To answer my own question, I powered on the PC . What could go wrong, right?
CPU shows up as a DX4 S with 100MHz but now I got a floppydrive error and some error from the SB CT2230.

There were minor differences between the AMD & Intel versions. Don't see why it shouldn't work if you jumper everything correctly. Intel was a 3.3V CPU.
Try going into CMOS setup and load the default values, save and exit, go back again and tweak for best performance

Kouwes wrote on 2023-05-22, 16:33:

To answer my own question, I powered on the PC . What could go wrong, right?
CPU shows up as a DX4 S with 100MHz but now I got a floppydrive error and some error from the SB CT2230.

Those issues scream something with write-back versus write-through internal cache

Hi Kouwes,

Okay, the Intel DX4-100 works. From the &EW marking and SK096 number on the CPU, I see that it is a rare DX4 with L1 cache Write-Back support. Great!
This explains the floppy drive and soundcard errors because they both use DMA which can create L1 cache coherency problems when the CPU is in L1 cache WB mode. So I fully agree with jakethompson1’s remark.
The cause of these problems is usually incorrect jumper settings or lack of BIOS support or both.

To help you find the cause, I have some questions:
- You write that the board is a GMB-486SPC V0, but could it be a GMB-486SPS V0?
- What is you BIOS version and ID-string?
- How are your CPU type jumpers JP4 up to JP17 set? Looking at the 486sps103 manual I would start with the settings at table 1G.
- What was you previous Am486 CPU type, NV8T or SV8B?

Greetings, Jan

Thanks for the replies!
Previous CPU was an Am NV8T. Someone wrote that on it, the top side has a heatsink glued on. I tried to remove it but that’s hopeless and I don’t want to damage the CPU.
The board really is a 486-SPC V0 and the jumpers are set as shown in table 1C from the SPC manual, I double-checked they are set correctly.
Table 1G are the settings for 486SX in this manual.

I took the board out of the case and as soon as it’s running again I’ll add the BIOS version.
But maybe a BIOS reset will do the trick.

I also have a 486DX50 which I could use…

Oh!

Since an Am486 DX4 (N)V8T behaves like a 3,45 Volt DX2 with L1 cache in write through mode, you need a different jumper setting for a write back enabled DX4 like the Intel &EW.

I just loaded the BIOS defaults and now it boots to DOS without errors, but I still need to install a heatsink with thermal adhesive.
The soundcard isn‘t installed yet, but I guess it will also work without problems.
However….now the video signal is weird, screen text is flickering.
There‘s a Trio64 installed so I swapped it for:
S3 Virge: no video at all, PC not booting up
Tseng ET4000 pci: same as above
Tseng ET4000 ISA: works fine

I also tried a different PCI slot with the Trio card but that didn’t change anything.

Seems the PCI slots are not happy. Odd that the other cards do not work at all. But then again, that Virge has some loose connection and gives black screens sometimes. And recently, I added 2 memory ICs to the PCI Tseng‘s SOJ sockets but removed them again - maybe something went wrong, I can’t tell.
Need to test those cards in another system.
I also have a Voodoo 3 PCI but always very careful to use Voodoo cards for testing.

I just disabled everything in the bios after the jumper settings (that took days to figure out) , then re-enabled one-by one , check out my 486 project.

Your board looks exactly the same! Until you take a closer look but the layout is about the same.
I haven’t had time to check my BIOS settings but will do so after work.
There‘s a PCI section I found in the manual.

I also found out this board doesn’t support 50MHz CPU clock so that 486DX50 goes back in storage.

Ok I found the cause: my 14“ Acer TFT (I often use it for testing) doesn‘t like this PC.
I connected the 486 to my Viewsonic and also to my HP LP2065: perfect!
Case solved, I‘d say.

mine supports 50mhz , I have the manual in link. - pages 7-9-12 (I had no problems on my Samsung TFT 19" syncmaster)