Useful info would be the motherboard make and model and the BIOS maker and version.

Personally, I'd want to check the 74F00 is okay, grounding the +5 on floating or low inputs shouldn't do much to it, but if inputs were high and from a larger current source, I'd worry that it might have had something backfeed through it to the unexpected ground. Then I'd probably want to check out what fed it, having their outputs grounded if high might have damaged them.

First check if the 74F00 still gets power, do two tests. When power is off there should be low resistance between pin 14 and +5V power connector (below 1 ohm), Also check the GND connection. Then, just to be sure, measure voltage when the system is running to make sure it's 5V. You should also check the capacitor, if it still has good connections.

If the power delivery looks ok then maybe the 'F00 chip got damaged. What kind of mobo is that? Something 386 or non-PCI 486 if it still has chips like 74F00 on it I think. Where is that chip located, somewhere close to the keyboard controller? Possibly it's a gate for clock and without clock the KBC will not respond to BIOS and the boot process will hang pretty early.

Thanks for your response.
The motherboard is Unichio 486WB 4407 (https://www.ultimateretro.net/fr/motherboards/6815)
The 74F00 is still powered, the resistance is about 250 ohm (power off) and the capacitor has still a goo connection.
The ship is located at the bottom left above the 6th isa port.

250 ohms is way too high, especially for F class chip. There's probably a burned trace and what you are measuring is the chip internal resistance (which might be that low for 74F).

There is a small ceramic capacitor next to that chip, probably a 100nF bypass/filter, it should be connected between GND and +5V and the +5 side should go directly to pin 14 of the 'F00. Measure that. There must be a small resistance (less than 1 ohm) between the capacitor and the chip.

If the resistance is small between cap and 'F00 chip, but both are 250 ohms to +5V, add an extra wire (not too thin) from nearest good 5V connection. The ISA slots should have good connections to power plane, and are nearby.
If the capacitor has good connection but chip does not, then just solder a short wire betwen the cap and the chip.

Hopefully this will fix the problem.

Problem solved, finaly it was the keyboard controler that had a problem.
Thanks for your help.

Deunan wrote on 2022-02-10, 21:16:

250 ohms is way too high, especially for F class chip. There's probably a burned trace and what you are measuring is the chip in […]
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250 ohms is way too high, especially for F class chip. There's probably a burned trace and what you are measuring is the chip internal resistance (which might be that low for 74F).

There is a small ceramic capacitor next to that chip, probably a 100nF bypass/filter, it should be connected between GND and +5V and the +5 side should go directly to pin 14 of the 'F00. Measure that. There must be a small resistance (less than 1 ohm) between the capacitor and the chip.

If the resistance is small between cap and 'F00 chip, but both are 250 ohms to +5V, add an extra wire (not too thin) from nearest good 5V connection. The ISA slots should have good connections to power plane, and are nearby.
If the capacitor has good connection but chip does not, then just solder a short wire betwen the cap and the chip.

Hopefully this will fix the problem.

Finaly, the problem was not solved.
The motherboard doesn't work again 😒
Sometimes I have no beep, sometimes I have beep and the post card stop at "0d 0c"
I don't understand, the day before it worked.

0b 0c is still keyboard initialization http://mrbios.com/techsupport/award/postcodes.htm

rasz_pl wrote on 2022-02-17, 17:38:

0b 0c is still keyboard initialization http://mrbios.com/techsupport/award/postcodes.htm

Yes but I tried with another controller and I have the same issue.

have you looked for fried traces? you could maybe buzz out keyboard and bios data pins to see if they are all intact. Keyboard controller doesnt decode its own address. It needs dedicated decode logic, either discrete (138 139) or signal from chipset (dont have unichip datasheet).

if you managed to make it work temporarily it might be burned spot making contact when slightly bending the board, or even depending on external temperature.

>On ISA port, on Addr 04 the signal is unstable . It goes from xxV to 0v. (with bios) and without bios the signal is stable.
most address signals should be flipping around while executing BIOS. What is 74F00 wired to?

Battery spill damage or a cracked via can easily cause such issues. There was, just until now, still some connection but very poor and now it's gone open ar high resistance, causing problems. There isn't any easy way to deal with it but inspect the PCB very closely. Stick to KBC area since this is what is causing the issue for you.