Acer E1-531 la-7912p motherboard no 3.3v

[Dannyx]

Good day folks. Back in the realm of laptop motherboard repairs, hoping to fix at least one of these, as so far I haven't been very successful This Acer laptop wouldn't turn on. Initially, I found two shorted transistors on the 5v rail (5vALWP) on page 48 on the right, PQ17 and PQ19. I removed them and after a long wait, I got my replacements off aliexpress. I replaced them, plugged in the charger and still nothing...no signs of life.

Now it's worth mentioning that I did something something stupid which I didn't realize until now upon taking a closer look at the schematic: those weren't the only failed components I found: PQ9 on page 47 also got hot and was shorted when I first got the laptop. I removed it and since I didn't have a replacement one on hand, I jumped across its drain and source pins thinking all it does is open up to send 19v to the rest of the components, so when I plugged in the DC jack, I heard a pop in the jack itself, most likely caused by the inrush current and while there were no sparks or smoke coming off the board, PU5 (page 48) now has a pinhole in it, so still no power or 3.3v/5v The transistors are not shorted funnily enough. I thought I'd push on and ordered myself some PU5 ICs as well. Not sure whether I should just go for it or is there something else to check beforehand...no shorts upon first check.

I don't know if PQ9 acts as some sort of regulator and by jumping across it I ruined everything after it by sending full adapter voltage into the board....I can never understand how those "input" circuits works....PQ9 is an N channel yet its drain is at VIN. There must be like a charge pump in that IC to get it going. Getting past that, if I get 19v after it, wouldn't that just flow straight through the body diode of PQ10 ? Why even have PQ10 then ? So disappointed...I though for sure it'd be an easy fix, but it's turned into a fiasco. So note to self, admitting PQ9 did not cause any additional issues, the controller IC must also be replaced when the transistors it drives blow, because this is the second time I've had this issue where replacing the transistors did not fix the issue and also caused the controller to pop... Worth it ?

EDIT: if anyone has a boardview for this that they can provide, it would be greatly appreciated

[caspian]

Re: Acer E1-531 la-7912p motherboard no 3.3v

all repairs on a laptop mainboard have to comply with electronics rules. Designing and applying a heuristic method of repairing which was not tested previously, will probably lead to a damage to the mainboard. thus, avoid heuristic methods and get used to standard methods of repairing a mainboard. those standard methods are the ones that are used frequently by members of this forum.

[piernov]

Re: Acer E1-531 la-7912p motherboard no 3.3v

PQ9 can be jumped between source and drain. PQ10 would need to be jumped as well. Not a good idea though. Better inject some voltage into B+ with a voltage regulated and current limited power supply.
PQ10 is a reverse input protection MOSFET. It still needs to be activated on its gate to reduce the loss.

The charger IC has indeed an integrated charge pump that takes the ACDRV voltage 6V above VIN (or something like that) to drive both DC in MOSFETs when all the conditions are met. You can find more information on this on its datasheet which is available.
Warning: if you come across the 247x7 variant (not the 247x5 as here), those are for P-channel MOSFET and will pull the DC in MOSFET gates low to enable them.

If PU5 got a hole in it after jumping PQ9, it was already dead anyway. Which is not surprising at all if PQ17/PQ19 were dead. At this point I'd replace PU5, and PQ16 to PQ19 for good measure.
When all those parts are removed, check the resistance on both +3VALWP and +5VALWP.
And pray no other parts got hit. Don't turn on the board with PQ16/PQ17 still on the board and either the dead PU5 present or removed, as they could send full B+ rail to the 3V/5V rails if their gate are triggered or left floating.

If unsure, always replace all the MOSFET and the buck controller when there's an issue on a buck regulator. If you're really on the cheap side (you shouldn't if it's a customer board or something less than 10 years old), you may get away with replacing only the buck controller if the MOSFET measure fine, or sometimes only the shorted MOSFET. If the buck regulator has doubled MOSFET, you need them matched though. And if the high-side MOSFET die, higher chance of the buck controller to get fucked, and something down the line too (sending B+ voltage to the line and onto the buck controller output and sense pins). A low-side MOSFET dying is usually not that worrying.
Finally, you can't assume that the buck controller caused the MOSFET to die, the MOSFET caused the buck controller to die, or some other random event happened. Both those parts can die randomly (MOSFET are more subject to heat and overload conditions though), and can cause each other death too.

PS: boardview may be available at sualaptop365

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

I had a look through the datasheet for that IC myself and it became clearer now, which will also help with further repairs since they all seem to utilize the same setup for VIN. It certainly put my mind at ease after that impromptu whack-job of jumping across PQ9. I removed that jumper anyway, determined to do it properly once I get PU5 replaced as well and will try it with my power supply rather than the adapter, so it hopefully cuts out if something goes haywire again. No shorts so far like I said, even in this non-functional state and with PU5 removed. I prepped the pads for a new one, so it hopefully was on its way out already and it just so happened to die now.

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

Good day again folks. Hopefully someone is still watching this thread. I got my replacement PU5s from Aliexpress the other day, I put one on there (the solder job looks satisfactory enough) and with a jumper across PU9 still, this time I gently increased the input voltage to 19v using my bench supply and although there were no funny side-effects, there was no power either....no current draw at all. No shorts that I could find...could be a bad PU5 ? Damaged from heat ? Do these ICs get damaged that easily ? I see my 19v coming into the "B+" point, so PU10 opens up correctly as far as I can tell...what else should I check ? I shall have another look at the schematic myself.

