Dead UPS--Won't Power Up, Even with New Batteries

I acquired an APC Smart-UPS (in fact, it used to be my UPS at work). When it was last working, one of its two batteries were failing, as it was notifying me so via the Powerchute software utility. When I took it out-of-service, I had some difficulty shutting it down. It did not want to respond to pressing the power switch on the front panel. I was finally successful in getting it to power off, but, as I recall, I wound up pressing both the power button (several times) as well as also hitting the TEST button. One of these combinations finally caused the unit to shutdown. Out of frustration, I think I could have pressed those button(s) pretty hard.... It then sat fallow in my cubical for at least 2-years before the company allowed me to "dispose" of it. My thought was I would purchase new batteries myself, and turn this into a nice UPS for home use. Well, I bought the batteries, and pulled the old ones, re-connected the necessary wires/and 60-amp fuse, buttoned it all up, plugged it in, and pressed the power-on switch, and absolutely nothing happened.... Just nothing. No sounds, no lights, (no sparks). Nothing. Dead. However, I refuse to give up on this thing, because I believe I can fix it if I could manage to find a service manual or something to aid me in checking out the unit. I have since acquired another exact same model, whose batteries had also long since died, (and the company was going to dispose of this unit, too). Therefore, I took it home, pulled the new batteries I had procured for the first unit--and put them into this second unit, and this unit powers up and works fine. This rules out the new batteries. There is something wrong internally in the first unit. As a capable electronics technician (US Navy), I know I could probably fix this unit (if I had access to the proper documentation), and really do not wish to send it off somewhere to be refurbished if I can, at all, possibly help it. I am supposing the power-on switch is as good as any place to start, but if someone on the receiving end of this request has experience and suggestions on exactly what to look for, and how best to proceed, I would be very happy to listen. FYI, I took the whole unit apart and gave it a pretty thorough visual inspection. I saw no evidence of destroyed components, overheating/melting, etc. The internals all look perfectly fine to me. Also, I metered the back panel fuse, as well as the 60-amp fuse in series with the 2 batteries. Both fuses read as a "short", as is proper for fuses :-) Any assistance afforded me in this effort would be appreciated. Regards, Mike Gentry