So it works fine as a 486-33 with the jumper in?

There isn’t a soldered 486 in there is there?

What brand CPU is your dx2-66?

Occasionally TI and Cyrix dx2’s would require a whole hoard of jumpers changed.

AMDs so called Advanced dx2 had write through and 3.3 volts which is problematic

If both chips are Intel 486 CPUs there is the possibility of damage .

Only 25mhz and 33mhz FSB should be available, you should check the true FSB results with the dx33 in.

If you have a dx2-66 Intel 5v chip with a heatsync and it’s having trouble at 66 you should be able to run at 50, it’s curious to me that the system works fine/stable with the dx33

https://www.ardent-tool.com/media/PS2_MOST/AC … ELL/2006026.PDF

So it will boot only when I remove the jumper on the 33mhz option, no jumpers on the board. Bios then says its a 486DX2-40 even though its stamped on the chip 80486DX2-66.

Nope, no 486 in there.

I had a Intel 80486DX-33 originally and then installed a Intel 80486DX2-66 in its place.

The only jumper options on the board are 25mhz, 30mhz, 33mhz and 40mhz. I would have thought leaving it on the 33mhz would have worked just fine, but I end up with no video and just beeps a few times and that's it.

camry_dude wrote on 2023-10-17, 17:55:

So it will boot only when I remove the jumper on the 33mhz option, no jumpers on the board. Bios then says its a 486DX2-40 even […]
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So it will boot only when I remove the jumper on the 33mhz option, no jumpers on the board. Bios then says its a 486DX2-40 even though its stamped on the chip 80486DX2-66.

Nope, no 486 in there.

I had a Intel 80486DX-33 originally and then installed a Intel 80486DX2-66 in its place.

The only jumper options on the board are 25mhz, 30mhz, 33mhz and 40mhz. I would have thought leaving it on the 33mhz would have worked just fine, but I end up with no video and just beeps a few times and that's it.

Most likely you are running the board with 20MHz, when it says DX2-40

Would there be a jumper to set the board to 33mhz?

camry_dude wrote on 2023-10-17, 18:03:

Would there be a jumper to set the board to 33mhz?

I don't know, I am only referring to what you wrote.
I don't own such a machine.
But you wrote you had a the DX33 MHz running, so that is the setting. And yes, a DX2-66 should run with the exact same setting as the DX-33.
Please consider to give some more information, like a high resolution picture of the mainboard.

Update on the issue. I did what I should have done and started to remove some the ISA expansion cards. After removing both the network card and Trident video card, I was able to use the integrated graphics and it booted to the DX2-66.
Why this worked, I am not sure though. Unfortunately when I close the case back up, it won't boot, so I am thinking something may be loose and causing issues when the cased it buttoned up.

Thanks for the feedback above!