Yeah, as mentioned, you really won't find a detailed service manual for that bike. Even back in the 70's when I was a teen the manuals you got for your bike were just generic things that applied to everything they sold. They sometimes told you how to do useful stuff but rarely.
Shimano has a good tech document site.
https://si.shimano.com/ And although they tend to make it hard, SRAM and others have the doc's out there that you need if you hunt for them.
But still, you aren't going to find a service manual that leads you step by step for troubleshooting. And the older your bike the more SOL you'll be.