Hi there.
On the music video of A-ha's 'Take on Me' the keyboard in use was the Sequential Circuits Prophet 5 which can be viewed at the link below. This instrument was also used when they played the song on the UK's Top of the Pops show as I have both videos and have checked (Huge 80's fan always will be lol).
http://www.synthmuseum.com/sequ/seqpro503a.jpg

Hope that helps