If you’re looking for a manual for an old synthesizer, here are some resources that may be helpful:

• • The Internet Archive offers hundreds of vintage synth manuals as free downloads. Most are available as PDF files.
  • Lojinx has a collection of operation manuals, service documents and schematics for a selection of analog synthesizers and electronic musical instruments.
  • Synth Zone features an index for owners manuals and documentation for vintage synthesizers.
  • Synthfo has a collection of vintage synth service manuals that you can download.
  • Tiny Loops has a collection of quick guides to vintage synth gear.
  • Vintage Synth Parts offers a collection of manuals, with an emphasis on service manuals and schematics.

These are generally available available as PDF downloads. The documents are of mixed quality, because many of them are scanned versions of vintage synth manuals.

The PDFs are of copyrighted material that’s unavailable, so get what you need and watch out for people trying to resell this stuff.

If you’ve got suggestions for other resources, leave a link in the comments!