Fly Back Converter - Fluke ScopeMeter 123 Service Manual

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.
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).
a. If no sawtooth voltage is present on R501, no current, or a DC current
b. If an incorrect sawtooth is present on R501 this can be caused by:
2. Check V552 and V553, 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 correct,
then check V552, and V553. If no square wave on TP526, then go to step 4.
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.
-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).
}
50...150 mV
Corrective Maintenance
7.5 Miscellaneous Functions
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