434 Fault Finding of Control Head with Graphical Display TM8100/TM8200 Service Manual© Tait Electronics Limited November 200714.7 Keypad Backlighting FaultyThe keypad backlighting LEDs are controlled by two FPGA signals and twotransistors (Q2), resulting in four intensity levels (off, low, medium andhigh). The keypad backlighting LEDs are arranged in two groups for themain keypad and one group for the on/off keypad, each group consisting ofthree LEDs.One LED orOne Group of LEDsFaultyIf one LED or one group of three LEDs is faulty:1. Send CCTM command 1003 x (where x is the intensity: 0=off,1=low, 2=medium, 3=high) to switch on keypad backlighting.2. Check the 13.8V supply voltage of the relevant branch.3. From top to bottom, check the resistor, the three LEDs, and thetransistor of the relevant branch for continuity.All LEDs Faulty orIntensity Faulty If all LEDs are faulty or the intensity is faulty:1. Send CCTM command 1003 x (where x is the intensity: 0=off,1=low, 2=medium, 3=high) to switch on keypad backlighting.Figure 14.7 Circuit diagram of the keypad backlighting circuitryR607KEY BRIGHT03V3Q2147K10K3V3Q2247K10KR611KEY BRIGHT113V8R615D608Q603D610D607R613R3013V8R618D612Q609D611D609R61713V8R619D618Q610D606D605R614FPGAMain KeypadOn/OffKeypadR30: 4.7ΩD607: 1.9V (on)D610: 1.9V (on)D608: 1.9V (on)Q603: 1.9V (on)R615: 56ΩR617: 4.7ΩD607: 1.9V (on)D609: 1.9V (on)D611: 1.9V (on)Q609: 1.9V (on)R618: 56ΩR614: 4.7ΩD605: 1.9V (on)D606: 1.9V (on)D618: 1.9V (on)Q610: 1.9V (on)R619: 56Ω