820-3115 Power rail issue

tlough75

Member
so I have an 820-3115 on the bench with a strange issue. PP5V_S5 cycles from 3.3V to 4.5V then to 5v over and over again. Is this caused by something opening and closing? doesn't seem like a short would do this. I'm going to keep looking. Board has no signs of liquid damage or corrosion.
 

2informaticos

Administrator
Staff member
Test the board alone on the desk, no TP/KB or other device connected.
USB mouse is good enough for test.

Any pulse at 3V3/5V, or other coils?
 

tlough75

Member
I'll have to check for pulses. I have the board on the bench by itself by the way. does it need RAM in the slots? I have one stick in. What would cause the 5V rail to oscillate like that?
 

2informaticos

Administrator
Staff member
If SMC detects a short or overcurrent, will switch off and cycles SMC_PM_G2_EN.

BTW, try to start in SMC bypass mode.
 

2informaticos

Administrator
Staff member
"Any pulse at 3V3/5V, or other coils?
--------------------------
I'll have to check for pulses."
???

"does it need RAM in the slots?"
No, for power tests...
 

tlough75

Member
Is it safe to inject voltage on this line? P5VSUS_EN_L IS bouncing around too. But I suspect everything on PP5V will be
 

2informaticos

Administrator
Staff member
"I found that PP5V_S3 is shorted to GND."
Exact ohm value?
Always allow 15-20s after power is removed, before measuring in ohm scale (or diode mode).
If low enough injecting voltage could help to find the culprit.
 

2informaticos

Administrator
Staff member
0.2 ohm at L7260 is really short.
But low enough to find the culprit with voltage injection method.
Start with 1V and go high a bit, if needed...
 

tlough75

Member
Will do. I'll post back with results. Should I remove L7260 to inject? or is there a better place. I am worried the memory slot plastics may melt.
 

tlough75

Member
Found the problem. It was a shorted cap next to the audio codec IC. Found it by injecting voltage and burning my palm :(. Was dissipating 7 watts... Replaced it and now I have a working board that boots fully to macOS. Thanks again for your guidance. On to the easy no Backlight 820-3115 and the pile of others from my client!
 
Top