Hi.
Most likely there is a tiny backupbattery at the motherboard, and sometimes they are soldered and not that easy to replace.
The Servicemanual is to find at PSR Tutorial too, hopefully it's OK to paste the link without any risk to be banned there or here:
https://psrtutorial.com/lessons/workshops/ServiceManuals/PSR-1000-2000_ServiceManual.pdf
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Cheers 🥂
GJ
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"Success is not counted by how high you have climbed
but by how many you brought with you." (Wil Rose)