Section Three Servicing Cases
3.3.1 Servicing cases
[ Example 1 ] symptom: machine not power on
Trouble description: after inserting power cord, standby indicator light is on. After pressing power
switch, green indicator light flashes once and the machine power off.
Analysis and troubleshooting: according to trouble symptom, we judge that trouble mainly lies in
power system. Firstly check whether power board can work normally. Test whether power board flat
cable +12V is normal when power on; unplug the 6P power supply flat cable from power board to main
board, after connecting with power, use multimeter to check whether 12V voltage output in power board
flat cable holder is normal; insert the flat cable from main video board to power board and check whether
voltage output is normal; press POWER button, after voltage skips to low level, machine power off, so
we doubt that power board cannot load. To further conform the causes, unplug power cord, connect a -
10 ohm resistor in power board flat cable 12V position to ground (or change a power board for
test), after connecting with power, voltage in this position changes all the time. According to the
symptom, we can judge that power board cannot load. The reason is that 12V filtering capacitor has
electric leakage/switch transformer has trouble, or the preliminary reverse clipping circuit cannot work
normally, or Ic1377 protects ahead of time. After test, we find that the secondary rectifying and filtering
circuit is normal, so we judge that trouble is caused by preliminary part. After power off, we check one by
one and find that Vd2 in reverse clipping circuit has cut off. Change diode with the same specification,
trouble is removed.
[ Example 2 ] symptom: power not on
Trouble description: after inserting power cord, indicator light is not on. Press POWER button and
machine cannot power on.
Analysis and troubleshooting: according to trouble symptom, we judge that trouble lies in power
circuit and system circuit. Firstly make sure whether power board has +12V voltage output; unplug 6P
flat cable in Cn6 position, check voltage and 12V output is normal, which means that power board works
normally. Press POWER button, check 12V voltage in Cn6 position and it is normal, which means there
is no load or short-circuit on main board. Check 5V voltage of pin 2 of main board U503 and it is normal.
Test after words and find that two ends of Ce503 have no 5V voltage output, so we doubt L503 has
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