Zanussi dryer blew lnk304 TWICE!

[gabri.peic]

Hi everyone. I am trying to help a friend that has a zanussi dryer. The unit is about 12 years old and worked fine until this year.
Around  3 months ago it was repaired by a technician who offered to repair the pcb. It worked fine until now, So this time the friend decided to call me since i do electronics repair as a hobby and have quite a bit of experience.
After taking the pcb out i found that the lnk304 that was already replaced blew again !
I can see that it was replaced by the sloppy but ok soldering job.
I checked the other components and cant find anything shorted or open, Even the capacitors check good esr and value.
Could it be a bad transformer ?
Or maybe the "tech" replaced the ic with a second hand part that was on its way out ?
So can anyone recommend what should be checked before i replace the components, I don't want it to blow up again.

[amyk]

Power surge. Those single-chip SMPSs are known to be quite fragile to them.

[gabri.peic]

But why would it blow a second time ?
There were not any lighting storms or power problems in the area.
As i said it was replaced 3 months ago and it is gone again now.

[gabri.peic]

I am wondering if it could be a bad emi filter ?
Is it possible that an emi filter failed and is not filtering voltage spikes when the motor starts ?
Also i noticed some scratches on the replaced lnk304 so maybe the tech replaced it with a used part that was alrady degraded.
I just dont want my friend paying for the parts just to have them fail again.

[Swake]

Many many possible reasons for that thing to fail. Used damaged part, fake part or wrong part. Or something else that makes it fail only under certain conditions or stress conditions.
A surge is something invisible. No need for a thunderstorm or other machines failing for a surge to occur.
A combination of factors might be the cause of the failure.
Ensure the type currently on the board was the original part, eventually look for an equivalent but 'stronger' part.

[coppice]

Look for any degrading capacitors that might lead to overstressing the chip.

[floobydust]

What model is the dryer? Pics of the board? A dozen things can cause the IC to blow. I think the PSU is running 24/7.
I would replace the electrolytic capacitors - you say they test OK and ESR is good, but make sure. Use a real LNK304, not a cheap knockoff.

[gabri.peic]

Thanks for the suggestions, I did some research and testing on the components.
I will post all of that tomorrow since it is almost  2 am over here so i need to get some sleep.
I just didn't have enough time today so see you tomorrow.

[gabri.peic]

Ok so i found a few interesting things so far.
First of all the psu is not on 24/7 the selector switch has a zero position that kills the power to the whole board.
I did some testing and found that  x2 across the line capacitor lost 90% of its capacity.
Other than that everything seems ok.
I am really suspecting that the tech used either a bad part or counterfit part.
My friend wants me to try repairing it so i will probably replace the across the line capacitor, mov and maybe capacitors and some other stressed parts along with a fusible resistor and the ic.
Here are some pictures of the pcb, model number of the pcb and the bad parts i found.

[gabri.peic]

I will post rest of the pictures later, i am having some trouble uploading...
But the model of the board is electrolux 0102_00048

[gabri.peic]

Here is the rest.

[Alti]

The unit is about 12 years old and worked fine until this year.

Maybe a technician replaced LNK304 but has not fixed the root cause of the problem. Also, it is 12 years old electronics so solder joint cracks wouldn't surprised me. Especially on bigger THT components that undergo multiple thermal cycles.

[mariush]

You'll probably want to re-solder joints of big components like relays, or whatever looks visually bad.  It's a dryer, I assume it spins at fast rpm, so you may also have vibrations, so you may have cracked solder joints or weak solder joints.
Check output capacitors, check diodes, replace the IC ... not much else you can do.

Alternatively, you could just figure out the output voltage (probably 12v because of the mechanical relays) and replace the whole thing with a classic transformer and a linear regulator, or a pre-made 12v power supply. It's not like you really care about the efficiency when you're washing so what's a couple watts wasted while washing, so even a classic transformer would work ... pcb transformers are relatively cheap, under 5-10 eur.

[coppice]

It's a dryer, I assume it spins at fast rpm, so you may also have vibrations, so you may have cracked solder joints or weak solder joints.

What kind of dryer spins at high speed? They are usually slow tumbling, low stress machines that function properly for many years, unless the life of something get cut short by the heat. Like, a capacitor, maybe.

[floobydust]

Zanussi tumble dryer, Electrolux 0102_0004B, PCB 712529-02 or Rev. 1 is -01.
Most of the soldering is shit for the through-hole parts. I'd touch up any fractured/rings that are visible with solder that has decent flux. The relays and jumpers look the worst. Then clean it with IPA and Q-tips.
R53 must be a fusible, flameproof type. Why is there black soot on the backside S14 of this part? Did it arc to something like the housing? That seems strange.
There is no surge protection for the LNK304 so it is vulnerable. The 10uF primary cap I would test as well.

[gabri.peic]

Thanks for the suggestions.
The soot marks are because as far as i can see the previous tech replaced the resistor with a regular metal film one rather than a proper fusible resistor... There is even a dark spot on the other side of the board where the resistor was so it probably got hot. The board was really black until i cleaned it off.

[gabri.peic]

The "soot mark" around s14 is actually heat damage. The board is a bit discolored but looks fine.

[gabri.peic]

Ok the pcb is repaired.
It seems to power up normaly on my bench.
Of course it is throwing up error codes because there is no sensors connected but it seems to work.
But is it normal that it produces a slight whining  noise. The noise is coming from the power supply area. I think it is normal since i heard that from some other switchers but i was just wondering if that is normal for lnk304

[floobydust]

Did you test the electrolytic capacitors? You replaced the low value X2 cap?

[gabri.peic]

All the electrolytic capacitors test the right values and really low esr well below 1 ohm.
The only electrolytic capacitor that is around 2 ohms is the 400v one at the input but that is normal since it is higher voltage.
The x2 capacitor has also been replaced.

[floobydust]

Fingers crossed it works for a while. Replacement boards are crazy expensive.
I wish it had an MOV to protect the LN304 against mains transients, or the other guy used a knock-off part.

[shakalnokturn]

They do fail easily, I'm sure the degraded X2 EMI filter capacitor didn't help.
If you look at the LNK304 application note in the datasheet there's a series inductor on the input side, that can help suppress transients, here they've cut the costs to a minimum.

The source of destructive transients could be poor contacts on a high current (heater) or inductive (motor) load in the dryer itself or external such as plug /socket or just about anything else on the mains.

[gabri.peic]

Well i hope it lasts now.
I will check some connections around the machine just incase.
I already warned my friend that there is a possibility that it fails again, but he wanted to try anyway.
If it fails again he is ready to get a new dryer.