There was no service manual issued for distribution for the FT-301/301D.
I looks like the same ugly bad scanned PDF of the instruction manual can be found on several web sites. The manual does include individual badly scanned schematics of the various boards and some pictures of the fully populated boards, but there are no board layouts included. Whatever service related info that can be had, comes from this manual.
If you want one that can be fully read and clear even down to the smallest font, I have it available from my web site.
Pete, wa2cwa
www.manualman.com