On The Hunt
That initial search did not take long: there are only three obvious electrolytic capacitors on the whole board, and the only one of them rated higher than 10V is this 68µF 25V can located in the CPU’s Vcore regulator area, which makes sense as the biggest load in the system would benefit the most from additional filtering on its regulator’s input. First test point identified; time to start measuring.
I hooked up the external adapter cable, plugged the adapter in, measured the voltage across my suspected system rail and what did I get? Absolutely nothing. Do we have a blown fuse, current measurement resistor or other component between the input jack and the system rail? Do we have a short across the system rail bringing it down to zero? Or could it be that I was wrong in assuming that the system rail was live at all times? Time for some more measuring to find out.
I decided to go with the short to ground hypothesis first, measuring resistance on a few strategic points around the board: the input capacitors of the various switching regulators and across the DC-in jack terminals. What did I find? Resistance across the DC-in input was 480kΩ, which initially made me suspicious of a blown input current sensing resistor (PR303 on the board, 20mΩ) but that checked out fine. Next on my suspect list was the 25V capacitor (PC815), for which my lowest reading was 0.02Ω, hinting at a dead short to ground.
My multimeter is showing 0.30Ω here because it is very difficult to apply sufficient contact pressure to get good readings on such low resistances while the board is propped for photography, and my hands are in an awkward position to hit the camera’s shutter button while attempting to keep everything in-frame. Forgetting to zero out my leads’ resistance (0.15Ω) for the photo didn’t help either.
How do you find which component is shorted? One popular method is to run a voltage-limited high current through the circuit and search for what heats up. I don’t own a high current adjustable power supply, so I went with resistance measurements to see if I could narrow it down with only one digit to work with, the idea being that I should get lower values the closer I get to the short-circuit’s origin. This is where a micro-ohm meter with proper Kelvin probes would have been handy for its much greater low-resistance resolution and reduced measurement uncertainty. I measured system rail resistance in the support rail regulator cluster where I got 0.04Ω, and I also went back to the system side of the battery management area where my lowest reading was 0.03Ω. This tells me that both test points appear to be further away from the short-circuit than PC815, the 25V capacitor. Conclusion? My short-circuit must be somewhere in the immediate neighborhood of the processor’s core voltage regulator.
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