My laptop AC adapter stopped working and I don't know where is the problem

Thread Starter

rambomhtri

Joined Nov 9, 2015
388
Hi, my laptop AC adapter stopped working suddenly. I heard no "POP" neither saw any smoke, so I proceeded to measure things hoping to fix it quickly (probably the cable). The input cord carries those 230V just fine, so there is no problem with that one. However, I measured the output 19.5V DC and I was reading just a couple of hundred mV DC. So I thought it was just the cable, but I was wrong. I have just opened and disassembled the adapter and the output cable is perfectly fine, there's continuity all along from the soldered connection to the end pin, both outer negative and inside positive.

I've been trying to spot a damaged cap, no luck, everything seems to be fine. I see no black spots, no explosion marks, no burnt areas, no corrosion, no dirt or dirty connections... So I am running out of ideas. I am gonna share a picture of the back plate and front plate of the disassembled adapter, as well as a video with my macro lens so you can "navigate" all along the different parts. In the upper right corner and bottom left corner you can see 2 missing soldered holes. That's the shield/earth, the metal case that's covering everything.

These cost around $40-50, so it is a huge success if I detect the problem and fix it replacing something for $3. And I basically do it because I love to fix things instead of replace them, unless the replacement is notably more efficient and way faster/better. Not the case.

back.jpg

front.jpg

Video: https://streamable.com/yttu3y

Oh, by the way, the adapter has a blue LED, soldered next to the output cable, and it lights up when plugged, so it is really weird. I though that the blue light was an indicator that told us the there were 20V DC reaching the output cable. It could totally be simply that there are 120/230V in the input cable, which would be way less helpful to diagnose these kind of things. I can tell you the values of some cap or anything you ask.

Also... how do you safely discharge all the caps and possible devices that may store energy and are able to give it very quickly (dangerous to the touch)?
If it is one by one, using a resistor and knowing which pins you should touch... what if it was a board with dozens of them and you don't want to go one by one?
 
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Thread Starter

rambomhtri

Joined Nov 9, 2015
388
I see a few bad solder joints. The LED lights up only when the input is plugged in or else?
Thanks!

Where do you see bad solder joints?
These have not been touched, so it's very rare if they don't work because there's a bad connection. In all the years repairing stuff, a bad solder join that makes a device stop working creates some black/burnt stuff somewhere, specially in power related devices. It could be, point them out and I will check them or upload closer pictures, but I guess it's just a component that stopped working internally, without leaving any trace. A cap that didn't explode or swell.

The blue LED lights up whenever you plug it in to the outlet, of course. There's something going on that is keeping the 20V from reaching the output cable pins. I don't really know how to troubleshoot this anymore... I can only desolder components and measure, but... it would take hours and hours, it would be blindly because I see no damaged parts, besides some components are stuck in the solid white-yellow "glue", such as big caps.

I have not tried to measure voltage with the adapter powered, I am scared that I am literally in front, a few inches away with my face and hands, of something that is protected by metal plates, solid glue and a big chunk of plastic case. I guess the only danger would be a cap exploding, but I have never experienced that in real life, and I don't want to like this, with no control.
 
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anniel747

Joined Oct 18, 2020
1,063
Thanks!

Where do you see bad solder joints?
These have not been touched, so it's very rare if they don't work because there's a bad connection. In all the years repairing stuff, a bad solder join that makes a device stop working creates some black/burnt stuff somewhere, specially in power related devices. It could be, point them out and I will check them or upload closer pictures, but I guess it's just a component that stopped working internally, without leaving any trace. A cap that didn't explode or swell.

The blue LED lights up whenever you plug it in to the outlet, of course. There's something going on that is keeping the 20V from reaching the output cable pins. I don't really know how to troubleshoot this anymore... I can only desolder components and measure, but... it would take hours and hours, it would be blindly because I see no damaged parts, besides some components are stuck in the solid white-yellow "glue", such as big caps.

I have not tried to measure voltage with the adapter powered, I am scared that I am literally in front, a few inches away with my face and hands, of something that is protected by metal plates, solid glue and a big chunk of plastic case. I guess the only danger would be a cap exploding, but I have never experienced that in real life, and I don't want to like this, with no control.
Exploding semiconductors are also fun.
 

