Sony KDP-51WS550 Training Manual page 16

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If the 9V line comes to proper level but the 5V line is 0V, check for a shorted condition in the circuits
the 5V line supplies.
2) The Q-Box Power Supply can be safely tested by unloading the +9V and 5V lines. Disconnect CN6902.
This will completely unload the power supply, but still send necessary feedback to IC6900 for proper
power supply operation.
3) If the power supply outputs proper secondary voltages after CN6902 is disconnected, suspect a shorted
+9V or 5V line
4) If the unit still goes into protection mode after disconnecting CN6902, check the following items:
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D6914, D6902, D6903, D6905 and D6904 (15V Zeners) for shorted condition.
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Check IC6900/pin 8 (B+ OVP) for proper voltage level (within 8V to 33V window).
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IC6900/pin 15 (this is midpoint for output MOSFETS Q6507 and Q6506). A 120Vpp, 90KHz
square-wave should be present. If the waveform pulses four times and stops this indicates that
IC6900 is attempting to operate and is most likely good.
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IC6900 Feedback circuit. The amount of current feedback can be determined by monitoring the
voltage drop across R6932. The minimum current feedback is 80uA, which is a minimum voltage
drop of 37.6mV. In normal operation, this voltage drop is approximately 223mV. If the voltage-
drop is below 37.6mV, suspect components in the feedback path e.g. PH6502 and IC6503.
Reference Figure 3-3 for proper voltages at these components.
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If no waveform is present when unit is turned ON, then check IC6900/pins 1 and 18 for proper
voltages (reference "IC6900 Startup Sequence" above) before shut down. If these voltages check
OK, suspect a defective IC6900.
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Check IC6900/pin 8 for proper voltage of 24V. This is the voltage used to power IC6900 after
initial startup (in the ON Mode).
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Check IC6900/pin 10 for 10.2V. This voltage is initially developed using the 296V at IC6900/pin18
and a voltage regulator circuit inside IC6900. If 10.2V is missing, suspect a defective IC6900.
Also check voltage at IC6900/pin 14 (Use battery powered DVM for this measurement, and
use IC6501/pin 15 (VS) as ground ref.). The voltage at IC6501/pin 14 should be 10V. If the
voltage measured at IC6900/pin 14 is incorrect, check D6901.
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3. AX-1 Chassis Power Supplies