This was my first time replacing all belts on my 1979 240D.
Took the old belts out, put the new belts in. The power steering pump belt I was able to retension just fine.

However, when it came to the alternator-water pump belt, I'm having problems with the tensioning screw, it seems to be slipping, leaving the belt very loose. What to do?

Also, in the case of the refrigerant compressor belt, how do I put the tensioning roller in tension before tightening the fastening screw? I have no idea how that can be achieved.
I've followed the instructions from my service manual CD to the letter (job 13-340 Tensioning V-belts)...it simply says (when covering the refrigerant compressor belt), "Tension V-belt by swivelling tensioning roller" then "Tighten fastening screw." But how do you do that, specifically?

TIA very very much for your help. Right now I have the car sitting in my garage, and I really need to get it going ASAP.

__________________
1979 240D, W123, 105K miles, stick, white w/ tan interior.