Lenovo 14ALC6 - NMD521 - No power, totally dead.

[Mike Russell]

Machine won't power on, no power led, no fan spin, completely dead.

Checked PQ5400 - AONS32314 and pins 5,6,7,8 are 20v (AC adaptor voltage) but pins 1, 2, 3, 4 are 0v.

Following Gate Pin 4 to PU5400 - BQ24780SRUYR.

VCC 20v
ACDET 2.8v
ACDRV 0v (goes to pin 4 gate of AONS32314).
ACOK 0v
REGN 6v

According to datasheet, conditions are correct for ACOK to be high but it is not.

Tracing ACOK it's connected to ACIN which is connected to a pin GPB0 of IT8227 but also shown as connected to +3VL via a 100k resistor (to pull it high).

It seems that +3VL is not present as it is low and BQ24780SRUYR shouldn't be pulling it low based on it's datasheet.

I am confused at where +3VL is generated as the information I have does not specify. The only mention of +3VL is that it is jumped with +3VLP at PJ6003.

Checking PJ6003 jump I see 0v and it appears not be shorted to ground.

My understanding is +3VLP is created by PU6001 - SY8386BRHC but it seems that it's input comes from B+ via jump PJ6001 but I don't have B+ (it's 0v) as the PQ5400 isn't turned on.

It is unclear to me where +3VL is to be generated, I assume it is generated outside of B+ as B+ depends on the PQ5400 being turned on and that depends on ACOK being high that depends on 3VL being present.

I would appreciate any help as I am quite new to this.

Thanks.

[piernov]

https://www.badcaps.net/forum/troubl...arging-circuit

[mon2]

Further to the above article read, remove all power.

1) Meter in resistance mode.

2) Measure the resistance across the mosfets to confirm if they are defective or not.

Measure:

source (1-2-3) & drain (5-6-7-8)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)

Start with the 2nd DCin mosfet @ PQ5401. Often, it is this mosfet that is defective which is causing the ACDRV voltage to be low. ACDRV should be = adapter voltage + REGN = ~25v to enable both, PQ5400 & PQ5401. Only if this main power rail is enabled, then ACOK will be pulled high by the LDO rail of +3VLP (pin # 13 on PU6001).

Each resistance check of the mosfet must be hundreds of K ohms else may be defective. If you are truly in Taiwan, you can probably find replacement mosfets at your local 7-Eleven stores.

[Mike Russell]

PQ5400
123 -> 5678 3.7 mega ohm
123 -> 4 3.5 mega ohm

PQ5401
123 -> 5678 3.7 mega ohm
123 -> 4 3.8 mega ohm.

Yes, I am in Taiwan but as a foreigner I don't always know where to get things and also being new to hardware it will be more the skills and equipment to do any replacement.

ACDRV is measuring practically zero. pins 5678 of PQ5400 is adapter voltage.

Should I expect that the mosfets are on and B+ is present even if ACOK is low (as my understanding that ACOK is dependent on 3VL/P which is dependent on B+ so B+ must be present to pull ACOK high)?

I have attached part of the TI datasheet for bq24780S that implies that power is only switched to using the AC adapter if ACOK is high, but how can that happen if there is no 3VL from another source?

[mcplslg123]

You're absolutely right in concluding that +3VLP aka LDO is generated by PU6001 and since it doesnt receive VIN,3VLP will be absent.
ACOK is an open drain pullup from +3VLP. So you are right in your conclusions.

However, ACDRV will rise only when ACDET/VCC/REGN/ACOK as well as CMSRC has valid input. In your case ACDRV wont rise as CMSRC is missing.

I'll suggest a few things:
01.Remove PQ5401 and see if PQ5400 starts working correctly.
02.Measure resistance to GND on PR5425
03.Measure resistance beween Pin1-2 and pin 19-20 of charging IC(No power attached obviously)
04.Replace BQ24780S.

[Mike Russell]

Yes, it confuses me because it seems impossible if ACOK is dependent on 3VLP which is dependent on B+, obviously it worked before so there must be something I am missing.

CMSRC is essentially missing, it's measuring fluctuations around 0.02v like the 5678 of PQ5400 and ACDRV.

I just checked resistance to ground at PR5425 and it is 5.6 ohms, so it appears B+ has a short to ground somewhere. I guess I should stop focus on PQ5400 and PQ5401 and focus on finding the short to ground on B+.

[Mike Russell]

Spent some time trying to find out what is shorting B+ to ground. Went through and disconnected jumps that connect directly to B+ PJ6001, PJ5701, PJ1010 and PJ7204 progressively looking for change and identified the short is within the PJ7204 area as with PJ7204 disconnected the short to ground on B+ is gone.

Plugging in the adapter with PJ7204 disconnected gives 20v at PR5425. Pressing the power button the power LED flashes (which is better than before as it was completely dead).

[mon2]

Remove all power. What is the resistance to ground of PL7203 (either side)? We are checking if there is a shorted part on this power rail. A low resistance on a low voltage rail is normal. Wish to confirm the resistance in ohms.

Is ACDRV now ~25v with PJ7204 open?

Keep the power off -> short PJ7204 again with solder. Meter in resistance mode. One meter probe @ PR5425 (either side is ok); other meter probe @ PL7203 (either side is ok). What is the resistance?

You are checking if the high side mosfet @ PQ7201 is defective. If there is a low resistance in this case then this board is likely a no fix since the high adapter voltage has travelled onto the very low voltage rail of 1v2 and most likely killed all downstream parts.

PS: Assuming the above to be the case, you are in Tapei. You should be able to locate a high grade repair shop that can fix this fault, if it is practical for you. We have many semiconductor firms we use in Asia and most of the HQs are in Taipei. Miss Tapei - the food court in the basement of Tapei 101 was very busy but good. You should attempt the Computex and Taitronics fair in Tapei - very good but getting smaller I hear fairs in the region.

[Mike Russell]

I managed to find that caps PC7229 and PC7230 were shorted and removing them from the board removed the short. Reconnecting PJ7204 and powering it up with those caps removed it boots successfully.

[mon2]

Well done!! The caps will probably not be missed but in your travels, hit up a local electronic parts store for closure. They are 10uf 0603 SMD size at a voltage that should be higher than the operating voltage. There are articles that report that the spec of the capacitor voltage should be close to the operating voltage else the capacitance will derate.

See attached. Good reading (if you are bored).

[mcplslg123]

Glad to know you solved the puzzle. Replace those caps with 10uF/25V ratings even though the system will work without the caps.
They were put for some reason by the manufacturer-so dont ignore it.