Analysis and troubleshooting: if there is disturbance after tapping machine, a certain or some
elements in circuit may probably be not contacted properly. According to trouble symptom, trouble lies in
the connecting board between display screen and main board. After checking machine flat cable, there
is no abnormality; then take down connecting board, check carefully and find that there is a certain
spring sheet in the socket on one end connecting with screen has sunk. Change connecting board flat
cable holder and trouble is removed.
[ Example 6 ] Symptom: black screen when power-on
Description: from standby to power-on, screen is black all the time and panel indicator light can
display normally.
Analysis and troubleshooting: according to servicing experience, if screen is black when power-on,
firstly place the head of oscillograph near high voltage of boost board and find that oscillograph has no
inductive waveform, so we can make sure that boost board has not outputted high voltage. Shown in the
figure 3.3.1.1, use multi-meter to test +12V power supply voltage of CN4 and it is normal. Use multi-
meter to test ON/OFF-INVERTER end and it is high level when in standby and low level in power-on in
normal conditions, test Q102 B-E and it is connected, so we doubt that triode Q102 has trouble; use
multi-meter diode level to test Q102 and triode Q102 has been stricken through. Change Q102 and
trouble is removed.
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Figure
3.3.1.1
Inverter components control circuit
[ Example 7 ] Symptom: remote control has no function
Description: when pressing buttons on remote control, TV screen has no response.
Analysis and troubleshooting: test voltage of 5V-IR pin of Cn105 and it is +4.96, which is normal.
When power is not on, test elements of L148,R342 between pin 1 of CN105 and chip U101 and they are
all normal. Test pin 4 of remote control receiving board Cn801 to pin 1 of IR sensor and it is normally
connected. After power on, test voltage of pin 1 of CN105 and it is +4.98V, which is also normal. Press