User's Manual

123
Service Manual
7-8
7.5 Miscellaneous Functions
7.5.1 Display and Back Light
Warning
The voltage for the LCD back light fluorescent lamp is >400V!
1. Connect another LCD unit to see if the problem is caused by the LCD unit. The unit
is not repairable.
2. Defective display
Check the LCD control signals on measure spots MS401...MS422 (near to X453).
Use a 10:1 probe with ground lead on the probe connected to the metal screening of
the UUT. Notice that MS407 is missing !
a. MS422: LCDONOFF for +3.3V.
b. MS420: DATACLK0 for 120 ns pulses
MS414-415: LCDAT0,1 for 250 ns pulses
MS417-418: LCDAT2,3 for 250 ns pulses
MS412 LINECLK, for 120 ns pulses, ≅16 kHz
MS411 FRAME, for 250 ns pules, ≅66Hz
MS409 M, for a ≅625Hz square wave.
c. MS406 +5VA for +5V
MS405 +3V3D for +3.3V
MS401 -30VD for -30V (from Fly Back Converter).
d. MS404 REFPWM1 for +3.3V.
3. Bad contrast.
a. Check MS403 (CONTRAST), see Figure below:
}
≅
50 mV
≅
0.8V
≅
15 ms
If not correct check FRAME signal on V401 for 0...3V, 250 ns pulses, 66Hz;
check PWM circuit (Section 7.5.14); check V401-V403.
b. Check MS408 (LCDTEMP1) for +1.6V at room temperature (to SLOW ADC).
If not correct, check R591 in SLOW ADC part.
4. Defective backlight:
a. Turn the test tool on, and monitor the voltage on T600 pin 3 or pin 5 for a 8 Vpp,
66 kHz, half rectified sine wave. If a half rectified sine wave, with an increasing
amplitude, is only seen for about 0.2 second directly after power on, then the
secondary circuit is defective. Install a new LCD unit. If this does not cure the
problem, check the resistance between T600 pin 10 and 11 for ≅300Ω, replace
V603, V605. Check C606!
b. Check T600 pin 3 and pin 5 for a 8 Vpp, 66 kHz, half rectified sine wave. If it is
present on only pin 3 or pin 5, then replace V601.