(RESOLVED) 820-3208-A, burnt cap on PPBUS_S5_HS_COMPUTING_ISNS, replaced, then 2 weeks later burnt cap on PPBUS_S5_HS_OTHER_ISNS

[joshAU]

Hi all.
I have an A1465 Macbook Air with a 820-3208-A logic board that is dead.

I recently had this unit it, dead, and found C7510 on PPBUS_S5_HS_COMPUTING_ISNS Rail was visibly damaged - burnt, and had shorted out that rail.
I removed it, the short was gone, so I replaced with another from a spares board.
The unit appeared to work perfectly, so I returned it to the customer.

Two weeks later, the customer brings it back in, dead again.

This time there is a damaged cap (C7284) on PPBUS_S5_HS_OTHER_ISNS, that is burnt, and shorted.

Both PPBUS_S5_HS_COMPUTING_ISNS and PPBUS_S5_HS_OTHER_ISNS show a short to ground, however after removing C7284, the short has disappeared.

What should I be looking forward that would cause this?
I could just replace C7284, but I suspect something is drawing too much current?
Or is it just bad luck/timing?
Any suggestions greatly appreciated.

 

[2informaticos]

Strange to happen so quick in time; but could be just bad luck.
Did you check the exact PPBUS_G3H, without battery connected?

 

[joshAU]

Hi 2informaticos.
Thanks for your input.

With C7284 removed, the short is gone.

PPBUS_G3H is low - it seems to fluctuate between 4.05 and 4.19 volts, every second or so - fast enough to notice on a multimeter.
Battery disconnected.

Magsafe light on connect is green then goes orange.
Unit does not power on.
Nothing seems hot to touch.

I guess I'll start looking at why PPBUS_G3H is low...

 

[2informaticos]

Start checking resistance between pins 17/18 and 27/28 of ISL6259.

Try starting in SMC bypass mode too.

 

[joshAU]

Update:
OK, above measurement was at F9700

If I measure PPBUS_G3H on pin 1 of F9700, the figure is ~4 volts.
This is the same as pin 2 of F7040.

HOWEVER, pin 1 of fuse F7040 measures 8.42 volts as it should for PPBUS_G3H.

I thought fuses either worked or they didn't?

I removed the fuse, and it measures 15 Ohms?
Weird.
I replaced it with another from a spares board, and unit is now running, boots to OSX, and seems fine.
I haven't yet replaced C7284

Thanks again for your help 2informaticos, I appreciate it.