820-02100 Turn ON Turn OFF cycle

[MobileXT]

Hello Everyone, I'm new on this board 820-02100 and having a problem on the charger 5v then 19v then back to 5V again in cycles. I don't know where to start. Any help greatly appreciate.

[mon2]

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

F5200 or F5201.

[MobileXT]

F5200 and F5201 resistance to ground 368kΩ
Diode Mode= .433v

PPBus_G3H = 12.34 to 10v Pulsing
PP3V3_AO = 3.3 to 0 Pulsing.

[mon2]

Flux and remove both of the fuses @ F5200 and F5201. Do not lose these parts. Power up again. Is PPBUS_G3H now stable? What is the voltage to ground from the ISL (producer side) of this power rail?

Power cycling / pulsing can also be linked to SPI flash rom corruption. First, rule out an excessive current draw fault. Update this thread with the above test. Also, remove all power, meter in resistance mode. Measure the resistance to ground of each and every inductor on the board. Each inductor is linked to a power rail. A low resistance can mean a shorted component.

[MobileXT]

Removed F5200 and F5201 Voltage steady @ 12.35V

Resistance to ground with without F5200 , F5201 at PIN2 = 6.90MΩ

Still no green or orange light on the MagSafe.

[mon2]

Remove all power. Locate the magsafe connector @ J5401. On J5401 is a diode @ D5403. Meter in resistance mode. Measure the resistance to ground of this diode (USBC5_CC1_PIN).

Also interested in the same resistance check of PPVBUS_USBC5 on this connector. No power to the board / no adapter during these tests.

If the diode @ D5403 is damaged from an ESD event, then this resistance will be low and a defective diode will shunt the communication over this BMC line used for the power delivery contract. Under this condition, only 5 volt will ever be selected by power on default to power this logic board = too wimpy of a power rail.

[MobileXT]

Okay! resistance of PPVBUS_USBC5 = 36.6KΩ
Resistance to USBC5_CC1_PIN = 4.8MΩ

[mon2]

No shorts here. Meter still in resistance mode. As per post # 4 - Measure the resistance to ground of each inductor on the board. Report any suspects with a low resistance.

[MobileXT]

I replaced UG800 due shorted on the ground, After replacement still have the same problem on the PPBUS_G3H Power rail pulsing from 12.35 to 10V. I removed the fuse again F5200 and F5201 and injected 13.0V on PIN2 and nothing is short. I decided to put it back the two Fuses F5200 and F5201 and pulsing for a second of and on off and on after couple restart finally I have a steady MagSafe orange light and steady PPBUS power and computer boots up. This is now FIXED. Thank you for your help.

[mon2]

Suspecting there was a shorted (partially?) capacitor on the PPBUS_G3H line causing the chirp mode pulsing. The injection smoked out the defective capacitor. Well done!

Scary that the laptop is relatively young and yet already faced with bad caps.