Teclast X21, Service Manual

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3.3.1 Servicing cases
Example 1 Symptom: power not on
Description: no screen display and no output
Analysis and troubleshooting: firstly check whether this trouble is caused by battery or by power
circuit, connect to power adapter and check whether charge is available and whether screen has display.
Power on and there is no display, check power supply 3.3V and 1.8V and find that 3.3V lowers to 0.6V,
and voltage in normal working condition should be 1.8V, 3.3VDC-DC switch circuit is shown in the figure
3.3.1.1, use power adapter to supply power, 5V input is normal, charge enable end is high level, which is
also normal, observe carefully and find that pin 6 of Ic9301 has false welding, after welding again trouble
is removed.
EN
U1
TPS79301 SOT 23-6
VIN
1
GND
2
EN
3
BYPASS
4
FB
5
VOUT
6
C6
102
R2
20K
1%
VCC33
C2
106/X5R
R1
33K1 %
C4
103
C1 22PF
C3
106/X5R
R3
68K
VCC33
P_GND
P_GND
P_VDD
P_GND P_GND
Example 2 Symptom: machine not switched on
Description: no screen display, no response after pressing ON/OFF button
Analysis and troubleshooting: firstly confirm whether it is machine not switched on or it is power not
on, connect to adapter or computer and check whether display on screen is normal, connect to adapter
and charge can display, which means that power and rear stage circuit working are basically normal,
ON/OFF circuit is shown in the figure 3.3.1.2 , the working principle is that after a light touch switch S2
closes, a loop appears and s signal Start produces, triode Q2 is on and gives a enable signal to power
control, test S2 switch, press down and check whether it is on, during test we find that S2 has false
welding. After welding again, trouble is removed.
Figure 3.3.1.1 3.3VDC-DC switch circuit schematic diagram
Section Three Servicing Cases
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