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LAMP SUPPLY (Figure 27)
The Projection requires a minimum of 330 volts to operate. To ensure that sufficient voltage
is present, a Boost Regulator is used. A Boost Regulator has a higher output voltage than
the input voltage. The Lamp supply is located on the Power Input board.
RAW-DC-LAMP is applied to the Boost circuit via Fuse 1511. Voltage is applied to Pin 1 and
Capacitor 2544 via the current limiting resistor Rs in 7502. When the charge on 2544
reaches 5.8 volts, the PWM circuit drives the internal FET. When the FET is On, a field
builds up in Coil 5510. 5509 is a Ferrite bead to reduce RFI. When the FET turns Off, the
field in 5510 reverses. This voltage is rectified by Diode 6501, which charges Capacitor
2534. This will continue until the charge on 2544 drops below 4.8 volts. The Startup cycle
will then repeat. When the voltage across 2534 reaches 330 volts, Shunt Regulator 7506
turns On, turning Transistor 7500 On. The Control Voltage from the Audio Power supply now
becomes the operating voltage for Pin 1 of 7502.
Regulation is accomplished by the monitoring of the 330 Lamp voltage by Shunt Regulator
7506. If the 330-volt supply increases, 7506 will turn On harder, turning Transistor 7500 On
harder. The increase in current will increase the voltage across the sensing resistor Re inside
7502. The Regulator connected to Pin 1 will keep Pin 1 at a constant 5.8 volts. The voltage
across Re will be fed to the PWM, reducing the On time of the internal FET to reduce the
output voltage.
If the Switch mode circuit for the Lamp supply failed to operate, the voltage to the Lamp
would be equal to the RAW-DC-LAMP voltage. The symptom is this case would be that the
lamp would not ignite. Another possibility would be that 7502 might short. There would not
be Lamp voltage in this case. A Low output voltage might be caused by the feedback circuit,
7506 and 7500, failing to operate. A Scope reading on Pin 1 of 7502 would indicate that the
voltage would be changing between 4.8 and 5.8 volts. Pin 7 of 7502 would be pulsing On
and Off.
Page 20
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