Hi everyone, and thanks for your suggestions. I finally changed the keyboard controller chip I was suspecting to be faulty, no change. I already tried with another (identical) CPU and RAM, nothing. I know the AT PSU works correctly too.
I'll precise the behaviour : when I turn on the computer, the PSU fan runs, always in any case. I got some POST codes on the post card, but the screen doesn't turns on at any moment, there is no beep at all, and it hangs with the 0D-0C code on the post card (0D is the meaning one, 0C the previous one) for maybe 1min then it turns to 00-0D code. As a precision, there is a "Powergood Select" jumper on the motherboard, when that jumper is on its initial configuration (the one who worked for years), "normal mode (default)", the post card gives the mentioned code sometime only, otherwise it show -- -- (no codes) like if the board doesn't run/do anything at all. In that situation, pressing the reset button can sometime trigger the initial behaviour with mentioned codes on post card (but still screen off and no beep). When I switch that Powergood jumper to "For 80486 New Version", the post card shows always mentioned code. I took a look on the signification of that "powergood", it seems to be related to power supply check before launching the initialization of the motherboard and components. That could means there is a power issue somewhere. But I checked the voltages on one ISA slot, it seems to be correct (5.1v on 5V lines, 12.3v on 12v lines). I don't really know how I can check capacitors more than check if there is not a shortcut on them, any suggestion on that ? Thanks.