TIMEBASE SERVICING

TIMEBASE SERVICING

In field timebases where the fault is complete lack of output (single horizontal line across the screen) the first essential is to reduce screen brightness to prevent damage to the phosphor layer. Once it is established that the operating voltages for all sections of the time- base are present, the next step is to ascertain which section – oscillator, driver or output stage – has failed, for which the oscilloscope is the best tool. Where the entire timebase or amplifier is embodied in a single IC which is proved faulty, check for destructive conditions before fitting and powering a new one. Typical of these are a shorted flyback diode (D305 in Fig. 10.3, D109 in Fig. 10.4), excessive sup- ply voltage, shorted capacitors or heavy output loading due to a leak- age or short-path to ground.

More often, field faults will take the form of various kinds of raster- shape distortion. Low supply voltage, faulty electrolytic capacitors or incorrect feedback conditions are the most common causes of these. A top foldover effect with teletext line superimposed is due to slow flyback. A cramping at picture bottom is generally due to an inability of the supply line or the output stage to furnish sufficient current to drive the scan coils fully; this may well be due to a dried-up or ageing electrolytic decoupling capacitor on the supply line to the field output stage.

Where the line oscillator, driver and output stages are not involved in the generation and control of power supplies (see Chapter 11) the diagnosis of a ‘no-go’ fault follows the same pattern of signal (here, pulse) tracing from oscillator via driver to output stage. Line oscillators and drivers are much more reliable than the output stage, which works with relatively heavy currents and high pulse voltages. A quiescent line output section is easy to troubleshoot with test-meter and oscilloscope – such things as dry joints, faulty base drive resistor or open base junction in the output transistor are usually responsible.

More often line output stage faults are manifest as excessive load- ing, leading to a high current drain on the power supply, which will usually invoke the latter’s overcurrent protection system; the end result, then, will often be a ‘pumping’ symptom, as will be described in the next chapter. First the e.h.t. rectifier or multiplier (if external to the l.o.p.t.) should be disconnected from the overwind. If the set now bursts into life replace the e.h.t. rectifier. Should the stage still not function, the output transistor and efficiency (where relevant, E−W modulator) diodes should now be checked for leakage. The can also be loaded by leakage in any other rectifier diodes it may feed, or by heavy loading of their outputs, e.g. a leaky or shorted A1 reservoir capacitor. If after exhaustive testing, and unloading of suspect components, the stage remains heavily loaded, the l.o.p.t. itself is suspect for short-circuit turns or (in diode-split types) faulty internal multiplier components. Shorting turns in the line scanning coils can give rise to similar symptoms, but this is rare; it can be checked by disconnecting the yoke.

During fault-finding it is often prudent to work with reduced volt- ages and currents in the line output stage to prevent damage, and this can be arranged by fitting a suitably heavy wire-wound resistor in the h.t. feed line to the stage, or by feeding the set from a variac – details in the next chapter.

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