A control unit is provided with a selector and a plurality of value indicators. One of the value indicators provides a coarse indication of an existing value of an adjustable control quantity. The other value indicators are in another state, distinguishable from the value indicating state. A rotary adjustor provides a manual adjustment facility for a user. Selection of the selector puts the adjustable control quantity in an adjustment mode. In this adjustment mode, rotary actuation of the rotary adjustor allows adjustment of the control quantity. A new value of the control quantity is indicated by a change in the display states of the value indicators.