Philippe, I'm not sure if there's someway of adjusting the tension, I'd have to try and find my De Vere service manual. Usually with Durst's there's a friction pad to stop enlarger creep and this wears over time and needs adjusting, but I've not had a problem with a De Vere.

You can lock the head or focus with the knobs at the back just above the baseboard at the back, where the column attaches to the base.