Dell laptop LA90PE0-01 charger repair

[Dannyx]

Good day folks. This may seem like a pointless thing to try and repair, but I've got a dead original DELL slim charger which I'm trying to fix and I figured it might be something others have stumbled upon, given that DELL is a fairly popular brand which others are likely to own. The model is LA90PE0-01 and the issue is simple: it doesn't output anything at all. I took it apart and after hours of fighting with that white silastic sh!t they felt the need to "paint" the whole board with, I eventually uncovered the board enough to take some measurement: I get around 325v on the main cap and 15 to 20v on pin 1 of the IC (goes up and down). This is where it gets slightly complicated and over my head: the IC is a LTA804N which controls BOTH the PFC circuit and the main switching one. I could not find too much about this thing, however I did find one source which says it's identical to TEA1751 (datasheet here). I was also lucky enough to find the schematic for the whole adapter here. Following along with the schematic and the pinout as best as I could, I do get some volts on pin 1 VCC of the IC, but nothing else elsewhere...I don't know if it's even pulsing those FETs on and off, because I don't have an oscilloscope to measure the gates The overcomplicated nature of that IC is also annoying. Resistors seem fine, except for one of the 1504 (1.5 megOhm, bottom left of schematic, which seems to be reading too low - only 800kohms). Already replaced C054, since it's in parallel with the VCC pin of the IC which sometimes causes issues, but it didn't change anything....any help would be greatly appreciated

[Dannyx]

Re: Dell laptop LA90PE0-01 charger repair

Also, I've just read that some other guy (from Poland I believe) was having the same issue, but when he tried probing the primary of the transformer (the one in the top right in my pictures), the power suddenly came on, so it could also be a bad joint somewhere which perhaps makes contact when he applied slight pressure with the probes.....haven't tried reflowing the thing yet....that would be one hell of a fix

[cpt.charlie]

Re: Dell laptop LA90PE0-01 charger repair

Do you have HV at pin 16?

[Dannyx]

Re: Dell laptop LA90PE0-01 charger repair

Do you have HV at pin 16?

I get 326v on pin 16

[Dannyx]

Re: Dell laptop LA90PE0-01 charger repair

Also, I get 3.5v on pin 7 which is the VINSENSE pin. According to the datasheet for the TEA chip, that should be enough to start it.

[CapLeaker]

Re: Dell laptop LA90PE0-01 charger repair

Resistors when they fail don't go 50% lower in value. They always go higher or open. So put that 1.5 M Ohm resistor back in. Could very well be that there is a bad solder joint and power isn't getting there where it is needed. Had bad solder joints on transformers before. I've also seen my share of a broken leg (broken flush on the top side of PCB) filter inductors even so they were glued. You would have to examen things.

You said you do get some volts on pin one of the TEA1751. What exactly do you get? Look at the schematic. There is a startup cap 35V 47uF.

[Dannyx]

Re: Dell laptop LA90PE0-01 charger repair

Resistors when they fail don't go 50% lower in value. They always go higher or open. So put that 1.5 M Ohm resistor back in. Could very well be that there is a bad solder joint and power isn't getting there where it is needed. Had bad solder joints on transformers before. I've also seen my share of a broken leg (broken flush on the top side of PCB) filter inductors even so they were glued. You would have to examen things.

You said you do get some volts on pin one of the TEA1751. What exactly do you get? Look at the schematic. There is a startup cap 35V 47uF.

I've just fixed it The issue ? A 0 ohm "jumper" resistor had gone open circuit. It's the one next to the optocoupler in the schematic (to the right of the main IC). I was following the traces for the optocoupler according to the schematic, until I reached that 0 Ohm jumper where continuity suddenly stopped. I pulled it out, measured it and indeed it read 2 megOhms - WAY above the intended value. I replaced it with an ordinary piece of wire and tried it out - works perfectly. Sadly, I was too excited about my victory to take some measurements and post them here to hopefully help other chaps who might run into this issue, but bottom line is: replace the small cap for the VCC pin of the IC and check all resistors, starting with that 0 ohm one. Cheers

[sam_sam_sam]

Re: Dell laptop LA90PE0-01 charger repair

I would put in one those resettable fuses
I will post a photo of one later

[CapLeaker]

Re: Dell laptop LA90PE0-01 charger repair

Yeah... I had another look at the schematic. If that 0 ohm resistor went open or high in value, it will sour your day quickly. As sam_sam_sam stated, they used that 0 ohm resistor as a fuse.

