430 Fault Finding of Control Head with Graphical Display TM8100/TM8200 Service Manual© Tait Electronics Limited November 200714.4 LCD Backlighting FaultyThe backlighting incorporated in the LCD module is controlled by a dataline 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 implementedby pulse-width modulation.For a circuit diagram and PCB layout, refer to Figure 14.3 on page 428 andFigure 14.4 on page 429.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” onpage 4263. 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 bereplaced.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 isopen or shorted. Check the signal at pin 8 of connector J102.If the signal is correct, replace the LCD.Q102 pin 1 (S1): 3.3VQ102 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)
