About Debugging Guidelines

In debugging – sometimes I got carried away with my “instinct”, although mostly it get me to the right conclusion in a short time – it is definitely too much of “black magic” to the newbies.
So my humble advice to the beginners is to think about every circuit or IC as a functional block – define the boundary yourself or from a schematic – make sense of the circuit does. To state the obvious, in a typical circuit block:

• will have a function name in the schematic - this contains important info regarding what the block supposed to do
• will generally has input, output, supply and control signals

identify which is which will generally give you a sense of how to debug the block

Once you know that the input/output relationship does not hold (assuming that you understand what the block supposed to do), here are some of the steps that might helps

1. Bias the circuit so that the block will have known state (or consider overwrite the input from a known source), use DMM to confirm all the DC pins, starting with supplies pins.
2. Confirm the input output relationship – if it is not correct, break this block into smaller block and repeat the process (take a look at binary search debugging post: http://electroniccircuitdesignsharing.blogspot.com/2013/02/debugbinary-search.html).
3. If the block is already an IC, check for part number, orientation, cold solder join, missing solder joints before “accusing” it as faulty part.
4. remember that sometimes certain failure mode needs more than a DMM to be used (see http://electroniccircuitdesignsharing.blogspot.com/2012/06/why-oscilloscope-is-needed-for-circuit.html) , so if DMM cannot give you convergence, you should consider to use another instrument (depending on what this block supposed to do).

so good luck with your troubleshooting!