Acer Predator GH51M LA-K862P NO POWER

[madjid911]

Hi, I have a laptop acer N20C2 (motherboard : GH51M LA-K862P ) that won't turn on, no signs of life.

I checked the motherboard, there is no short circuit, no liquid damage, just the graphics card which is only 2 ohms (procesor 4ohms and mosfet are ok), so in doubt I disconnected its coil of the graphics card. I checked all the mosfets, all are good.

The 19v arrives on the first mosfet PQB11, but does not come out, the mosfets have good resistance, even at the exit of the input mosfet PQB12 I have 270 ohms. So no shorts, I don't understand why BQ24781 does not give the signal to close PQB11 and PQB12. So I replaced BQ24781 but nothing changed.

strange also that I have no voltage on BQ24781 apart from the 19v power supply on pin 28 Vcc.

Can you help me resolve the problem please?

[MOD EDIT] Link to Schematic - https://www.badcaps.net/forum/troubl...862p-schematic

Reminder: rules and organization of this section - https://www.badcaps.net/forum/troubl...f-this-section

[mcplslg123]

Resistance to GND on PRB3? Are you sure you have 270 Ohms to GND at PQB12 Drain(Pin5-6-7-8)? If yes,then resistance to GND on PRB3 should be also 270 Ohms. Confirm it.

If this is true,then your +19V rail is partially shorted.

[madjid911]

Résistance à GND sur PRB3 ? Êtes-vous sûr d'avoir 270 Ohms à GND au drain PQB12 (Pin5-6-7-8) ? Si oui, alors la résistance à GND sur PRB3 devrait également être de 270 Ohms. Confirme-le.

Si cela est vrai, alors votre rail +19 V est partiellement court-circuité.

270 ohms its PQ12 pin 5
PRB3 : with GND mode diode is 135OHMS, mode normal OL.
PQB12 123: With GND mode diode is OL and mode normal OL, pin 5 : diode 270ohms and mode normal OL
I don't understand, I do have 19v coming out of the connector however and arriving on PQ11 pin 5

[mcplslg123]

I dont understand your measurements. Set your meter to 200K scale,put black probe to GND and red probe on target where you want to measure resistance to GND. Give us these readings as aked in post#2.

Something is definitely wrong woth your measurements as Resistance to GND on PRB3 and PQB12,pin5 has to be same.

[madjid911]

I dont understand your measurements. Set your meter to 200K scale,put black probe to GND and red probe on target where you want to measure resistance to GND. Give us these readings as aked in post#2.

Something is definitely wrong woth your measurements as Resistance to GND on PRB3 and PQB12,pin5 has to be same.

Yes I'm sorry, "PB1" and "PQ12 pin5 " are identical 288ohms

[mcplslg123]

The two pics shows different measurements on the same components. How? Unless you can answer the queries perfectly,no one can help. Afterall the mb is on your table and we need to suggest with your inputs. When input it self is shaky,you know what a disaster it can be.

[piernov]

Diode mode != resistance.

[mon2]

Review your meter face. The dial is in the DIODE mode. For this setting, you must continue to use with the RED meter probe to ground; Black meter probe to the point to test.

Then the meter will show the forward mode voltage drop across the internal diode so for the first measurement, the voltage drop is 288 mV = 0.288 volts. If this measurement is closer to 0 volts then the 2 points are considered to be a short.

A good validation is to next shift the meter face to resistance mode and check again. In resistance mode, the meter probe colors do not matter. Again, if the resistance is too low, the points are considered to be a short condition. For resistance checks only, you can reference in ohms. But in diode mode checks, it is in volts or mV (milli-volts.).

[madjid911]

Review your meter face. The dial is in the DIODE mode. For this setting, you must continue to use with the RED meter probe to ground; Black meter probe to the point to test.

Then the meter will show the forward mode voltage drop across the internal diode so for the first measurement, the voltage drop is 288 mV = 0.288 volts. If this measurement is closer to 0 volts then the 2 points are considered to be a short.

