What is the importance of a diode mode measurement in troubleshooting Macbook logic board circuitry?

Posted Tuesday, January 13, 2015

What makes diode mode measurements different from resistance mode measurements?

Diode mode measurements allow me to quickly get an idea of what the entire circuit is doing and compare it to a known working board. Diode mode measurements are very similar to resistance measurements, but they are much faster. Resistance is measured in Ohms, and tells you the resistance between two points in the circuit. Diode mode measurements tell you the voltage drop across two points in the circuit. The difference between diode mode vs. resistance measurements is that diode mode measurements can be performed much quicker. Measuring resistance takes the multimeter a while. Measuring the voltage drop takes less than half a second. It is a much quicker and more accurate way to measure an entire circuit.

How does measuring the voltage drop to ground at different points in the circuit help us?

How does this work? Every circuit has a connection to ground at some point. If the circuit changes in any way, the way it connects to ground will change. So what we do is measure the voltage drop between a specific point in the circuit, and ground. We can then compare these values with a known working device and have an idea of where a problem may lie. Let’s say we’re looking at the output of the LCD screen backlight power rail – I expect 0.521 to 0.546 between output & ground on an LP8550 based board. If I see 0.0 or 0.021, there is a short to ground – too litle resistance to ground. If I see 0.474 to 0.511, experience tells me the feedback via is corroded. If I see 0.521 to 0.546, I know the line is correct, and backlight may not be coming on because of a corroded connector, or a signal not present.

I can also measure the individual pins of a chip to get an idea of where the problem lies.

Measurements will always be a little it different based on the different tolerances of the different components. There is no specific guideline on how to tell EXACTLY where the problem is in a circuit based on these measurements; you will still have to use your brain. However, it can be very useful if you are totally lost – these diode mode measurements make an excellent tool to push your focus in the right direction if you have overlooked something obvious.

Check out a real world application.

Here I take the example of a non-functioning trackpad. I overlooked something simple and silly that I should have checked. I went around each pin of the trackpad connector with my multimeter in diode mode, red probe on ground, black probe on each pin of the connector. Then I compared it to a known working board. One was very far off from every other reading, so I followed along that line extra carefully, and found a missing component on an 18v line! Doh.

This is one of many tools that helps me remain professional.

Doing this type of work, it isn’t about being a genius. It’s about having a methodology that allows you to occasionally be an idiot and still resolve the problem. Professionalism isn’t about always being right, having ESD safe mats, accepting AMEX and never making mistakes. It’s about being able to produce repeatable results for your clients that are positive even when curve balls get thrown into the mix. It’s about finding ways to get around the reality of life. You will make mistakes when you don’t get enough sleep, have a bad day, or just miss something because you’re human. You are not a robot. Diode mode measurements assist me in hitting these curve balls out of the park even when I am having an off day, and contribute to the reputation of my business by keeping my success rate high.

Find the tricks that help you remain professional to a WORTHWHILE definition of professionalism.