There's a full scan of my printed A3000 service manual freely available on my webserver schematics page.
If you've already looked at that, get it again as I just replaced the PDF with my original scans. Cosmos offered to clean and compress them for me a couple of years ago, which in this case resulted in loss of detail and a compression from 3.8MB to 14.7MB (WTF?!). Must go through and compare the rest of my scans and Cosmos' cleaned versions and keep the better of the two.
If you're after a 'diagnostic manual', I've never seen such a thing for the A3000. There's an A500 service training guide here which gives some insight, but realistically it's not easy or practical to produce a guide to magically diagnose every conceivable fault for such a relatively complex machine. Besides, it keeps electronics engineers such as myself employed 
If you get stuck with it, contact me for a repair estimate. I have a collection of instruments and in circuit hardware debuggers for fault tracing and repair to more depth than the basic checks you can do yourself.
Thanks for the update scans, wish SAMS made a manual for the 3k and other Amigas. When a 3k goes black screen and a dim LED it is turning into a real pain to fix.
Are you repairing Amiga's 3k's with these symptoms? I'm going to see what I can do to fix it myself right now, shipping to NZ from US would be quite expensive.
Matt
