Mac Pro 2008 almost dead

I own since many years a Mac Pro early 2008. Few days ago it stopped working. It starts, fans spin, but no chime. Initially, I thought it was a RAM problem, but soon understood that the problem is "before" RAM is used. I already did the usual checks (battery removed, internal SYS_RST button, all the non-necessary boards removed etc.). I also took it apart, disconnecting and reconnecting all the connectors. No change. Diagnostic LEDs are all OFF, but when i press the DIAG button, I get LED 2 (STDBY) and LED 8 (PWRGD). Keeping the main key pressed, the Mac switches off normally. The SYS_RST button has no effect at all. Did anybody have a similar experience? Thanks