A good validation is to next shift the meter face to resistance mode and check again. In resistance mode, the meter probe colors do not matter. Again, if the resistance is too low, the points are considered to be a short condition. For resistance checks only, you can reference in ohms. But in diode mode checks, it is in volts or mV (milli-volts.).

Yes, the value I gave of 288ohms is in diode mode, in normal mode (black probe on GND) with the gauge at 200K, the value is 48k Continuously increasing

[jasko_jacker]

The Bq24781 is the successor to the bq24780s and it may happen that even with all the correct voltages if it is not receiving the right signals from the host it may not work properly. Personally I would first test the system by bypassing this chip and injecting the 19v directly (be careful that on some boards it is necessary to also supply the voltage to the connector as there could be a line that powers some components before the bq24781 intervenes). If the system works correctly then I would investigate the part that concerns the chip in question. If you don't understand the problem well, it's always better to avoid desoldering or replacing components because afterwards it's difficult to understand what works or what doesn't work.

[madjid911]

The Bq24781 is the successor to the bq24780s and it may happen that even with all the correct voltages if it is not receiving the right signals from the host it may not work properly. Personally I would first test the system by bypassing this chip and injecting the 19v directly (be careful that on some boards it is necessary to also supply the voltage to the connector as there could be a line that powers some components before the bq24781 intervenes). If the system works correctly then I would investigate the part that concerns the chip in question. If you don't understand the problem well, it's always better to avoid desoldering or replacing components because afterwards it's difficult to understand what works or what doesn't work.

II put 19v at the output of the pqb12 mosfet, I have 19V everywhere, the sio chip is powered by 3V3, 20V cpu gpu mosfet, when I press the power button no indicator light no sign of life does not start but I find all voltages after pressing, 5V coil 3V3 coil, 3V3 chipset, 3V3 eeprom. Does the machine have to be controlled by a BQ load manager for startup? I have bq24780s, can I use it?

[madjid911]

Review your meter face. The dial is in the DIODE mode. For this setting, you must continue to use with the RED meter probe to ground; Black meter probe to the point to test.

Then the meter will show the forward mode voltage drop across the internal diode so for the first measurement, the voltage drop is 288 mV = 0.288 volts. If this measurement is closer to 0 volts then the 2 points are considered to be a short.

A good validation is to next shift the meter face to resistance mode and check again. In resistance mode, the meter probe colors do not matter. Again, if the resistance is too low, the points are considered to be a short condition. For resistance checks only, you can reference in ohms. But in diode mode checks, it is in volts or mV (milli-volts.).

I understand better thank you

[mcplslg123]

BQ24780S can be replaced as substitute for BQ24781.

[madjid911]

BQ24780S can be replaced as substitute for BQ24781.

and the BQ24781 can be replaced by the BQ24780S? Because the original on the motherboard is the BQ24781

[mcplslg123]

and the BQ24781 can be replaced by the BQ24780S? Because the original on the motherboard is the BQ24781

yes. Can change in any direction.

[jasko_jacker]

https://e2e.ti.com/support/power-man...81-description There are some difference

[mcplslg123]

https://e2e.ti.com/support/power-man...81-description There are some difference

I know but it works.I've done it on 3-4 occassions.

[madjid911]

I know but it works.I've done it on 3-4 occassions.

I checked everything, all the resistors are good, none short, I don't understand why the chip doesn't give the signal to close the mosfets, I ordered BQ24800

[mon2]

The replacement BQ24781 is soldered ok? Please share a close up pic of this part.

carefully measure the voltage to ground of each pin on this charger ic. Post each measurement for a review.

[madjid911]

Here is the photo of the chip, I always check my soldering, I even tried with a BQ24780S chip but it still doesn't work. Something is missing from me, Only PR12 gives a voltage of 19.8V. All the other voltages from bq are at zero volt, I checked all the resistors are good and no short