Idzerda
This sign was on the wall in the studio of broadcast pioneer A Steringa Idzerda. It means "When lit: shut up!" It is still in the broadcast museum in Hilversum.
(Thanks to Martijn Reneman)
In Studio Amstel
Upper two: Silence please, radio transmission
Lower two: Please close doors softly
(Thanks again to engineer Geert Kramer who took a camera to Amsterdam to make these pictures)
In the former AVRO building
(Thanks again to engineer Geert Kramer who saved this sign before it disappeared for ever)
Around TV studios