TRACING INTERMITTENT FAULTS
Of all equipment malfunctions, intermittent faults are the most difficult and frustrating to trace and diagnose. Provided that operating conditions are not ‘borderline’, i.e. weak tacho pulses, low supply- line voltage, PSU starting resistor gone high resistance etc., they usually have their origins in mechanical or ‘thermal’ effects. Bad jointing on PCBs or within components, as well as hairline board cracks etc., can usually be traced by flexing, probing and tapping of PC boards – once the fault has been made to appear, use test equipment to prove its origin for a definite diagnosis.
Intermittent faults which are temperature-dependent can often be traced to a single component (very often a semiconductor) by the alternate use of gentle heat from a hairdrier and an aerosol freezing spray. The most vulnerable and suspect components for thermal faults are diodes, transistors, ICs and to a lesser extent ceramic and electrolytic capacitors. ‘Mechanical’ intermittent problems are most often due to dry joints (sometimes concealed under an apparently good solder blob), potentiometers, switches, plug/socket interfaces and board-print faults. The most vulnerable joints are those which carry heavy (e.g. scanning and primary power) currents, and those which support heavy or iron-tagged components like transformers and large electrolytic capacitors.
The main aim in diagnosis of intermittent faults should be to get positive indications of the identity of the faulty component or section. Thus a solder-up of suspect board joints or replacement of suspect components followed by a fault-free test period is not as satisfactory as a cross-substitution test of stages, panels or individual components, in which the fault can be reproduced in isolation. Whether this takes the form of transferring an intermittent blue picture to an intermittent green one by interchanging B- and G-output transistors, or an indisputable meter reading at some crucial point in the circuit during the presence of the fault, it gives complete confidence in the diagnosis and the repair.
It is important to glean every clue from the equipment user in cases of alleged intermittent faults. Analysis of these can considerably narrow down the field of search. In difficult cases, it is necessary to leave the equipment with diagnostic instruments (ideally redundant and aged test gear saved for the purpose) hooked semi-permanently on key test points.