820-01700 (A2141) R7900 failed, power cycling after R7900 bridged

[craig1410]

Hi,

My wife's 2019 MBP died a couple of weeks ago and I have been trying to figure out what is wrong with it. I have an electronics background and equipment but never tried to diagnose a modern MBP before. I have the board view and schematics already and have been climbing the learning curve steadily!

Initial fault was no power with USB ammeter showing 5.19V @ around 12 to 35ma. I won't go into all the initial debug I did because ultimately I found R7900 to be high resistance when it should have been a 0ohm link.

So, going from the power sequence chart in the schematics I get:
PPBUS_G3H = 12.29V
PPBUS_HS_GPU = 12.29V
PP3V3_G3H_SSD0_SNS = 3.38V
PP3V3_G3H_T = 0.02V (this led to R7900 being diagnosed as o/c)

PP3V3_G3H_RTC_X is also 3.38V which feeds the upstream side of R7900.

I linked out R7900 with some wire wrap for now and that now causes the USB charger to power up and shut down continually and I can't get a reading.

I tried removing the R7900 bridge and injecting voltage to the PP3V3_G3H_PMU_VINRTC_R line which R7900 feeds and it takes 3.65A at 3.3V. I only ran at this level for a second because the U7800 Power IC starts to warm up even at 2A current draw at 2.65V injected. When injecting power I didn't see any voltage on the U7800 outputs.

I checked the T2 power rails as per logi.wiki and got the following. Took resistance measurement plus diode mode in brackets (red lead on ground):

PPVDDCPU_AWAKE = 15.9ohm (0.021)
PP0V82_SLPDDR = 34.5ohm (0.032)
PPVDDCPUSRAM_AWAKE = 212ohm (0.637)
PP1V1_SLPS2R = 7.8kohm (0.402)
PP0V9_SLPDDR = 57.3ohm (0.054)
PP1V8_SLPS2R = >20kohm (0.390)

General observations - no signs of liquid damage and no corrosion. U7800 has a somewhat "mottled" appearance under the microscope but no signs of burns. The PPGFX_S0_GPU line is very low resistance around 1.6ohm but not sure if this is normal.

So, my guess is that U7800 is faulty but I'd appreciate a second opinion. I have a Quick 861DQ hot air station and Hakko FX-888D solder station and have some experience at using hot air but would appreciate nozzle size, airflow and temperature guidance for removing U7800. I would also appreciate any help in finding a source of a new or used/reballed chip. It says it's a 338S00267-A0 (1939PHGU) but I don't know if these are generic or specific to each individual MBP model.

I'd really appreciate any help you can offer.

Many thanks,
Craig.

[mon2]

Re: 820-01700 (A2141) R7900 failed, power cycling after R7900 bridged

PPBUS_G3H is low.

Remove the battery. Attach the official power adapter.

Do you see a 20V power delivery contract with the type C port ?

If not, the ACE controllers need to be reviewed. Each ACE controller must be functional before 20v will be selected.

https://boards.rossmanngroup.com/thr...-5v-dfu.61490/

[craig1410]

Re: 820-01700 (A2141) R7900 failed, power cycling after R7900 bridged

Hi, thanks for the quick response.

Sorry, should have mentioned, the logic board is already removed from the chassis and so the battery is not connected. My USB ammeter doesn't show any readings before it resets itself. I only see the backlight on the ammeter come on briefly then it goes off again and this repeats every ~3 seconds.

Before I bridged R7900 the USB ammeter was only showing 5.19V and drawing 30mA or less. It was never going to 20V.

I am using the official 96w power adapter and official USBC charging lead.

During original debug I tried powering up all 4 USBC ports and looked for any differences - there were none. I couldn't find any shorts around the CD3217s either. I did take some voltage readings on U3100 before I found the R7900 issue. Here they are with the power adapter powering the U3100 port.

PPVBUS_USBC_XA = 5.17v
PPDCIN_XA_G3H_F = 5.17v
PP5V_S4_X_USBC = 0.00v ( I think that's to be expected for an S4 line?)
PP3V3_G3H_RTC_X = 3.38v
PP3V3_UPC_XA_LDO = 3.38v
PP1V5_UPC_XA_LDO_CORE = 1.59v (a bit high maybe?
PP1V8_SLPS2R = 0.00v (tracing this led to discovery of R7900 o/c)
PPBUS_G3H = 12.29v

Edit: Also, PPBUS_G3H is now oscillating between around 3.5v and 11v+

Edit2: Just measured PPBUS_G3H with my oscilloscope and actually the voltage is ramping up from 3.7v to 12.6v over the space of 170ms and then sustaining 12.6v for 70ms before dropping rapidly to 3.7v again.

[piernov]

Re: 820-01700 (A2141) R7900 failed, power cycling after R7900 bridged

No point in measuring random things when a problem is already found. Something is shorted to ground causing overcurrent situation on PP3V3_G3H_PMU_VINRTC_R through U7800, either U7800 itself or one of its downstream power rail.

[craig1410]

Re: 820-01700 (A2141) R7900 failed, power cycling after R7900 bridged

Thanks @piernov - yeah I've had a good look at the outputs from U7800 and can't find anything shorted but will have another look this evening. I'm pretty sure the issue is U7800 itself but I'll need to try to source a chip before I can attempt replacement.

