Fluke 123B/S Service Manual - Page 121

Fly Back Converter, incorrect

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7 Corrective Maintenance 7.5 Miscellaneous Functions 5. Backlight brightness control not correct: Check the TP605 (BACKBRIG, supplied by D-ASIC D471) for a 25 kHz, 3.3 V pulse signal. The duty cycle of the pulses controls the back light brightness. The backlight brightness increases with an increasing length of the high pulse. Check V604, R604. 6. Measure the voltage on the collctro of V605: - correct voltage 1.5 V - >1.5 V : N600 defect - 4.8V. 3. Check TP552 (FLYGATE) for a square wave voltage of at least some volts (for a correct Fly Back Converter 50...100 kHz, ≅10 Vpp). a. If a square wave is present on TP552 (may be not the correct value), then: 1. Check the voltage on N501 pin 55 (FLYSENSP). For a correct converter this is a saw tooth voltage of 50...100 kHz, 50...150 mVpp). } 50...150 mV a. If no sawtooth voltage is present on R501, no current, or a DC current flows in FET V554. The primary coil or V554 may be defective (or interrupted connections). Check R504, R506, R507 (battery current sense resistors); these resistors may be fused due to a short in FET V554. b. If an incorrect sawtooth is present on R501 this can be caused by: -overloaded outputs (Frequency low, e.g. 100 kHz;

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Corrective Maintenance
7.5 Miscellaneous Functions
7
7-9
5.
Backlight brightness control not correct:
Check the TP605 (BACKBRIG, supplied by D-ASIC D471) for a 25 kHz, 3.3 V
pulse signal. The duty cycle of the pulses controls the back light brightness.
The
backlight brightness increases with an increasing length of the high pulse.
Check
V604, R604.
6.
Measure the voltage on the collctro of V605:
- correct voltage 1.5 V
- >1.5 V :
N600 defect
- <1.5 V : secundary circuit defect (V606, V603, replace both if one is defective!)
7.5.2 Fly Back Converter
1.
Check the voltages on TP572 (+5V), TP573 (+3.3V), TP574 (+3.3V), TP576
(-3.3V), TP577 (-30V) on the POWER part.
a.
If one or more voltages are correct, then check the rectifier diodes
(V561...V564), and coils (L562...L567) of the incorrect voltage.
b.
If none of the voltages is correct, then the fly back converter does not run
correctly, continue at step 2.
2.
Check TP504 (VBATT) for
>4.8V.
3.
Check TP552 (FLYGATE) for a square wave voltage of at least some volts (for a
correct Fly Back Converter 50...100 kHz,
≅
10 Vpp).
a.
If a square wave is present on TP552 (may be not the correct value), then:
1.
Check the voltage on N501 pin 55 (FLYSENSP).
For a correct converter
this is a saw tooth voltage of 50...100 kHz, 50...150 mVpp).
}
50...150 mV
a.
If
no
sawtooth voltage is present on R501, no current, or a DC current
flows in FET V554.
The primary coil or V554 may be defective (or
interrupted connections).
Check R504, R506, R507 (battery current
sense resistors);
these resistors may be fused due to a short in FET
V554.
b.
If an
incorrect
sawtooth is present on R501 this can be caused by:
-overloaded outputs (Frequency low, e.g. <<50 kHz; 250 mVpp)
-underloaded outputs (Frequency high, e.g. >>100 kHz; <<100 mVpp)
-bad FET V554 (Sawtooth voltage is not linear).
2.
Check R570 and VCOIL connections.
b.
No FLYGATE square wave is present.
Check TP526 (FREQPS) for a 50...100 kHz, 3.3 Vpp square wave. If no square
wave on TP526, then go to step 4.
4.
Check TP528 (PWRONOFF) for +3V.
If not correct, see Section 7.5.13 Power
ON/OFF.