Repair Tips - Philips 20PT643F Service Manual

Repair Tips

Below some failure symptoms are given, followed by a repair tip.
Set is dead and makes hiccupping sound. 'Main Power Supply' is
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available. Hiccupping stops when L5561 is de-soldered, meaning that
problem is in the 'Main Power Supply' line. No output voltages at LOT, no
horizontal deflection. Reason: line transistor 7460 is defective.
Set is dead, and makes no sound. Check power supply IC 7520. Result:
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voltage at pins 1, 3, 4, 5 and 6 are about 180 V and pin 8 is 0 V. The
reason why the voltage on these pins is so high is that the output driver
(pin 6) has an open load. That is why MOSFET 7521 is not able to switch.
Reason: feedback resistor 3523 is defective. Caution : Be careful
measuring the gate of 7521: circuitry is very high ohmic and can easily be
damaged!
Set is in hiccup mode and shuts down after 8 seconds. The blinking LED
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(set is in SDAM mode) indicates error 5. As it is unlikely that the
microprocessor 'POR' and '+8V protection' happen at the same time,
measure the '+8V' supply. If this voltage is missing, check transistor 7480.
Set is in non-stop hiccup mode. Set is in over-current mode; check the
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secondary sensing (opto coupler 7515) and the 'Main Power Supply'
voltage. Signal 'Stdby_con' must be logic low under normal operation
conditions and goes to high (3.3 V) under standby and fault conditions.
Set turns on, but without picture and sound. The screen shows snow, but
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OSD and other menus are okay. Blinking LED procedure indicates error
11, so problem is expected in the tuner (part reference number 1000).
Check presence of supply voltages. 'Vlotaux+5V' voltages at pin 5 and 7
are okay; 'VT_supply' at pin 9 is missing. Conclusion: resistor 3460 is
defective.