Specifications

3-14
3.7.1 Hum and Other Noise Problems
If you hear undesirable hum from the speakers when you apply power to the CP500
and other projection room equipment, check the following list for possible causes.
1. Equipment grounding. All equipment including the CP500 is grounded to
the rack. To ensure good ground contact, install a starwasher to one
mounting screw per piece of equipment. Installation of star washers is
strongly recommended because electrical contact may not be achieved since
modern powder coat paints can be very tough.
2. Ground loops caused by audio signal wiring, especially to power amplifiers.
Be sure to check the booth monitor installation.
3. Projector power wiring. All mains wiring should be properly grounded.
4. Room lighting dimmer controls (SCR-TYPE).
5. Power amplifiers. Disconnect from the CP500 and ground the inputs to
determine if the power amplifiers are causing hum problems.
6. Solar cell wiring. (Analog film sound format selected). Check the shield
connections. Cell wiring should be placed away from mains and other
wiring. Cell wires must not be connected to the frame of the projector.
7. Exciter lamp power supply. Check for ripple on the DC power supply
outputs. Some old exciter lamp power supplies and emergency supplies
provide AC to the lamp. The resulting hum makes them totally unsuitable
for a Dolby film sound system. Such exciter supplies must be replaced.
8. Projection room lighting/solar cells. Ambient lighting, especially
florescent tubes, can leak into the solar cell area and cause hum.