Troubleshooting complex boards

When troubleshooting complex boards, your troubleshooting toolkit must contain many tools.

I define complex as those boards being SMD, multilayer, analog combined with digital, and high-speed designs.

Here are some scenarios I have run across.

1) If you have a dead short (rail to common), a few methods are available:

• Lowest resistance IC by IC check. A four-wire or a milliohm meter is preferrable over a DMM. Some high-precision bench DMMs in high-count and manual range mode, zeroed (or nulled) can be used if solid (good quality) probes are used.

• You can power up the board if it was powered up before, and quickly check the board over with a liquid crystal sheet or thermal imaging to see which chip heats up. NOTE: the chip that heats up may be the chip which is actually attempting to drive (output to) a shorted chip.

• You can apply low-voltage current to the board and check for heating with a crystal sheet or thermal imaging. This has the benefit of being able to also diagnose physical (copper) shorts. The risks include a)overheating a component, b) burning out a component, and c)burning out the copper short without knowing where it happened.

2) You are seeing an higher current draw, and the current is increasing.

Turn off power immediately. Check polarity of all capacitors while wearing eye protection. An electrolytic or a tantalum may have been installed incorrectly.

3) The output of a circuit is clamped to a lower voltage than nominal. Or, voltage is not reaching a proper level.

Check output TVSes for a correct rating.

4) Logic devices are resetting, not working, or operating abnormally.

See my other article. You probably have transients on common (one of the many "grounds"). This may be caused by a circuit elsewhere requesting too much current periodically, or dumping current into common.

5) Which methods can be used on a large board?

• Disable chips one by one. For computable logic, you can place the chip into reset. Be careful if you decide to lift power pins to a logic chip. Many logic families have input protection networks which will not like the absence of power. Some chips will actually become powered by drawing current from inputs thru the input protection diodes.

6) For condicted emissions or noise, you may use a spectrum analyzer with a close-field wand to narrow the problem to a small area. For those who cannot afford the fancy instrument - make a small