820-00840 shorted on board, now 5v cycling and no 20v

Atomrepair

Member
Board came in clean looking, no corrosion spotted anywhere so far. It drew about 2,5 amps at ~4 volts and didnt switch to 20V.
PPBUS_G3H and PP20V_USBC_XB_VBUS_F were shorted to GND

Q3100 had an internal short, bridging PPDCIN and PP20VB. replaced it.
Q3200 replaced just in case. pads for both of these looked bad. Pitted, dull grey. Probably from heating up a lot?

U3100 replaced from identical donor board - no damage found underneath, but did find some weird glue-looking stuff. reflowed U3200.

Short is gone, machine now sits at 5.18V with 20 mAmp.
measurements
PP3V3_UPC_XA_LDO3,4V
PP3V3_UPC_XA_SXcant find
PP1V8_UPC_XA_LDOA1,8V
PP1V8_UPC_XA_LDOD1,8V
PP3V3_UPC_XB_LDO3,37V
PP3V3_UPC_XB_SX
PP1V8_UPC_XB_LDOA1,8V
PP1V8_UPC_XB_LDOD1,8V
PPBUS_G3H0V


I'm not (ever) completely sure if my U3100 replacement was a sucess. I have no stencil yet and am not looking forward to putting solder balls one by one on one of these again to rule it out. Is there anything else I can check to verify if U3100/U3200 are working properly? Anything else to measure with these symptoms?

U3100:
2019-12-02_04-17-27-.jpg2019-12-02_04-14-35-.jpg2019-12-02_03-34-52-.jpg
 

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2informaticos

Administrator
Staff member
Compare diode mode readings between both CD3215 chips.
Better to compare with an working board.
You must be sure chip is correctly sioldered and maybe need to change the other too.
 

Atomrepair

Member
I checked nearly all pins in diode mode (some I couldnt reach a measuring point under a shield by U2800 or by J3300) and only this stood out:
pin 62 U3100&3200: 0.951 vs 0.969 on working board
pin 84 U3100: 0.985 vs 0.967 on working board
pin 94 U3100: 0.990 vs 0.968 on working board
pin 95 U3100: 0.987 vs 0.968 on working board

I'm not a heavy diode mode user, but these differences are very minor and not worth checking out, right?

I still need to check a bunch pins that read 0L on the board im working on to see if those are the same on the working board.

Right now I'm thinking reball U3100 and put it back again. But I think I'll hold off on that until I get my stencil. This board is my own, so no real hurry.
 
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