Fluke 123B/S Service Manual - Page 120

Miscellaneous Functions, 5.1 Display and Back Light

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123/124 Service Manual 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: DISPON for +3.3V. b. MS420: MS414-415: MS417-418: MS412 MS411 MS409 DATACLK0 for 120 ns pulses LCDAT0,1 for 250 ns pulses LCDAT2,3 for 250 ns pulses LINECLK, for 120 ns pulses, ≅16 kHz FRAME, for 50 us pulses, ≅70 Hz M, for a ≅625Hz square wave. c. MS406 MS405 MS401 +5VA for +5V +3V3D for +3.3V -30VD for -30V (from Fly Back Converter). d. MS404 REFPWM1 for +3.3V. 3. Bad contrast. a. Check MS403 (CONTRAST) for a 60 kHz sinewave of 200 mVpp that rides on 0.8 Vdc. 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 6 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. c. Check TP601 and TP602 for a 7Vpp, 66 kHz, square wave. If not correct then check TP604 (TLON) for +3V3. If TLON is correct, then replace N600. d. Check (replace) V600, V602. 7-8

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123/124
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:
DISPON 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 50 us pulses,
≅
70 Hz
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) for a 60 kHz sinewave of 200 mVpp that rides on
0.8 Vdc.
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 6 for
≅
300
Ω
, 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.
c.
Check TP601 and TP602 for a 7Vpp, 66 kHz, square wave.
If not correct then
check TP604 (TLON) for +3V3.
If TLON is correct, then replace N600.
d.
Check (replace) V600, V602.