A1990 820-01814-A dead not charging - need help with diagnostics.

[master_haze86]

Good day everyone,

Let me preface this with letting you know that I have a smalltime computer repair shop but do not have a lot of experience when it comes to board diagnostics. I am good at soldering etc but finding the issue if there isn't an obvious burnt component is not my strong suit.

The other day I got a customer come in with a macbook A1990 (motherboard 820-01814-A) that would not turn on and had previously been getting the warning message that the battery needed servicing. (Most folks seem to ignore that message, just like the check engine light in vehicles.)

First thing I did was make sure the usb-c ports were clean which were dirty so I cleaned those. Once clean, I then checked if the power was reaching the board by measuring voltage on the PP20V_USBC vbus test pads (XA, XB, TA, TB).
While the charger is connected to the pad's corresponding usb port, I get 5.1V instead of 20v while showing 0V on the non-corresponding test pads.
This goes for all of the test pads, except for the XA test pad which shows 2.6V-2.7V while the charger is connected to any of the non-corresponding usb ports and 5.1v when the charger is connected to the corresponding usb port.

Then I checked F7000 and F7001 both of which have unstable resistance readings, starting out in the 1 to 3 mega ohms range while slowly lowering, then jumping down to 1 kilo ohm and slowly lowering, then jumps to 40 to 60 kilo ohm range and slowly rises, then jumps back up to the 1 to 3 mega ohm range and the cycle sort of repeats from there. Voltage to ground for both F7000 and F7001 with only battery connected is 17.6mV and with the charger also connected, the voltage to ground becomes 12.26V.

Hope someone here has the patience and time to guide me through diagnosing this board to find the problem. (if the board is the issue to begin with and not the charger, I do not have another original apple usb-c charger to test with, just a lenovo 19.5V usb-c charger which I am hesitant to even test with.)

Sincerely,
Rick.

[mon2]

Remove all power. Disconnect the battery. Meter in resistance mode. Measure the resistance to ground of:

SMBUS_3V3_BATT_SCL
SMBUS_3V3_BATT_SDA

This is to confirm if the TVS ESD dual diode @ U6950 has triggered or not. If yes, this can prevent a boot. Post the measurements.

[reformatt]

Pull the board out and check each wing for corroded caps, often one will be shorted. You get an accumulation of dust as the Intel machines run quite hot and the fan draws in so much air. Since your PPBUS_G3H isn't shorted, you might be lucky and just have a shorted cap on the 2.5V SSD rail. Depending on the SSD size, you may only have one side populated. The most common fault is failure of one of the SSD 2.5V TPS buck converters, but that usually results in a simultaneous short on PPBUS_G3H and the SSD 2.5V rail.

[master_haze86]

Remove all power. Disconnect the battery. Meter in resistance mode. Measure the resistance to ground of:

SMBUS_3V3_BATT_SCL
SMBUS_3V3_BATT_SDA

This is to confirm if the TVS ESD dual diode @ U6950 has triggered or not. If yes, this can prevent a boot. Post the measurements.

Measuring resistance to ground using the SMBUS_3V3_BATT_SCL and SMBUS_3V3_BATT_SDA test pads I get an unstable reading that will not settle. It acts a bit weird, the first moment I touch the probes to the testpad I get a reading of about 10 kilo ohm and rising fast, when I then touch the other pad I get a reading of 1.3 mega ohm and dropping slowly, when i then touch the first pad again I get the same mega ohm reading that slowly lowers. If I then wait a bit and measure again, the same thing happens again, first about 10 kilo ohm and rising fast until one switches pads at which point the resistance shows as 1.3 mega ohm and slowly lowering.

Pull the board out and check each wing for corroded caps, often one will be shorted. You get an accumulation of dust as the Intel machines run quite hot and the fan draws in so much air. Since your PPBUS_G3H isn't shorted, you might be lucky and just have a shorted cap on the 2.5V SSD rail. Depending on the SSD size, you may only have one side populated. The most common fault is failure of one of the SSD 2.5V TPS buck converters, but that usually results in a simultaneous short on PPBUS_G3H and the SSD 2.5V rail.

After taking readings of SMBUS_3V3_BATT_SCL and SMBUS_3V3_BATT_SDA I took a closer look at the wings and I think I might have found the issue. Around Q3100 there is some slight corrosion present both on the left row of caps and the row of caps and resistors to the right of Q3100. Also looks like CC772 and R2836 have corroded to the point they fell off the board, they are missing and the pads look corroded. Since I cannot afford a microscope with the small number of clients I have, I guess I am going to have to find a colleague to help out. >_<

[mon2]

Try to use your phone to zoom in on the same region for inspection. Some of the phone cameras are amazing for this line of work. Last year in China, found at least one vendor with a slip on lens that allows the phone to be used as a microscope. Was under $10 USD if I recall correctly.