I have an Acer Nitro 5 AN517-51, Board: LA-J891P. The device does not start. No +3VLP LDO voltage. +5VLP LDO voltage is present. I have 19V at the input of the PU301 and PU501 (PIN 2-5). I have 4.5V at pin 11 (ENLDO_3V5V) of the PU301 and PU501. No short circuit at the output of the +3VLP LDO (PIN17)
-
-
Not at my desk but each always on regulator will have enable pins (EN). Check their voltage to ground to confirm if the rail is being enabled. Often the voltage is created by 2 resistors acting like a voltage divider off the 19v rail. Also test the local ldo low current rails. Are they working?
Test the voltage to ground of:
PU301 - pin 17 = LDO +3VLP. This rail is used to power the EC and the EC is the one who enables the switching rails (via 5V_3V_EN) that are missing from PU301 & PU501.
Is the +3VLP rail present?
What is the voltage to ground of ENLDO_3V5V? This signal is created from the +19VB rail using the 2 resistors onboard (PR301 & PR302). If this always on signal is absent, review the same 2 resistors.Last edited by mon2; 02-04-2025, 07:45 PM. -
Thank you for your input mon2
My problem is that the 3V LDO rail is not working. I have already replaced the PU301. I have also checked the two resistors. There is an anomaly. According to the circuit diagram, the PR301 should have 499k and the PR302 150k. I measured 120k on the PR302 and only 260k on the PR301. When I sound out the PR301, it has 499k. I have also replaced it before. Whenever it is soldered in, it has a different resistance, which I can't explain. Based on the circuit, the behavior is also illogical.
Comment
-
Remove all power. No battery. Meter in resistance mode. Measure the resistance to ground of the +3VLP LDO rail. If the resistance is low, then there is a shorted part on this rail and this will explain why the LDO rail is absent.
The resistance of resistors can vary when the part is IN CIRCUIT versus OUT OF CIRCUIT. Other parts on the line influence the resistance.
Best to test if the resistance is correct by carefully fluxing and removing the resistor so it is out of circuit. Then are the resistors of the correct value?
Next, with both of these resistors onboard, what voltage to ground do you have at the ENLDO_3V5V rail? If this rail is of a high enough logic level then the LDO rail on pin # 17 should enable, provided the LDO rail is not faced with a shorted part on the rail.
Share the voltage measurement.Comment
-
I have 260k ohm at PIN 17 (+3VLP LDO) of the PU301. I have 4,5V at PIN 11 (ENLDO 3V5V) of the PU301. +3VLP is NOT present
I have 700k ohm at PIN 15 (+5VLP LDO) of the PU501. I have 4.5V at PIN 11 (ENLDO 3V5V) of the PU501. +5VLP is present.
I have disconnected the +3VLP rail in the direction of the EC (UB1) at jumper JPB1. According to the circuit diagram and boardview nothing is now connected to the +3VLP rail.
Comment
-
Flux and carefully remove and replace PU301. Test without the jumper you have isolated for the EC power. Apply the jumper after the ldo is confirmed to be working.Comment
-
What is the resistance to GND on pin 17,pu301?Comment
-
Post #5:
have 260k ohm at PIN 17 (+3VLP LDO)Comment
-
I know that everything indicates that only the PU301 is defective. I still have a working LA-H501 (Acer Nitro 5) here. The mainboard is slightly different, but the 3V and 5V rails are the same with the same ICs as the LA-J891P. I desoldered the SY8288BRAC from the LA-J891P and soldered it onto the LA -H501. The +3VLP rail worked. I soldered the working chip from the LA -H501 onto the LA-J891P, +3VLP no function. With the LA-J891P I have 300k ohms at PIN 17 of the PU302, with the LA -H501 it is 600k ohms. I checked the +3VLP rail on the BoardWiever again. Although I have disconnected the jumper JPB1, the +3VLP rail still goes to the power controller (PUB1) via resistor PRB22.Comment
-
+3VLP also goes to UG108,pin27. Try disconnecting the track for RG3019.
If this fails, you need to remove all caps connected to 3VlLP one by one.
Recently had a case with HP LA-G07DP where resistance to GND on +3V LP was more then 100K and all requirements of 3V regulator was ok but still + 3VLP was missing. Turned out to be a leaky caps on +3VLP.
Good Luck.Comment
-
Hello technician,
Thanks for your input mcplslg123. RG3019 is not present on my board. The position is open. But as you write, I can't avoid checking the components on the +3VLP rail. I hoped that I wouldn't have to do this, but it was unavoidable. As the resistance was slightly lower than it should be, I assumed that the short circuit must be somewhere further along the rail, behind some resistors.
I have now found the fault.
