Might be a chipset limitation.
You mind dumping the original AMIBIOS of your motherboard?
I presume you have an PROM reader if you were able to replace the original BIOS.

Here it is.

Don't know if there's two different PLL-chips for generating FSB, need to search those. Or, is it possible that Chipset makes the clock, don't know.

I don't own this board, however looking on photos from the link you've provided it looks like clock being generated with U44. On photos there is ICW42C07. So it's all around this IC which could make 50MHz for you

Need to check that one again, I think it's just too small to be a PLL.

@40MHz it's not slow compared to other DLC-40 systems.

dj_pirtu wrote on 2023-06-25, 12:57:

Need to check that one again, I think it's just too small to be a PLL.

@40MHz it's not slow compared to other DLC-40 systems.

The datasheets for the OPTI chipsets are available online - should be easy to confirm. I know that the 82C206 also has a clock generator built in.

If I recall, overclocking a 486DLC to 50 MHz is really pushing the limits of that CPU. At 40 MHz it already runs incredibly hot (too hot to touch) compared to a 486DX-33. Suggest some sort of thermal solution (heatsink) to cool things down a bit.

Since this is a hybrid board, you'll get much better performance by just installing a 486DX-33 or 486DX2-66, in conjunction with a VLB graphics card.

I do have a few 486-machines already but just wanted to take the shit out from a "386"-system, just for fun.

And yes, CPU has a heatsink. When I run the CPU @50MHz with my old board (which had some other issues) I put a fan to it.

So I need to investigate traces of those FSB-jumpers...

Got an great idea, I want to make switch, or switches and connect those to jumper positions by wire and then I can choose which CPU socket (and CPU) to use by switch 😁

Edit: It didn't post with two CPUs in place. So I'll forget that and leave it as is.

Strange thing is that I can't get it to work with 32MB RAM, not even 20MB. Only 4 sticks of RAM works. BIOS counts memory and then black screen, or tries to boot DOS and crash.

Memory problem solved, needed to raise wait states even if memory is 60ns.

Tried Kingston Turbochip (Am5x86/160) and did some benchmarks. And yes, the legend of Opti chipset slow memory access is true. My SiS471 based 486-motherboard is much faster accessing memory with all bios settings set to tightest. It shows in Doom benchmark and most noticeably in Quake (~12fps vs ~17fps). When using 386 CPU it's not that crucial.