412 Fault Finding of Control Head with Graphical Display TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
14.4 LCD Backlighting Faulty
The backlighting incorporated in the LCD module is controlled by a data
line from the FPGA, which switches a transistor on MOSFET Q102.
Note The LCD backlighting has four brightness settings: off=GND,
on=3.3V, and two intermediate settings which are implemented
by pulse-width modulation.
For a circuit diagram and PCB layout, refer to Figure 14.3 on page 410 and
Figure 14.4 on page 411.
If the LCD backlighting is faulty:
1. Make sure that LCD backlighting has been enabled in the program-
ming application.
2. Check the 3.3V supply voltage at pin 1 (S1) of Q102.
If the signal is correct, continue with Step 3.
If the signal is not correct, refer to “Power Supply Faulty” on
page 408
3. Send CCTM command 1004 3 to switch on LCD backlighting.
4. Check the signal at pin 2 (G1) of Q102.
If the signal is correct, continue with Step 5.
If the signal is not correct, visually inspect pin 2 for open contact.
Otherwise the FPGA is faulty and the control-head board must be
replaced.
5. Check the signal at pin 7 (DA1) of Q102.
If the signal is correct, continue with Step 6.
If the signal is not correct, replace Q102.
6. Visually inspect whether the contact of pin 8 of connector J102 is
open or shorted. Check the signal at pin 8 of connector J102.
If the signal is correct, replace the LCD.
Q102 pin 1 (S1): 3.3V
Q102 pin 2 (G1): GND (with backlighting switched on)
Q102 pin 7 (D1A): 3.3V (with backlighting switched on)
J102 pin 8: 3.3V (with backlighting switched on)
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