I got one Migitronics TK-82C491/386-4N-D02C board with soldered Am80386DX-40 CPU, missing BIOS chip and no cache chips. The board is in pretty good visual condition (no broken traces, no battery leakage, clean and with no missing elements, except the BIOS and cache chips) and I thought - this will be one easy fix, a matter of new BIOS chip insertion plus some SRAMs for cache. And as always it turned out that things were much more complicated.

I grabbed one Winbond W27C512 (64 K x 8 ) EEPROM chip, burned the AMI BIOS dump from TRW site for this model motherboard, installed missing cache (one TAG SRAM chip (8 K x 8 - 12 ns) and four cache SRAM chips (32 K x 8 - 15 ns) for a maximum of 128 kB L1 onboard cache), installed 8 MB FP-RAM and some ISA VGA and I/O+ATA cards, set the jumpers according to ABIT AK-3/4 (similar board) "manual" from TRW site and some pictures of this model board, which I found online, and... system booted, but there are some problems.

The "turbo" jumper doesn't switch to 40 MHz mode and the board doesn't "see" any cache regardless of "external cache" setting in this AMI BIOS from TRW site.

I checked and traced with multimeter the TB-SW ("turbo swich") jumper traces on the PCB and everything seems OK with them - one of the TB-SW jumper's pin goes to ground and the other leads to pin 5 on the UM82C491 chipset chip, which is labeled "TURBO" in the datasheet. So, the culprit for the missing "turbo" mode i.e. 40 MHz for the CPU isn't the TB-SW jumper itself.

Also changed and tested total of 16 known good and tested SRAM 32 K x 8 chips for cache and two known good and tested SRAM 8 K x 8 ships for cache's TAG RAM in different combinations, but the board still doesn't "see" any cache.

I also tried all the AMI BIOS ("3umm002.bin" - this one is identical to TRW's dump for Magnitronics TK-82C491 board; "3umm003.bin"; "3umm005.bin"; "3umm006.bin"; "3umm007.bin" and "4umm002.bin") and Award BIOS ("3umw001.bin" - identical to TRW's dump for Abit AK-3/4; "3umw002.bin"; "3umw003.bin" and "3umw004.bin") dumps for 386/486 UMC chipset from this site and the board refuses to boot with all of the Award and some of these AMI BIOSes. Also tried the Abit AK3/4 Award BIOS from TRW and as expected the mobo doesn't boot at all. One of these BIOS dump files ("3umm002") was binary identical to the one from the TRW, which makes me think that the original BIOS for this motherboard is untraceable on the internet.

With some of the tested AMI BIOS dumps the board does boot, works in "non-turbo" mode and when I short/close "turbo" jumper the system hangs.

I think I've tried enough combinations of BIOS dumps and SRAM chips, but still no luck with this mobo. It just refuses to work in "turbo" mode and to use any onboard cache.

Please, if someone owns Migitronics TK-82C491/386-4N-D02C motherboard to upload here a dump file of its original BIOS. Thank you!

P.S. Or maybe I have to try some MR BIOS dump, but I don't know which one.