Saab: heater panel 900se 1997

Hey,

The knob which controls the direction of the heater seems to 'free fall',
I.e will more without actually changing the direction of the flow of
hot/cold air. Unfortunately for me it is stuck in the 'down' position!
Therefore makes clearing the windscreen that little bit more challenging on
these cold mornings!

I removed the lid and can manually move the white coloured arm around
manually, but again it appears to be in free fall, so it is not a broken
knob so to speak. Any ideas if this unit is repairable? Can I pull it
apart easily and establish what the problem is or do I have to replace the
whole lot?

many thanks in advance.
A