Teclast X21, Service Manual

Page: 31 / 76

EN
Q2
MMST3906
R12
100K
C14
475/X5R
R19
2M
Q5
MMST3904
Q3
SS8050LT
D4
1N4148WS R14
47K
R17
100K
R10
100K
R15
470K
R23 1M
R13
5.6M
D3
1N4148WS
R9
10K
R8
10K
P_GND
P_VDD
OFF 2
P_GND
VCC33
START 3
START
START
Figure 3.3.1.2 ON/OFF circuit schematic diagram
Example 3 Symptom: computer not connected
Description: when connecting to computer, no reaction and o screen display
Analysis and troubleshooting: firstly confirm whether charge and power on are available, and the
result is normal. Use diode position of multimeter and anode of pen to ground, test whether the
resistance value to ground of D+, D- data signals in the position of USB terminal is close to several
hundred omh, short-circuit could not appear between D+ and D, when testing we find that D+ is normal,
D- resistance value is infinite, USB terminal has false welding. After welding again, trouble is removed.
Example 4 Symptom: only one side of headphone has output
Description: only one side of headphone has output and the other has not.
Analysis and troubleshooting: firstly decide the trouble lies in tuning components or in common part,
whether only one side has sound when listening and result is normal. Tuning circuit is shown in the
figure 3.3.1.3, tuning part components reaches the main board through a flat cable holder, radio station
program can be received and this shows that power supply, data and clock are all normal, use
oscillograph to test left and right channel output of FM directly and find that FM-OUTL has no waveform
output, then check tuning board output part and waveform is normal, circuit at back is connected through
a flat cable holder, press the flat cable holder and it is ok, after losing, trouble appears again, so we
decide that flat cable holder has trouble, after changing it trouble is removed.
IIC_SCL
IIC-SCL
FM_3V3
FM_OUTL
IIC-SDA
FM-OUTR
D_GND
CN1
CON2X4
8 7
6 5
1 2
3 4
IIC_SDA
FM_ANT
FM_OUTR
FM-OUTL
Figure 3.3.1.3 Tuning circuit schematic diagram
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