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 conducted 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-size tuned LC circuit, and observe its output with a scope while moving it over the board. For the creative types, you can use beat frequency to create an audible noise pickup probe.
7) For a circuit with many rails and commons, you can isolate a transient or noise problem to one rail by attaching external decoupling caps to the rails, and seeing if operation improves. Build a small external board with a string of low-ESR capacitors (large-capacity electrolytics, small capacity tantalums, and low-capacity ceramics all connected in parallel to eliminate all noise frequencies).
8) Which methods can be used to troubleshoot a malfunctioning part of a circuit?
For complex circuits, you can use the following tools:
- Power-off DMM compare measurements. Compare the measurement with either the schematic or a known good board. Be aware the ICT (in-circuit test) component values will not always match schematic values.
- Compare signals of the malfunctioning circuits to signals of a known good one.
- Observe the operation and signals of a malfunctioning circuit, and use your knowledge of its operation to guess at the problem. To do this properly, save all measurements such as scope screens, DMM measurements, etc. Preferably do this on a large-size printout of the schematic diagram. Take measurements of many points around important places in the circuit, and record all data.
- If you are new to troubleshooting, then I need to the the first to tell you about cracked SMD components. These are hard to find without an extensive visual inspection under magnification, or a direct in-circuit / out-of-circuit measurement.
- For very difficult problems, do not discount the possibility of PCB physical problems such as broken traces and broken vias. If you are tracing a problem down a circuit, and the signal is different on one node than it is on another interconnected node of a schematic, then check all tracer and vias connecting those two nodes together.
- For an in-circuit-test which is more sophisticated than a DMM, specialized RLC test equipment exists. One equipment of this type is a Huntron, which can visually display the relationship between the three fundamental electronic properties - Resistance, Capacitance, and Inductance. It shows all of this information in a single plot on the screen for a comparison with a known-good unit.
- Do not discount the method of replicating parts of the circuit off-board (such as with thru-hole devices on a proto-board), and replacing the internal part of the circuit with an external clone for difficult issues such as noise injection, EMI, physical board problems, and the like.