820-00923 5V 0 Amps

[JdYoda]

Hello.

Mac A1706 doesnt turns on.
3 of 4 CD3215 rebooting, LDO 1.1 present.
UB400 steady 5V, 0Amps, no LDO 1.1v After changing UB400 no changes.
Found out that all PPBUS lines are missing (ppbus_g3h, ppbus_hs_cpu, ppbus_hs_oth5v )
PPDCIN_G3H - 5v
PP3V3_G3H - 3.3
Diode mode for ppbus lines is 0.4
Diode mode for all LDOs of UB400 matches other CD3215
Changed U7000 - no changes.
Since there are no short on ppbus i've tried to inject 13 volts to PPBUS_G3H. With injected voltage - 3 CD3215 switching to 20 volts and rebooting, UB400 is not reacting for charger at all. USB-C meter doesn't turns on.
Checked the board with thermal cam - nothing gets hot. But looks like there is a short circuit somewhere...How can i find it?

 

[2informaticos]

You have to fix the problem on UB400 area first.
Other things around the chip affected; if the new chip is good 100%.

I recommend to try another CD3215 first.
Once removed, check the traces for its pads.

 

[JdYoda]

CD3215 is good 100%. Checked it on another board. Also traces is ok, checked each trace under UB400

 

[2informaticos]

Change QB400.
Check all resistors and small coils related with UB400.

 

[JdYoda]

I found the problem, but it's a little weird...
I decided to check all the pins under UB400 in diode mode, and found out that pin 36 ( UPC_TA_UART_RX ) has very low resistance, just 26 Ohm. This pin goes to resistor RB305, and it should be 1 MOhm. I removed RB305, puted back UB400 and mac works.
What is weird - i thought when resistor goes bad - resistance can only get higher value, not less. Is it possible that 1M resistor became 26 Ohm?
Or RB is not a resistor?
And one more question: is it necessary to solder back new RB305, or i can just remove it and forget about it?

 

[2informaticos]

Corrosion on a resistor can easy drop down its ohm value; especially for high ohm resistors.
Is like connecting another (parasite) resistor in parallel with it.

You should put back new 1M resistor.
I think UB400 boots from UB300, through UPC_TA_UART_RX/TX lines.
UB300 boots first from UB090.

 

[JdYoda]

Thanks for explanation