I overlooked the fact that the +3VLP rail goes via PR101 to DH2. I desoldered DH2. The short circuit was gone, +3VLP was now present. I measured further behind the DH2. DH2 is a twin diode that connects +3VLP and 3V from the RTC battery and passes on to resistors RH30, RH50, RH52. All three go from the resistors to the chipset UH1.
RH30 goes to PIN407 of the UH1
RH50 goes to PIN 448 of the UH1
RH52 goes to PIN 490 of the UH1
And this is where the short circuit is. So the chipset is defective. I was just hoping for a defective capacitor. Unfortunately that was nothing. I will order a new chipset and replace it and then let you know. I hope it's nothing more.
Thank you all for your support. If anyone needs any data on the mainboard, please let me know.
zz0.j8tnh407pzizz
Comment
-
With DH2 removed off the board, what is the exact resistance to ground at PCB pad #1 of the removed DH2 dual diode?
RH30 is most likely not the fault since this is a 1M ohm resistor.
RH50 & RH52 are most likely not the fault since they are each @ 20k ohm resistor.
Perhaps UH1 is shorted due to the direct connection of VCC (BC49 & BD49) to the +RTCVCC rail.
As a quick test and for giggles, flux and remove the 2 caps that follow the removed DH2 dual diode @ CH47 and CH48. Then test the resistance to ground again of this power rail. Any changes or still a low resistance to ground. If still low, UH1 is probably defective.Comment
-
Hello mon2
What do you mean by "Perhaps UH1 is shorted due to the direct connection of VCC (BC49 & BD49) to the +RTCVCC rail." I can't find BC49 and BD49.
But you are right, something is still wrong. Was I perhaps premature? I have checked again and have removed
RH30, RH50, RH52, DH1, CH47, CH48
RH30 PAD2 to earth 4Mohm (seems to be okay)
RH50 PAD2 to earth 4.1Mohm (seems to be okay)
RH52 PAD2 to ground 2 ohms it leads to PIN 490 (PCH_RTCRST#) of UH1 and to QB6 PAD1 and to CH19 PAD2
I have removed QB6 and CH19 and always have only 2ohm at RH52 PAD2. Only PIN 490 of UH1 is still in the rail.
At DH1 PAD1 to earth I also only have 1.3 ohms from there it only goes via PCB layer PIN 1172 to UH1 PIN 449 and 429 (+RTCVCC) and to PR212. PR212 has 98k ohms, so it is OK.
I would say that the problem comes from the RTC battery, as low resistance values are only present in the +RTCVCC rail of the UH1. I have also measured the RTC battery and it is also dead. I believe that the RTC battery has damaged the UH1. If the RTC battery was dead, it overloaded the +RTCVCC section of the UH1 during operation. Now it remains to replace the UH1.Comment
-
See attached.
Flux and remove CH18 = 1uf that is also on the same netname of PCH_RTCRST# - see attached partial schematic. Perhaps it is shorted.
Comment
-
Hello mon2
The CH18 was also removed. In the Net: PCH_SRTCRST# I have 4,2Mohm so no short circuit.
I have short circuit only in the:
Net: +RTCVCC = 1.32 ohms
Net: PCH_RTCRST# = 2.52 ohms
I have removed all components in the affected nets. Only UH1 is still present in both. It is possible that UH1 has discharged the RTC battery and not the battery has damaged the UH1. I now definitely believe that the chipset is defective, or what is your opinion?Comment
-
Quite a common fault where +3VLP gets shorted due to shorted PCH(RTC Section). Pch is for replacement for sure going by the resistance to GND on pin1 of DH2.
Usually if +RTCVCC is shorted ,chances of PCH being defective is very high unless u are lucky to find some leaky cap on RTC circuit.Comment
-
Hello mcplslg123
Thank you for your contribution and your experience with the device. As I already wrote in the post of 09.02, I have removed all capacitors in the affected circuits and in the +RTCVCC and PCH_RTCRST# nets there is only the UH1 left. It is interesting why the UH1 is damaged by the short circuit of the RTC battery and not just some component in the +3VLP LDO rail. I will remove the UH1 and then measure whether there is still a short circuit.
THXComment
-
It appears that UH1 may be defective.Comment
-
I have ordered the chipset. As soon as it arrives I will install it and let you know if the problem has been solved.Comment
-
I removed the chipset. I carried out measurements on the +3VLP LDO rail and the short circuit was gone. The new chipset has already arrived and I have installed it. After installation, I checked the +3VLP LDO rail again and everything was OK, no short circuit. I tried to start the device, but it did not start and the short circuit on the +3VLP LDO rail was there again. I have no idea why it happened. Actually, the chipset can only be damaged by an overvoltage. However, I can't see such a problem anywhere. Given the circumstances, I now assume that the RC battery was dead because it was shorted by the chipset and this did not damage the chipset. I hate the device.
Comment
Comment