Don't 'cha hate those places that sell you User Manuals (of once freely available PDF downloads)?
Generally, the original supplier should just support these legacy devices by hosting a few PDF files for the tinkerers and hackers to help give a nice long life to some older devices. But alas, most business' quickly sever support for older devices, so they can focus on creating a new product to sell you. So much for legacy archival formats destroyed by the internet...
I've run into the issue with my second-hand DAT machine, where I couldn't find any free downloads, only paid. Of course, if you really hustle, everything is available free on the internet.
Sony DTC 690 DAT Recorder
Found the User Manual still available on Sony's Site, and archived here for my needs later...
SONY DTC 690 USER MANUAL (4.7mb PDF Download)
Then, I found the service manual available here, surprisingly as a multipart RAR file, but it was quick and easy to download all 7 parts and get a nice new PDF out of it: