Logical Fault Finding Tutorial

YOU SHOULD KEEP YOUR ONE HAND IN YOUR POCKET WHEN WORKING ON HIGH VOLTAGE EQUIPMENT.

MAKE SURE THAT YOU KNOW OF THE HAZARDS FROM A LIVE CHASSIS, CHARGED CAPACITORS, FINAL ANODE VOLTAGES, THE AC SUPPLY ETC.

The POKING AND HOPING method of fault finding on electronic equipment is ok if there are only a few components which can be changed one at a time, but is useless where a large number of components are involved. A more logical method is necessary.

Begin by observation using the senses.

• Can you smell overheating? With experience one can recognise the type of component by its smell.
• Can you see any burnt components, are any particular colours missing from the television picture?
• Can you feel any components overheating? Can you feel static on the screen?
• Can you hear any distortion, whistles etc?

Have you heard of the game Twenty Questions? One person thinks of something and the others have to guess what it is by asking questions. They receive only YES or NO answers.

If the first question IS IT AN ANIMAL? and the answer is YES then all non animal items in the universe can be ignored.

If the second question IS IT HUMAN? and the answer is YES then all other animals in the universe can be ignored.

If the third question IS IT FEMALE? and the answer is NO then only questions related to men need be asked.

After twenty questions most items in the universe can be discovered!!

A similar system can be applied to fault finding. This is called the HALF SPLIT method.

A transistor radio has several STAGES and the signal from the aerial passes through these and is emitted from the loudspeaker as an audio signal.

The volume control is about half way along this chain. If I inject an audio signal at this point and hear noise from the loudspeaker then I know that all stages and components after this point are ok and the fault lies before this point.

From this one measurement we have proved that half of the components are ok and that the fault lies in a certain area. Further HALF SPLIT measurements will enable us to locate the precise stage in which the fault lies.

If we had started at the aerial end and the fault was in the loudspeaker then we would have wasted much time and effort before we found it.

These tests are called DYNAMIC MEASUREMENTS and enable us to locate the stage or area of the fault. Signal Generators and Oscilloscopes, Logic Pulsers and Logic Probes are frequently used to make these tests.

To find the actual faulty component we use STATIC MEASUREMENTS. These usually require the use of a Voltmeter to make measurements on the faulty stage. The measurements obtained are interpreted to obtain the identity of the faulty component.

For example, the base to emitter voltage of a good silicon transistor is 0.6 volts. If it is not this voltage then it is possibly this component at fault.

Beware that a faulty associated component could possibly give the same readings. If you haven't had much experience at interpreting voltage measurements then remove the suspect component and check it by resistance measurements or substitution with a known good component.

Since the faulty stage has been located and only a few components are usually involved then POKE AND HOPE is more permissible!!