Just a few days ago I was also searching for TOE 7404 schematic, service manual.
I sent 2 mails to Toellner, but received no reply.
(the manual on that link posted by coromonadalix was uploaded after that)
I have the newer model with frequency counter, and a completely different all smd PCB.
There is a 6 page long pdf that I've found with a quick search, that should have 16 pages with schematic and calibration instructions, but unfortunately it is cropped.