Iarsin wrote on 2023-03-27, 13:27:

Could it be, that the 1990 AMI BIOS isn't able to boot from c: directly, and this PC setup was meant to use a boot floppy?

I'd consider that to be highly doubtful that I'm gonna answer no to that. it should boot from hard drive.

my normal sequence for this would be
boot from floppy
remove all partitions, primary & extended & logical drives
exit to dosprompt
restart
boot from floppy
create primary partition
exit
restart
boot from floppy
format primary with system ie format c: /S
at dosprompt
restart without boot floppy.

After formatting i also run fdisk /mbr command, to be sure the mbr is ok. This may be the problem sometimes, wouldn't be my first.
Also, you may want to try with different dos versions if this won't do. Maybe there's something fuzzy going on when running this specific sys command.

I'll backup first the HDD, before trying that as a last resort.

I also tried 560/6/21 c/h/s values instead of type 33 (AMI). Then it comes with a message C: not ready or the like. So I have to use the translated values of Type 33.

AMI Setup claims, that it's not a AMI BIOS, but a NEC one. I'm not sure, because 1069 stands for NEC, but the bios string comes with 6069, which stands for octek.

Maybe due to incompatibilties it throws the message that the cmos crc doesn't match, and it is corrupted. Other tools are fine with that bios though.

I read, that a depleated flat CMOS battery might be the culprit of such ssues. The Varta Ni/Cd accumulator was leaky. Therefore I desoldered it from the mainboard. I'm astonished, that the capacitors seems to hold the current for at least one day, to keep the CMOS Values intact. But nevertheless it could be, that a lacking battery is the culprit here. I'll try to use a battery set instead. Do I have to use a diode, to keep the cmos save? Or what is that diode for, if one replaces the accumulator battery with an alkaline?

Iarsin wrote on 2023-03-29, 16:34:

The Varta Ni/Cd accumulator was leaky. Therefore I desoldered it from the mainboard. I'm astonished, that the capacitors seems to hold the current for at least one day, to keep the CMOS Values intact. But nevertheless it could be, that a lacking battery is the culprit here. I'll try to use a battery set instead. Do I have to use a diode, to keep the cmos save? Or what is that diode for, if one replaces the accumulator battery with an alkaline?

Varta NiCd batteries are rechargeable. The computer always recharges the battery when it's on, and the battery discharges maintaining the cmos setting & RTC.
The diode is required to stop the computer from charging a non rechargeable battery like a CR2032.

I don't get this, but with xfdisk it boots just fine from c:. I tried everything like fdisk /mbr, sys c: and so on, with the same MS-DOS version, I found in C:.

Maybe there is an explanation for this, but I'm wondering what it could be ...