there's a blown smd capacitor on the top left corner of the bottom side picture by the red and black wires.

[MiataMuc]

I have a look at it. I'm apparantly to blind to find that myself 

[MiataMuc]

I don't think it is connected to the Y-Cap. The Y-cap is connected between neutral and live, while this one is connected to earth and "-" after rectification.. or am I wrong?
just checked it out.

[MiataMuc]

sorry, I just confused X and Y. this cap seems to be okay..

[Grapsus]

I'm not really getting what you're saying about X and Y caps here. The burned cap is on the secondary side, it is connected between the negative output and earth ground (green wire running to the mains connector). It must be some kind of interference suppressor. Maybe the transformer isolation was breached and current from the primary is getting to the other side, but I 'm not a specialist. Does the brick work better with the burned cap removed ? Have you tried measuring the resistance between the primary and the secondary winding of the transformer ?

By the way the quality of that power supply is pretty scary. Looks like they use good brands for the components but the soldering job is piss poor and the whole design looks too simple for pumping 150 Watts...

[MiataMuc]

Just a short update: after probing around a little bit more, I decided I'd buy a replacement. The pc, for which this power brick is being used, has to work on Monday..
Just had to change the connecting cable to the pc.

Thank you all for your help! 

P.S. the replacement looks cheaper..

[NiHaoMike]

Try replacing the small electrolytic on the primary side.

[ConKbot]

Whats with all the gripes about build/soldering quality? Its a single sided board that was wave soldered with lead free solder, what all do you want?