There is a chance that the contents of the BIOS flash chip has been degraded over time. Try backing it up in an external programmer, then flashing with a good known BIOS.
If you don't have a good known BIOS image then upload your backup here, maybe someone will check it for damages and possibly revert them.

Do you have a POST card? If you do, could you write the sequence of the codes that you get during the boot process?

What happens if you don't press Del?

Have you tried resetting the CMOS configuration data by shorting the jumper on an unpowered board or removing the CMOS battery?

What happens if you press tab, does that show the post screen?
Basically checking if any key press is recognised.

And checking the basics are you using PS2 or USB keyboard

Did you removed the Bios-Battery? Try it with the battery removed.

Many thanks for the suggestions so far.

- I don' have a POST card unfortunately.
- If I don't press Del, the screen goes dark (no blinking cursor etc.)
- I tried resetting CMOS and removing the CMOS battery
- I am using a PS2 keyboard
- Pressing tab does not show more information, however it seems like del is recognized

I do now have a T48 to flash firmware on the PLCC32 chip, but I have trouble finding a suitable bios. The ones from following link don't seem to work, i.e. flash fails and no beep/screen stays dark:
https://theretroweb.com/motherboards/s/bcm-in440zx#bios

Could you share the backup of your flash image? Probably that could help in identifying the correct image to reflash.

I think I messed up - looks like I overwrote my backup copy. Not sure what to do from here, I checked archive.org for bios on micronpc.com and bcmcom.com, but the links to the bios are not archived, e.g.:
http://web.archive.org/web/20001003005853/htt … 110-xx&Include=