Here's a simple tip for avoiding costly mistakes when measuring values on a powered-on board: cover up your meter probes!
Meter probes are oh so shiny and conductive but they also like to slip as well as touch other parts you don't want touched. This sort of unwanted touching often leads to nasty sparks and subsequent release of the "magic smoke" form your components.
It may seem "duh" but wait until it happens to you! Things like vintage power output transistors and old chips are expensive to replace. Shorting an output transistor induces big current flows through other parts resulting in collateral damage. One slip can result in multiple burnt parts.
Step 1: Horror! Short Circuit About to Happen!
Here we are trying to get to a pin nearest the voltage regulator heatsink. We're dangerously close to shorting the pin to the heatsink.
Step 2: Materials
This instructable uses heatshrink tubing. I'm pretty sure mine came from a RadioShack assortment. Choose a piece that will go over the biggest part of your probe.
Step 3: Measure
Slip on the heatshink tube and pinch it to find the end of your probe. Then slip the tube down just a bit and cut it off right where the probe tip ends.
Step 4: Shrink
Shrink your heatshrink with a careful application of heat. I used a butane lighter flame waved around the heatshrink.
Step 5: Done!
Do the other probe(s) and you're done.
What a difference! There is now so much less chance of destroying things if the probe slips. Worst you can do is short the adjacent pin. This should happen only briefly due to the small area of the tip and the fact that you slipped so it's moving.
I hope this simple tip saves you both money and grief in your circuit testing future.