[craig1410]

Re: 820-01700 (A2141) R7900 failed, power cycling after R7900 bridged

Anyone know where I can source an ideally new or reballed U7800 PMU IC for the above machine? Part number is 338S00267-A0 (1939PHGU)
Thanks

Edit: Looks like the A1989 (820-00850) U7800 chip might be the same. From a quick check of the respective schematics, the BOM details are the same and the pin assignments seem to be the same from what I've checked so far (mostly the LDO section). Even the neighbouring passive components have the same designations and values. I also checked A2159 (820-01598) but it has a different U7800 chip with part number 338S00466 which has different pin assignments, and from checking a BGA stencil from see-ic on eBay, this chip has a different stencil to the 338S00267. The stencils are both 18x18 (324) pins but the pitch on the 466 is very slightly larger than the 267 it seems.

see-ic seem to be able to supply the 383S00267 chip either new or reballed, or I could pick up a donor 820-00850 board and extract the IC and do the reball myself if I can get a suitable stencil. Since the only place I've found so far to get a stencil from is see-ic then I might as well just get a new chip from them instead. I might get a chip and a stencil just in case I don't get the installation right first time!

[craig1410]

Re: 820-01700 (A2141) R7900 failed, power cycling after R7900 bridged

Hi,

Just wanted to give an update on the above.

I've replaced U7800, which was a bit of a struggle as I was installing a brand new chip with lead-free solder balls and just wasn't using enough heat initially. A subsequent attempt to install a donor chip reballed with leaded solder was more successful and resulted in a transition to 20V on USBC and 12.6V on PPBUS_G3H. Unfortunately the machine still won't boot although I was able to successfully enter DFU mode via keyboard method and successfully did a revive three times before switching to restore which I ran two times. In all cases the process seemed to be successful and got to the lock icon in Apple Config 2. But still no display on the LCD and still no signs of life in general.

You can see the long story version of this process over on https://boards.rossmanngroup.com/thr...-bridged.63851

Hopefully I'm not breaking any forum rules by posting the above link but mods please feel free to remove and inform me if I am. I imagine most people on here frequent both sites.

I've gone through the power sequence diagram as posted on repair.wiki and it seems to be good right through to 2.5V and 0.9V on the SSD. I also see two brief pulses on PP3V3_S5 and PP1V8_S5 a few seconds after power up but it then stays low.

My next step is to replace the PMU again and try to ensure 100% connectivity this time but if there is anything else I can try first then I would really appreciate suggestions. Here are a few details I posted on the other forum:

I tried a second revive which also completed successfully but again with nothing shown on the display. I then did a DFU restore which also completed through to the lock symbol on the host computer but again no change. I just did that restore a second time with the same result.

I noticed that during the DFU revive/restore, the 2V5 and 0V9 SSD voltages appear as well as the 1V8 and 1V1. But when not in DFU revive/restore, only 1v1 and 1v8 are present.

I did diode mode readings for the following lines:
PP3V3_S5 = 0.326
PP2V5_NAND_SSD1 = 0.390
PP2V5_NAND_SSD0 = 0.380
PP1V8_NAND_SSD1 = 0.369
PP1V8_NAND_SSD0 = 0.350
PP0V9_NAND_SSD1 = 0.405
PP0V9_NAND_SSD0 = 0.363

I checked voltage on all main coils and only the following had any voltage:
L7030 (charger) = 0.60V (varying a bit)
L7600 = 5.13V
L7660 = 3.35V
L8410 (backlight) = 0.02V

USBC ammeter is sitting at 20.56V and 0.015 - 0.025A

If I press the power button the current drops top 0.000A within 2 seconds and the another ~8 seconds recovers back to 0.020A or thereabouts. This is repeatable. I wonder if this indicates a short in one of the power rails that only activates when the machine turns on.

PP1V8_G3S is at 1.799V as measured at L6301
PP3V3_G3S_T is at 3.34V as measured on C5051
PP3V3_G3S_X is at 3.38V as measured on TPC4H5

L7700 is 0.398 DM and 0.000V
L7701 is 0.091 DM and 0.000V
L8102 is 0.099 DM and 0.000V

U7800 diode mode readings
V3P3/VOUT_RTC = 0.135
LDO_CORE = 0.359
LDO0 = 0.213
LDO1 = 0.468
LDO2 = 0.523
LDO3 = 0.593
LDO_RTC = 0.361
HIO_SW = 0.220
VPUMP = 0.572
V3P3_SW1 = 0.638
V3P3_SW2 = 0.326

Okay so the diode mode readings from the Power FETs are as follows:

U8215 (PP3V3_G3S_X) = 0.340
U8210 (PP3V3_G3S_T) = 0.423
U8220 (PP1V8_G3S) = 0.302
U8240 (PP1V2_S0SW) = 0.281
U8297 (PP3V3_S0SW_TBT_X) = 0.529
U8295 (PP3V3_S0SW_TBT_T) = 0.527

Note that sense resistors R5720 and R5730 are both not stuffed by design as the PCB has a link between the pads.

My next step is to reball my new, lead-free U7800 PMU with leaded solder and try another replacement but I'll wait to see if anyone has any other suggestions first.

Thanks,
Craig.