A Citation Single pilot experienced a cabin altitude warning during his climb; declared an emergency; descended and returned the aircraft to the service center where it had recently undergone routine maintenance service. After a safe landing the pilot discovered the Air Source Select knob had been placed in the 'OFF' position; possibly during the previous maintenance visit; thus depriving the system of the air required to control cabin altitude. Subsequent flights with the knob in the 'BOTH' [sources] position have proven trouble free.

Narrative

During climb my 'Cabin ALT' annunciator was activated. My cabin pressure had increased to approximately 10;000 FT. Under normal operations my cabin altitude should have been below 8;000 FT.I was already using the supplemental O2 system.I declared an emergency and requested an immediate descent which was approved along with a turn back toward my departure airport when able. I began an emergency descent then turned to the heading given by ATC. At that time I was unsure as to the cause of the sudden rise in cabin pressure and was focused on making a controlled descent while on supplemental O2. I then requested a return to the Cessna Citation Service Center airport where my airplane had just completed routine scheduled maintenance. It had not been flown since the maintenance was completed other than a short flight back to my own airport.After landing I began trouble shooting the probable causes associated with a 'Cabin ALT' annunciator and determined that the 'Air Source Select Knob' had been set to the 'OFF' position. I then confirmed with the Service Center that switching the Air Source Select Knob to 'OFF' would result in a lack of pressurization consistent with what I had experienced. I have since flown the aircraft at flight levels without incident.Normal operation of the aircraft dictates the Air Source Select Knob always is in the 'Both' position; although the Cessna approved checklist indicates that the knob be set 'AS REQUIRED.' I believe the Air Source Select Knob was turned off during the recent maintenance at the Cessna Citation Service Center by the avionics technician - probably while he was completing an RVSM documentation that was due. I feel declaring an emergency was an appropriate action.I have added the word 'BOTH' next to the 'AS REQUIRED' on my checklist.