PMDG 747

Does anyone know why the Display screens go blank after about 1 minute? i do start the APU but they go blank before its running.
thanks.
thanks.
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Do you switch the (cant think of the name to save my life) switch to Auto? (The one on the far left of the panel, on the same level as the APU start)
And you've engaged the APU Generators? (Lights illuminate about 30 secs after APU start is initiated)
Alex