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Fundamentals of Troubleshooting – Electronics & Photonics

By Daniel J.M. Guibord

Proper test and measurement instrumentation for the type of equipment that you are troubleshooting is a must. With today’s circuitry, at minimum you need an oscilloscope. A multimeter is to the electronics & photonics technician what a blindman’s cane is to a blind person. An oscilloscope is your eyes. 

All electronic equipment can be divided into two fundamental blocks:

· The power supply

· The electronic circuitry fed by the power supply

First, conduct a visual inspection of the electronic circuitry’s components. Look for signs of overheated or burnt components. Then, verify the integrity of the power supply’s voltages. Begin at the input(s) of the power supply, and work your way to the output(s).  If the supply’s voltages do not check out OK, disconnect it from the electronic circuitry that it feeds, the measured voltages’ values will give you a clear answer as to whether the problem is with the power supply, or with the electronic circuitry fed by it. If the supply’s voltages check out OK*, then apply a signal (or signals) to the input(s) of the electronic circuitry and trace the flow to the output(s). Look for the signal at about half way between input(s) and output(s), it will determine which direction you need to go from that point on, forward or backward. Use the half way approach again (e.g., say the signal was not present half way between an input and output, go half way between these two points now, and so on, until you reach a point where the signal is OK on one side of a circuit’s node, and not OK on the other side of that same node). Wherever the signal is not what it should be, that is where the problem is. Now you have to narrow it down to the component level, right? Perhaps not. It depends on the dollar value of the components where the signal that you were tracing got lost or altered. It is up to you to judge the situation. If the components involved are worth less than $10, simply replace all components in the vicinity where the signal got altered (how much is your time worth?).

[*There is however a relatively infrequent case where the output voltages of the power supply unit are correct without load, but incorrect under load. It may turn out appropriate to test the power supply unit with a suitable load in order to eliminate this possibility.]

However, once you verified the integrity of the power supply, there are a few things that you can check before spending your time tracing signals through myriads of components. You should bear in mind that:

· 70% of failures of electronic equipment are due to metallic contacts (e.g., oxidation of switches, connectors and relay’s contacts, potentiometers’ wipers, etc.).

· 20% of failures are due to semiconductors. Those which fail most often are:

o Power devices

o Peripheral chips (chips located typically at the inputs and outputs of a circuit block).

· 5% of failures are due to capacitors, either opened or shorted; or worst, leaking or slightly out of specifications.

· 2% of failures are due to resistors (opened in the vast majority of cases).

· 1% of failures are due to drifts in components’ values.

· 1% of failures are due to borderline engineering (a diplomatic word for quack engineering).

· 1% of failures are due to a number of causes usually not easy or obvious to pin down, which, in most cases, result in intermittent problems. These are, typically:

o 1- Electrical noise conducted or radiated.

o 2- Mechanical vibrations.

o 3- Cold solders. If you suspect a cold solder, do not try to pin down exactly which solder joint is the culprit, simply resolder every solder joint in the vicinity, unless you want to spend an inordinate amount of time searching for what may turn out to be, in the end of it all, a metallization pad’s problem inside an integrated circuit.