Delta Prime

Joined Nov 15, 2019
1,014
Hello there! :)
With the unit unplugged disconnected from outlet.
1)450 volt capacitors short the pins with a 5 Watt one kilo ohm or higher resistor value, may spark.
2) T3. 15A 250V slow blow fuse check continuity beat good, no beep no good. Replace.
3)U1 acquired data sheet take note of pin out, power pins. Stop! report back!
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
388
Exploding semiconductors are also fun.
Uh? Never heard of that. I though they would burn, not explode. How does that happen?

Is this related to your tripping breaker?
No, not at all, that tripping breaker post is from a friend's house, and I guess that even after I switched off the hot wire, the neutral was still connected to all the neutral circuit in the house, and basically to the breakers. When the tip of my iron, which is earthed, touched the neutral, that triggered the breaker. I still don't know if it was because there was a tiny current going on, or if the moment you short earth and neutral, even in my case if there were no current at all, it still triggers the breaker.

This post is about a power adapter suddenly stopping working, and how can I fix it.

Hello there! :)
With the unit unplugged disconnected from outlet.
1)450 volt capacitors short the pins with a 5 Watt one kilo ohm or higher resistor value, may spark.
2) T3. 15A 250V slow blow fuse check continuity beat good, no beep no good. Replace.
3)U1 acquired data sheet take note of pin out, power pins. Stop! report back!
Hi!
Where is the 450V cap? Oh the blue one?
The brown one is 420V. How do I know what's inside the yellow-white stuff, by the way?
Anyway, both brown and blue are discharged, the T3 is working fine (no resistance).

I don't understand number 3. What is U1? A voltage?
 
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anniel747

Joined Oct 18, 2020
1,063
Uh? Never heard of that. I though they would burn, not explode. How does that happen?
I see them all the time the top of the casing blows out leaving a crater. I have a IPM on the bench right now that you can see the 3 MOSFET dies. Pressure buildup following a shot circuit is usually the culprit.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
388
I see them all the time the top of the casing blows out leaving a crater. I have a IPM on the bench right now that you can see the 3 MOSFET dies. Pressure buildup following a shot circuit is usually the culprit.
Can you point me to a video?
I have see videos of capacitors exploding, and batteries, but nothing else. I wanna see other accidents with other components to know what's normal and what to expect.
 

Delta Prime

Joined Nov 15, 2019
1,014
My mistake! let me put it in another way. Any electrolytic capacitor you find in a laptop power supply above 35 volts, for safety while servicing the unit,it's good practice to discharge stored energy in the capacitor. When dealing with capacitors always consider them charged always handle them by the sides of the capacitor.
 

anniel747

Joined Oct 18, 2020
1,063
My mistake! let me put it in another way. Any electrolytic capacitor you find in a laptop power supply above 35 volts, for safety while servicing the unit,it's good practice to discharge stored energy in the capacitor. When dealing with capacitors always consider them charged always handle them by the sides of the capacitor.
I see a lot of field technicians that blow the tips of their probes with caps.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
388
My mistake! let me put it in another way. Any electrolytic capacitor you find in a laptop power supply above 35 volts, for safety while servicing the unit,it's good practice to discharge stored energy in the capacitor. When dealing with capacitors always consider them charged always handle them by the sides of the capacitor.
Yeah, I've done what you said, now what do you suggest?
Everything seems to be working so far... could it be the transformer coil that got broken and there's no secondary coiling continuity and that's why there is no 20V DC?
I don't know...
 

anniel747

Joined Oct 18, 2020
1,063
Yeah, I've done what you said, now what do you suggest?
Everything seems to be working so far... could it be the transformer coil that got broken and there's no secondary coiling continuity and that's why there is no 20V DC?
I don't know...
SMPS transformers are extremely rarely defective.
 

Delta Prime

Joined Nov 15, 2019
1,014
I see a lot of field technicians that blow the tips of their probes with caps
I have said nothing about probes I am not a field technician. You have to make a decision would you like assistance in troubleshooting an external power supply to a laptop? there is a logical process in the procedure itself, one must possess a command of electrical theory so let's cut the chit chat shall we?
 
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