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OM8838/TDA8842 series color TV sets rely more on the “soft start and slow stop”
function set inside the IC chip and the “automatic fault diagnosis” or logic protection function
on some ports to realize protection for the circuit. In case of a fault in horizontal scanning,
vertical scanning and video amplification circuits, the whole machine will be in the faulty
status of “three no’s” fault.
The primary meaning of “soft start and slow stop” function is that, in case of oscillation
stop in horizontal scanning output, excessive electron beam current, oscillation stop in vertical
scanning output, it will immediately enter the “slow stop” process from the internal horizontal
triggering output, and finally cut off the triggering output, resulting in the “three no’s” fault.
Because of the internal protection provisions, even if the microcomputer-based monitoring
system fails at this time, it will not damage other elements. Therefore for the repair of this
“three no’s” fault in the chassis, it is permitted to cut off the power supply protection system
of the microcomputer (CPU), then to monitor the change in voltage at various critical points
quickly after “forced on”, so as to analyze the location of the fault. The “soft start” means in
each turning on, there is a transition period from start to normal, without exception even for a
TV set with fault. “Slow stop” means there is a transition period from normal to turning off
once a fault functions. During the transition period, the voltage at a point without fault
changes in the order of starting, normal and waiting, and the voltage at a point with fault
changes in the order of starting, abnormal and waiting. Therefore each time after turning on
again, check is started, the variation range of instrument measured values are observed in time
within the valid time period, then any circuit or element with fault is determined according to
the monitoring results during the transition period following turning on.
For the flow diagram of “three no’s” fault, see Fig. 13.
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