Dolby Laboratories CP65 - Manual (Page 23)

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4. Power amplifiers. Disconnect from the CP65 and ground the inputs to
determine if the power amplifiers are causing hum problems.
With CP65 FADER turned up and format 04 selected:
5. Solar cell wiring. Check the shield connections. Cell wiring should be
placed away from mains and other wiring.
6. 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 stereo playback system. Such exciter supplies should be replaced.
7. Projection room lighting/solar cells. Ambient lighting, especially florescent
tubes, can leak into the solar cell area and cause hum.