820-00928 Touch Bar not powering on

EsteTron

Member
Device came in liquid damaged and had missing R701 to CPUVIN and RA603 to PPVIN_S0_GFXIMVP_VIN. Replaced and device boots up normally now. Performed a QC test and gave USC. Everything was working normally with board loosely installed. After screwing down logic board screws to frame, The touchbar stopped working. Unscrewed logicboard screw next to Left Hall Sensor and touchbar came back. Was unable to find anything grounding out board, so I assumed Left Hall sensor was bad. Replaced and still had the same issue.

Installed board into a known good topcase with a different touch bar and still had the same issue.

When I boot the board with the original topcase, touchbar is no longer working at all now. Tried booting into safe mode and a USB test os and still had the same issues.

Measured voltage on J4401 and J4402 and getting 1.8 on PP1V8_S0SW_DFR and DFR_DISP_RESET_L. Seems like device is getting proper power but perhaps a data line is missing?

Pull up resistors for DFRDRV_I2C_SDA R3918 and DFRDRV_I2C_SCL R3917 both measuring proper resistance of 4.7k ohms
Pull up resistors for DFR_TOUCH_ROM_I2C_SDA R3916 and DFR_TOUCH_ROM_I2C_SCL R3917 measuring 2.2k ohms

Getting 0V on DFR_TOUCH_LID, which from my understanding is suppose to be 0V when SMC_LID is 3.42v on Q4400

Not quite sure where to go from here. Thank you in advance.
 

2informaticos

Administrator
Staff member
Check for knocked off components, or broken traces.
Possibly to have more areas affected by corrosion, or crap underneath some BGA chip (U3900?)...
 

EsteTron

Member
Wasn't able to find any missing components, but everything keeps circling back to U3900. The only thing is that I do not have a stencil for this IC at the moment.
 

2informaticos

Administrator
Staff member
Heat it with good flux.
Just to burn eventually crap, no more than 180-200 degrees; no need to melt the balls.
 
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