PRACTICAL SERVICING
In general the servicing process consists of three distinct steps: (a) diagnosis of fault; (b) repair or replacement of faulty component or assembly; and (c) setting-up, alignment and test. All three apply equally to mechanical, electronic and optical aspects of TV and video equipment. It is important to separate these three aspects of service work as far as possible. While diagnosis by substitution may be necessary where information, expertise, test equipment or (in some circumstances) time is lacking, it is far better to carry out careful analysis of the symptom and a logical process of reasoning – aided by progressive test readings – to identify the faulty component.
There are several exceptions to the rule, especially where (as is usually the case) the labour time in diagnosis must be held to the absolute minimum. Where the fault is an intermittent one, cross-substitution (e.g. of picture-tube cathode feeds, or of L and R channels in audio systems) is invaluable, especially where – as in the examples given – the fault can be made to give a positive indication of its origin when it appears. For complex faults the problem area can be considerably narrowed down by substituting complete sections or PC boards where possible. In cases where components (ICs, tuners, modules etc.) are pluggable and a substitute available, it is expeditious to test in this way. On the other hand, haphazard trial-and-error methods of fault- finding are likely to damage printed circuits and delicate components, multi-legged ICs and wound components, especially where tightly packed PC boards (many double-sided) are used in miniature and portable equipment. The time spent in this pursuit can often be more profitably used in analysing TV-screen images, voltages and waveforms.
It is good practice to spend the first few minutes (or more) of service attention in a careful and close visual examination of all sections of the equipment – a high proportion of failures are due to ‘physical’ causes like corrosion; damaged, split or cracked circuit boards; burnt, broken or misplaced components; intermittent joints or contacts; faulty plug/socket connections; and spark or corona discharge of one type or another. If no obvious problems can be seen, the next logical step is a check of supply voltages to appropriate sections of the equipment – not only in respect of correct voltage, but (particularly in the case of digital circuits) the degree of ripple and spurious noise rid- ing on them. The presence of input signals, and correct operation and connection of peripheral components should next be checked for. Failure to do this could result, for instance, in a long diagnosis session in a videorecorder servo only to find that the control track head is dirty; the replacement of a TV tuner because the aerial has been blown down; or the replacement of a TV picture-tube whose heaters are not being energised! Only when it is proved that there are no physical problems, power lines are present and correct, input signals present and output or load devices connected need fault diagnosis begin in the signal-, power- or data-handling stages themselves, in the sequence: signal in, operating conditions, signal out. Reference to the OEM’s service literature is essential at this stage, not only to establish the layout and operating mode of the circuit and components, but to refer to the d.c. voltages and waveforms quoted for normal operation.
ICs are always direct-coupled internally, so that an incorrect pin voltage should lead to a check of the externally derived voltages on associated pins (refer to the IC block diagram) before condemnation of the chip itself. Where a clamping or gating action takes place inside an IC, incorrect ‘static’ voltage levels may well be due to a missing or badly shaped keying, trigger or gating pulse. In digital and especially processor chips, all relevant inputs should be checked in the event of an incorrect or missing output, bearing in mind that apparent loss of an input signal can be due to a stuck-low input gate within the chip itself.
Many circuits, analogue and digital, have feedback loops. While this may appear to complicate the task of fault-finding, it can often help, in that under fault conditions the two ‘loose ends’ of the loop move (electrically) in opposite directions in an attempt to restore normality. Thus the loss of an FG feedback signal will usually vastly increase a videorecorder’s capstan speed to aid diagnosis; a low gain UHF tuner in a TV will result in a ‘go-to-full-gain’ a.g.c.-line mes- sage to the tuner even with high applied r.f. input level; the chopper transistor in a switch-mode PSU will have a high duty-cycle waveform applied to its base in the event of no output due to its collector lead opening, and so on. Again, analysis of all symptoms, and rational thought, should lead to a speedy diagnosis.
Experience, both of diagnosis and fault patterns in general, and especially of the particular equipment under service, counts for a great deal. Many a problem has been very quickly solved on the basis of ‘hunches’ and a knowledge of the habits of various classes of components. Large electrolytic capacitors tend to dry up and reduce capacitance, especially when mounted in hot places; some sorts of transistors are prone to go b-e open-circuit in certain circuit configurations; heavily loaded drive belts in video decks will slip under heavy loading conditions – the list is very long. An experienced engineer is aware of all these things, and thus often able to short-cut much of the diagnostic procedure.
Some quick checks are easy to understand and follow. A 10 kΩ wire-wound resistor with 200 V across it is intact if it is very hot, and open-circuit if it is cold to the touch. A multirange meter’s prods form a handy and instant shorting link with the meter switched to its 10 A range. An analogue test meter, provided its sensitivity is known (e.g. 20 kΩ/V d.c., 1 kΩ/V a.c.), is a useful switchable substitute resistor provided the applied voltage will not overload the movement. The test voltage on an ohms range will switch on a transis- tor junction. These are but a tiny selection of time- and trouble- saving techniques.
In support of the approach to fault-diagnosis suggested above, the off-screen photos on the following pages have been chosen not as simple cause-and-cure cases (diagnosis by ‘stock-fault’ lists is seldom relevant to modern equipment) but as illustrations of logical testing and analysis, and of how much information can be gained by careful study of picture symptoms. The same principles apply to situations (i.e. deck malfunctions, digital logic faults etc.) where the convenience of a ‘screen’ fault display is not present, necessitating close observation or the use of test instruments at the outset.