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Methods of Checking
Repairs will be considerably simplified by careful observa-
tion of the defect and its location to a specific part of the
circuit. Once this has been done it can be said that 90%
of the repair has been completed.
To do this an understanding of the outline of the ICs used
in solid state colour T.V. will be of great assistance in
quickly
diagnosing the cause of trouble.
When checking, the following procedure is recommended:-
that the defect is in fact caused by the receiver and not
by a fault in transmission, by switching to alternative
channels.
circuit in the receiver.
ensure that these are correctly set. If adjustments have
no effect on the fault be sure to re-set to the original
positions.
block and, if defects are found, replace the transistors,
(a) Check the collector voltage and waveform of
transistors.
transistors.
(Generally 0.3 - 1 v for silicon
transistors and 0.1 - 0.5 v for germanium tran-
sistors).
Measure the terminal voltages and waveforms of
Check resistors, capacitors and coils.
When removing components from the PWB dis-
Note:
connect the leads by heating the soldered portions
with a soldering iron, removing surplus solder
with a desoldering tool.
COPPER FOIL
Figure 15.
TROUBLE SHOOTING GUIDE
PART
TOOL
Melt the solder and suck it
up by the desoldering tool.
PRECAUTIONS TO BE TAKEN DURING REPLAIR
In servicing, some of the methods used for valve systems
do not work on solid state colour T.V. receivers. Keep in
mind that:-
momentarily.
In the case of valves, damage is rarely caused even if the
anodes and screens are severely overloaded, provided
that this condition is not maintained for very long.
Transistors and ICs, however, should not be subjected
to overload even for a moment.
Be sure to switch off the power source when removing
or soldering leads or measuring resistance.
The test and replacement of power transistors must be
carried out in such a way that the transistor is kept in
its heat-sink. Valve checking is simple but with tran-
sistors testing is complicated by the fact that screws and
solder must be removed. Never test a power transistor
which is not securely fixed in its heat-sink otherwise
damage will be caused by overheating as power tran-
sistors consume large current.
To check small transistors, remove as quickly as possible
with a soldering iron avoiding excessive heating of the
transistor.
When checking voltages or waveforms take care not to
produce shortcircuits with the test probes or test clips
of the meter or oscilloscope.
During operation: -
DO NOT short the emitter resistor.
DO NOT short or open circuit the bias resistor.
DO NOT short the collector resistor.
DO NOT allow adjacent legs of ICs to come into contact
with each other.
Removed.
If the vertical output circuit is operated with parts
removed a large flyback pulse may be produced which
can cause short-circuting or deterioration of transistors.
oscillation such, as sparking the high voltage circuit and
stopping the oscillator circuit must be avoided.
If the high voltage circuit is checked by sparking the
driver or the input pulse is cut off abruptly with the
horizontal output circuit operated, the output transistor
may be damaged. The output transistor is designed to
withstand high voltages but if it is heated or subjected
to abnormal conditions for long periods the resistance
against voltages' will be considerably reduced, and the
transistor may be damaged if subjected to an input
circuit shock (cutting off the input instantly) or output
circuit shock (sparking the high voltage circuit).
Wires must be carefully arranged after check and repair.
(Be sure to connect them to the original places taking
care not to induce high voltages or other circuitry).
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Troubleshooting
