Here's the maintenance manual circa 1970:
http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp8/pdp8s/F-87S_8sMaint_Oct70.pdf It has schematics at the end.
Bitsavers has a wealth of documentation on old machines.
And as far as cost per performance, the 8S and the rest of the 8 line existed because there was a market for a computer that a lab could afford. The price was the first consideration, performance the second. As a laboratory instrument, even a slow computer could collect, analyze, and record data faster than a graduate student. Labs (academic, industrial, and government) was the market target for a number of the little guys at the time. (DEC wasn't entirely alone.) The 8 and 11 series showed up everywhere from nuclear blast detectors, to animal environment controllers and recorders, numerically controlled machine tools, access/perimeter control, academic computing services, coal mines, and even the logo/light controller for the Westinghouse building in downtown Pittsburgh.
I think there were two things that made the line successful in the market: It was relatively inexpensive, and priced so that it wouldn't require approval in most companies to go all the way to the CEO or board. And the PDP-8 and PDP-11 series were made for their peripherals. There were lots of peripherals in the catalog, and some were created as building blocks for customer designed stuff. DEC made a lot of money from peripheral sales. Yet they were more than eager (in my personal experience) to make it possible to build custom modules for both lines. The documentation was pretty good, and the Omnibus and Unibus specifications were easy to read and easy to build to. (Modern eyes may find Omnibus and the earlier IO signaling schemes a little hard to grok. Take a deep breath. Forget all you know about TTL and CMOS. Forget modern notions of synchronous two-phase design. Embrace the current loop. Imagine you're in a large woolen mill in 1966.)
matt (Digital 1983 to 2001 (Digital/Compaq/Intel))