Mower deck just started throwing the belt immediately on startup. Put it back and it immediately throws it again.
All spindles appear not be bound up and when the transmission is engaged with the belt off the pulley seems to spin true.
I don't see any obstructions below or atop the deck and the blades can be turned via the spindles. I am presuming that the three blades don't require synchronization to avoid each other.
I can't recall when the belt was replaced but I don't think it is original as it it looks ok. Maybe replaced once since 1997 but it's not cracked or frayed.
Deck has been stored outside since new in 1997 and is in original condition except for noted belt.
The idler tension spring doesn't seem to require significant muscle to remove and replace it so I wondered if that might be the issue.