so.. if you do not care how much spending..
You will need a good quality tester like
https://www.aliexpress.com/item/4001155350991.htmlseveral other models also available - this is plain standard.
Basic faults on PANELS are LED lamps and/or input mosfets..
Rare cases you have bad caps. they are high quality. Made to last.
Input is over stressed by LAMP panels and heat backside ..
booster diodes and fault gates also fail.
You need a schema . just google around best close match
Poking the primary side of SMPS panels is DANGEROUS
You need a proper understanding.
Paul