[Dannyx]

Re: Dell laptop LA90PE0-01 charger repair

Yeah... I had another look at the schematic. If that 0 ohm resistor went open or high in value, it will sour your day quickly. As sam_sam_sam stated, they used that 0 ohm resistor as a fuse.

Ok, but (so far at least), nothing's really complaining...it might fail again sooner or later though. What exactly is that "fuse" protecting and what was its cause of failure ? I see it's in series with the AUX winding, but I couldn't find any shorts - none at all...

[budm]

Re: Dell laptop LA90PE0-01 charger repair

https://www.badcaps.net/forum/attach...1&d=1485884098
Can you show us where that '0' Ohms resistor on the is?

[Dannyx]

Re: Dell laptop LA90PE0-01 charger repair

https://www.badcaps.net/forum/attach...1&d=1485884098
Can you show us where that '0' Ohms resistor on the is?

Above the top-right corner of the PFC inductor (that's the yellow square at the bottom in the picture you linked), there's a hole - you can see a "0" right above that hole, placed vertically in the picture. Sorry, I didn't feel like downloading the pic again and marking it in MS Paint

[budm]

Re: Dell laptop LA90PE0-01 charger repair

OK, I see that now.

[sam_sam_sam]

Re: Dell laptop LA90PE0-01 charger repair

I would put in one those resettable fuses
I will post a photo of one later

This is the kind of fuse that I was taking about
Polyswitch Resettable Fuse 170mA 120V 2 Pin bel0017RM PTC | eBay

For some reason the link doesn't work

[delaware74b]

Re: Dell laptop LA90PE0-01 charger repair

Did you reflow the broken solder joint I circled on the back of the board?

[Dannyx]

Re: Dell laptop LA90PE0-01 charger repair

Did you reflow the broken solder joint I circled on the back of the board?

Yes I have. I've actually reflown MANY other joints for good measure

[eckibear]

Re: Dell laptop LA90PE0-01 charger repair

Hi all,

I found this blog very helpfull, thank you guys!
I've encountered yet another zero-Ohm "fuse" in my power supply of this type. This 0R resistor does not appear in the schematic. It is placed between the upper winding of Transformer T050 and the anode side of diode "JSW" & R-52k3, on the very top of the schematic. It is marked up by the yellow circle below (the red circle shows the zero Ohm Resistor/"fuse" discussed above).
BTW.: My supply was way more filled up with the white silicon putty, hardly any area was left open. I guess it had been very cheap on that day of manufacture.

Cheers,
eckibear

[MPFI]

Re: Dell laptop LA90PE0-01 charger repair

I just registered to thank you for the solution. It helped with my DELL charger as well. Done some measurement. I concluded that the chip was good since the PFC was OK, I got 300V no problems. but measuring arund the optocouples ... got lost. I found a 0 jumper opened. Wondering if the boar design, that has two major holes to fit the low profile transformers (on coil and one transformer) just weakened the board and could crack SMD components. Could be the reason for the silastic flood (YES ... I spent a lot of time cleianing this LOL).

Many thanks !!!!

[stj]

Re: Dell laptop LA90PE0-01 charger repair

those resistors are used to pass over other tracks, it's possible the person who designed that forgot to take into account how much current was flowing when they chose the resistors.

[MPFI]

Re: Dell laptop LA90PE0-01 charger repair

those resistors are used to pass over other tracks, it's possible the person who designed that forgot to take into account how much current was flowing when they chose the resistors.

Could be, but in this case are related to the optocoupler cricuitry (output side) I dont tink that the current could be that high.