Thinkpad P72 EP720 NM-B723 laptop no power, unlucky repair attempt - Page 1
Forum section: electronics / repair

author: mita
author_rank: Newbie
timestamp: April 28, 2025, 04:47:18 am
content: Hi,Try to repair a Thinkpad P72 EP720 NM-B723 MB with I7-8850H processor and N18E-Q1-KB-A1 (P3200) GPU with 6GB memory. Unfortunately there is noinfo what happened to the laptop. I got it disassembled with parts missing, like keyboard, battery.Unfortunately I have no schematic available for me but got the boardview file.Symptom: after connecting to the charger the power led light up for one-two second and switch off. After a couple secons the power led light up for half a second and switch off. No battery, no IO board, no memory, no keyboard connected to the MB.Checked the resistance on the power rails. Measured VCCPUCore resistance to ground 2 ohm, VCCGFXCORE_D 0.1 ohm. I have no experience on these resistances but I thought the GFXCORE rail is shorted.Thermo camera was not able to pinpoint any parts with increased heat. The only visible change was the small heatup of the VCCPUCORE inductors and drivers.Checked the NCP81274 phase buck controller controlling the VCCGFXCORE drivers with scope. PWMs are present, VCCGFXCORE goes to 1.25V and dorps to zero withing half a second. Power good PGOOD signal goes up and down accordingly. On the other hand according to the boarview file the PGOOD signal pulled up but not routed anywhere else.Checked the NCP81215 three output controller controlling VCCPUCORE with scope. PWMs are present VCCPUCORE goes about to 1.3V and drops zero within a second.Injected 0.9V (5A limit) on VCCGFXCORE. I got no voltage drop and 2 amper but there was not any heat up parts on the thermo camera. The GPU slowly warmed up to 32Celsius. I do not really understand this short. If it would be short the current should have reached the limit but not.Same injection to the VCCPUCORE rail. No voltage drop, 1A current, no heated parts.Removed the four coils, big tantal caps from the VCCGFXCORE rail measured 0.4 ohm resistance to ground, increased from 0.1ohm, it is quite a lot. Measured the lo and the high side resistance to ground of the drivers, seems to be OK, in couple Kohm range.At this point I am asking help how to proceed forward. Do the GPU or the CPU toasted? Injection is not supporting this theory. These amperes are within the limits, there are no under and over voltage situations therefore the controllers should operate continously.Unfortunately I do not know the power sequence of this laptop. As far as I see the power management is controlled by one of the two Lenovo ThinkEngine chip, but there is no documentation on the net about this. It could be possible there is an another reason the rails are disabled within a short period of time.If anybody knows these power controller ICs thsn please advise how to measure the possible reasons of the rail disabling.Any advice is appreciated in advance.

author: amyk
author_rank: Super Contributor
timestamp: April 28, 2025, 05:24:45 am
content: Even at idle that CPU consumes ~2W with ~0.6Vcore so seeing sub-ohm resistances on the power rails is normal.There are sites claiming to have the schematic download for a fee but this overview might help too:https://laptop-schematics.com/userfiles/schm/EP720_N18EQ13_BD.jpgThe EC is an MEC16xx series, the MEC1632 has a public datasheet you can refer to for reference. Other laptops with Coffee Lake CPU should also be similar.

author: mita
author_rank: Newbie
timestamp: April 28, 2025, 02:12:27 pm
content: Thank you your quick response and the provided information. I found the MEC1653 datasheet on a pay site. The most important info would be the power sequence and the power schematics. I think a logic analyset should be used in order to detrmine what power rail fails first because when the first fail the embedded controller quickly disables all the other power rail.Regarding the documentations behind pay walls I am a bit concerned to use it as I am not bale to verify them. Maybe I will try later on, report back if I find a site with no scam.

author: mita
author_rank: Newbie
timestamp: May 01, 2025, 11:58:02 pm
content: I was able to move forward because bought the schematics and got the boardview file from the forum.I replaced the 330uF tantalum capacitors on the VCCGFXCORE_D rail. Soldered back the coils as well.The situation became worst: no power at all, not for one second. I have attached the schematic of the DC-IN section to better understand the "strange" situation.Symptom: there is no voltage at the output of the PQ1 P-channel mosfet because its gate always on DCIN_PWR20_F, ie. at 19V. It seems the Thinkengine switches off the MB immediately with PWRSHUTDOWN signal pulled to low. PQ1 pin 5, output resistance to ground is in megaohm range, no output short present.Measured PR8 resistor's both legs with scope at the same time and got the picture attached. Yellow line on pin 2, blue line on pin 1 of PR8. Pin 1 went to high and to gnd for about 1ms and to high again. 1ms wide 3V PWRSHUTDOWN signal switches PQ8 which turns on PQ6, grounding PR8 pin1. But. How on earth the PR8 pin2 do not follow this change? PR8 pin2 is on steady 19V instead of about 6.3V for 1ms.To simplify the situation I removed the resistors with red circles eliminating the PQ1 gate and the PQ4 effects.The situation remained the same, PR8 pin 2 did not react the change of pin1 voltage drop to gnd. We have a voltage divider PR3 (200k)-PR8(100k) which is not working. Between PR3 and PR8 about 6.3V should be present for 1ms but not, it is 19V all the time. It is like there is no current flowing through PR3. The resistance between PR8 pin1 and PR3 pin 2 is about 300kohm wich is ok based on the schematic.If somebody has an explanation why the voltage divider is not wokring please share with me. What would be the next try to figure out what is happening with the gate voltage?Other question is why Thinkengine PWRSHUTDOWN output is high for 1ms only. If it is low the main power rail disabled. PQ1 is never open therefore main rail overcurrent can not be the reason to shut down the main power rail. But the first question is the PQ1 gate voltage. If the PWRSHUTDOWN signal would be high all the time the PQ1 gate voltage still remained high and the main power rail voltage would be zero.