[caspian]

Re: Acer E1-531 la-7912p motherboard no 3.3v

So this laptop has got multiple problems. If PQ9 and PQ10 do not allow the power to pass, you should fix this problem first. Just then, you can focus on PU5. jumping PQ9 and PQ10 is a very bad idea. If the new PU5 does not get hot and there is no short, it is possible that this new PU5 is healthy. Thus, focus om fixing PQ9 and PQ10 first.

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

Only PQ9 is jumped (and missing from the board entirely). PQ10 is fine and power does get through to the current sense resistor PR41, so B+ is "up"...a scope would've enabled me to see if PU5 is operating or there's a bad solder joint on the new transistors I put in. I haven't measured the 3.3v LINEAR regulated output of PU5 to see if it outputs anything - should be 3.3v (+3VLP pin 8 in the schematic, page 48)

[caspian]

Re: Acer E1-531 la-7912p motherboard no 3.3v

so measure all pins of PU5 to check.

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

Did some digging and also found the boardview I actually had to create an account on another forum to grab it and it seems they're actually well stocked when it comes to boardviews, since I needed one in the past and it redirected me to the same site, so now I can hopefully find more if needed. So big thanks to the chaps over at SUALaptop365...don't know if I'm actually allowed to promote other platforms like this, but they're worth checking out. Cheers.

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

Ok, back on this project, here's where we stand: the enable voltage on pin 13 is present and so are Vreg 3 and Vreg5, so PU5 appears to be functioning to a degree. Could be a badly soldered transistor out of the four that are being driven by PU5 or some of these components got damaged by the heat, though it didn't seem like I was being too rough with them - it just seemed like the normal temperature I solder SMDs at...

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

Not having made much progress on this board due to lack of time, I found THIS video which explains something I missed: a "sub" power rail which I didn't take into account (page 46 of the schematic). It shows a +VSBP controller by another MOSFET. I'm not sure whether it's the same issue or not, because towards the end of the video, the chap shows he's replacing PQ9, which coincidentally is blown on my board too, though I had a different approach: I simply jumped across it, so ultimately I think the problem is STILL in the standby circuit.

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

More to come, another thing to check: it could be related to the ENTRIP pins of PU5. I had no idea how these operate at first, but doing some more digging and losing a couple of hours of sleep I think I got its operation down: there's a resistor divider formed on the ENTRIP pins which only gets "activated" when PQ20A and PQ20B are "on" and completing the circuit to GND. Otherwise, the pins are kept high by internal pull-up of PU5.

In order to enable the resistor divider, PQ20A and B must turn on and the way this happens, I THINK is the following: PU4 (page 47) first sends its ACOK signal on pin 5 which then arrives at pin 2 of PQ22 in the very bottom left of page 48. With its base now high, PQ22 conducts and pulls the gate of PQ23B to GND. Since this is an N-channel FET, it no longer conducts, allowing pin 2 of PQ21 to go up to a high state. PQ21 now conducts and also turns on PQ20A and B....could be wrong though...

[piernov]

Re: Acer E1-531 la-7912p motherboard no 3.3v

About PU5, the ENTRIP should not be driven low or high, and PR71/PR72 sets the current limiting. So PQ20 should not get activated. So the gate shoud stay low. So PQ21 gate should be high. Apparently all the lines going to PQ21 are no stuff except the PR80, meaning PQ21 is driven by MAINPWON, which should go high.
MAINPWON should be driven by the EC, but apparently R893 was changed over revisions so check if it's on your board.

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

How can you tell something is "no stuff" ?

I see an arrow pointing AWAY from PQ21, so wouldn't this mean the node at the base of PQ21 is driving something, rather than BEING driven ?

[SMDFlea]

Re: Acer E1-531 la-7912p motherboard no 3.3v

How can you tell something is "no stuff" ?

I see an arrow pointing AWAY from PQ21, so wouldn't this mean the node at the base of PQ21 is driving something, rather than BEING driven ?

The No Stuff ones have the @ symbol .

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

Aha, thought so myself

So now I need to actually measure the ENTRIP pins again just to see what I actually get there....sure hope the KB9012QF is not damaged in some way. I see that MAINPWON signal leading to it...

[piernov]

Re: Acer E1-531 la-7912p motherboard no 3.3v

I see an arrow pointing AWAY from PQ21, so wouldn't this mean the node at the base of PQ21 is driving something, rather than BEING driven ?

No. That would actually be the smart way to draw things on schematics so that's not how the "engineers" do it. (well I'm criticizing but could be worse…)
The signal is connected to a gate of a MOSFET, so the gate is driven by the signal. The cannot cannot drive the signal.

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

So it all comes down to whether or not the KBC is still functional at this point. I'll also replace PQ9 since at the moment it's still a piece of wire there, though I believe the two STB circuits 3.3 and 5v should work even without it. That's for inrush limiting.

[piernov]

Re: Acer E1-531 la-7912p motherboard no 3.3v

EC should be powered by the 3V LDO (+3VLP) when off on pin 111. Also I think EC_RST# should be high, and EC_ACIN as well. Maybe EC_ON… Schematics is a bit confusing and we never know what are the requirements for the EC.
Could also be a bad EC or EC firmware.
But yeah first fix the charging circuit.

[Dannyx]

Re: Acer E1-531 la-7912p motherboard no 3.3v

That doesn't really change the fact that 3.3v and 5v ALW should be there, or at least I THINK so.