Teclast X21, Service Manual

Page: 33 / 76

Example 7 Symptom: no screen display
Description: screen is not on and there is no display
Analysis and troubleshooting: firstly confirm the trouble appears in power supply and screen itself,
after power and listen, the sound is normal, so we preliminarily decide that power supply 3.3V, 1.8V are
basically normal. Screen display control circuit is shown in the figure 3.3.1.6. Test voltage on two ends of
L25 and it is about, which is normal, then test voltage on two ends of D8 and find that anode has no 13V,
remove screen and there is still no13V. Check input end voltage and enable end is basically normal,
then check feedback pin FB voltage and it is on low side obviously, and capacitor C68 also has trouble.
After changing it, trouble is removed.
CN4
GND
L
C
D
C65
16V/104
R72
0R
R71
100K
L24
601
L25
CDRH3D16-4R7
R63
4.7K
VOLED
R74
0R*
R75
27K 1%
C68
22P
C71
475/X5R/6.3V
C63
103
C72
106/25V/1206
R64
4.7K
C67
104
R73
0R
P_VDD
VCC33
GND
L
C
D
_
3
V
3
EN1
2
_
R
E
S
E
T
2
L
C
D
_
R
D
2
L
C
D
_
D
B
1
2
L
C
D
_
D
B
4
2
L
C
D
_
D
/C
2
L
C
D
_
D
B
0
2
L
C
D
_
D
B
7
2
L
C
D
_
D
B
3
2
L
C
D
_
W
R
2
L
C
D
_
C
S
2
L
C
D
_
D
B
2
2
/R
E
S
E
T
1
,2
L
C
D
_
D
B
6
2
L
C
D
_
D
B
5
2
V
O
L
E
D
R65
2M
L26
0R
R70
240K 1%
D8
MBR0520/SOD123
U10
TPS61040/LM2703
EN
4
VIN
5
FB
3
GND
2
SW
1
V
D
D
2
0
D
/C
#
1
5
D
5
7
N
C
1
V
C
O
M
H
3
D
3
9
W
R
#
1
4
IR
E
F
4
R
E
S
#
1
6
R
D
#
1
3
V
C
C
2
B
S
2
1
8
D
1
11
C
S
#
1
7
G
N
D
2
1
B
S
1
1
9
D
0
1
2
D
6
6
D
4
8
D
2
1
0
D
7
5
Example 8 Symptom: screen trouble
Description: screen display has error and disorder
Analysis and troubleshooting: servicing of screen trouble is similar to that of no screen display and
the difference is that power supply may not be considered, that is, power supply should be normal.
Firstly confirm the trouble lies in screen itself or the circuit, change display screen and trouble still exists,
then use oscillograph to test 8-bit data on OLED display screen, we find during the test that flat cable
holder of display screen is false welding, after welding again trouble is removed.
Example 9 Symptom: noise when recording
Description: when playing record files, there is noise.
Analysis and troubleshooting: recording circuit is shown in the figure 3.3.1.7 and composed of
microphone and microphone biasing circuit.
Figure 3.3.1.6 Screen display control circuit schematic diagram
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