Miscellaneous Functions; Display And Back Light - Fluke ScopeMeter 123 Service Manual

123
Service Manual

7.5 Miscellaneous Functions

7.5.1 Display and Back Light

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
2. Defective display
3. Bad contrast.
4. Defective backlight:
7-8
is not repairable.
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
LINECLK, for 120 ns pulses, ≅16 kHz
MS412
FRAME, for 250 ns pules, ≅66Hz
MS411
M, for a ≅625Hz square wave.
MS409
c. MS406 +5VA for +5V
MS405 +3V3D for +3.3V
MS401 -30VD for -30V (from Fly Back Converter).
d. MS404 REFPWM1 for +3.3V.
a. Check MS403 (CONTRAST), see Figure below:
≅ 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.
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.
Warning
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≅ 50 mV