Troy bolt 30" deck popping belt off

[PopPop2Spud]

Hello folks first time posting and I'm looking for some assistance, I've never worked or owned a riding mower before so I'm looking to y'all for help.

I bought this mower used and it worked the 1st time I used it and then the 2nd time the belt kept popping off so I got a new belt and it's still popping off.

I'm not sure what's up with it, I e been searching around the web and can't seem to find anything to help but I did notice this spring isn't attached and I'm not sure where it goes and it seems it may be an issue.
The spring is attached to the back of what I believe is the belt tensioner.
I'll add a photo and hope some of y'all could chime in and help me out.

 

[bertsmobile1]

Well that narrows it down to about 50 different mowers since 1990
Under the seat is an ID tag with a number starting with 13
Take a picture of the complete tag & post it please so we can work out which variation you have & what deck

 

[PopPop2Spud]

Here's a picture of the tag.