Mower deck Husqvarna CTH 173 deck belt problem

Hi all.

Since I bought this ride on new 2 years ago, the deck belt drops off the pulley every time I stop the cutting blades.

The only way to get it back on is to stop the engine, and manually replace the belt on the front pulley.

A real pain in the a**!

Is there a tensioner? I've had the deck off several times to clean it and find a cure, but no joy.

Last time I used it I got so fed up, that I put a cable tie across the front pulley U shaped guide bracket. It has stopped it dropping off, but not a very good fix.

Any other suggestions?

Thanks in advance.

Russ.