The Mesaba Avro RJ-85's are not equipped with the air stairs, however there is a switch on the FA panel for them. This leads me to believe that they are mechanical. Mesaba uses the stair storage space for crew bags.
Not sure about the RJs but on the 146s airstairs were an option for both forward & rear doors. They slide out of the stowage to the aft of the forward door & forward of the rear door. They are lowered by gravity against hydraulic resistance & are raised by a hydraulic pump & slide in & out of thier stowage on a double track on the door sill. Here in the UK, FlyBe's 146s have forward airstairs & the exAirUK/KLMuk/Buzz aircraft had them at both forward & rear doors.
Worked on - Caravelle Mercure A300 A320 F27 SD3-60 BAe146 747-100/200/400 DC10-30 767 777 737-400 757 A319 A321
AWAC Does have the airstairs on a select few of their BAE-146'S. They are at both ends of the aircraft, and they work mechanically. Good for EGE and other ski resort cities.It makes it easy to offload a 100 seat BAE-146-300 from both ends, saves time.