What could happen?

Depending on the installed oscillator crystal, your CPU designed for running at 20MHz will be run at 33MHz or 40MHz. It's not going to boot. Potentially it might get damaged, but probably won't due to not even booting.

Also curious, from a former DTK user: my very first IBM PC compatible was a DTK with 80386DX-20; I often wondered if it was possible to upgrade it simply by switching to a higher-clocked CPU.
On the other hand, upgrading RAM required a different jumper setting back then.

The CPU still lies quietly in my collection today. Most of its other components were long gone, though.

It might work just fine.

Back then I gave a friend a bugged 386DX-20 and he used it at 33MHz for a few years.
It died then, though. Putting a cooler onto it would probably have been a good idea, as it got scorching hot at 33MHz.

dionb wrote on 2021-09-07, 11:23:

What could happen?

Depending on the installed oscillator crystal, your CPU designed for running at 20MHz will be run at 33MHz or 40MHz. It's not going to boot. Potentially it might get damaged, but probably won't due to not even booting.

Thanks so much for answer. In that case i wont try it at all, but to be 100%, is it any chance the motherboard and the computer could run this 386 dx-20mhz cpu at 20 mhz ? ( I'm not interested at all to try to run it at 33mhz ) If not, without messing with the crystal oscillator, do you have any idea to get it working at 20mhz, even by modifying something, my father is a electronic technician. I'm curious about this cpu, because when i was young, i had a 386 dx-20 computer for 5 years, and now i want to do some benchmark with a 386 dx-20, for compare and for fun vs dx-33 and vs sx 16. But i dont want to buy a 3rd 386 computer. I also have a 386 sx 16, but the cpu is soldered on the motherboard, so i cant install a 386 dx-20 cpu in it. If you have any idea, let me know, thanks a lot ! ( i suppose i could change the crystal oscillator for a lower one in mhz to get it working at 20mhz, but i'm not really interested to do it now, maybe in future if i have no other idea, but i will need more infomation to exchange it. I dont know at which mhz the crystal oscillator need to be to run the cpu at 20mhz. I think i would keep both crystal oscillator and use a switch button for each cpu )

A picture of the board would help, but usually for boards of that era there is an oscillator that determines the CPU frequency, in some cases it's 1 to 1 (e.g. 33MHz oscillator to run the CPU at 33MHz) but sometimes there's a divider (66MHz oscillator for 33MHz CPU). So if you want to change the CPU speed you need to swap the oscillator accordingly, the rating of the CPU itself doesn't determine anything, it's more like an upper bound, it will just run (or try to run) at the clock frequency dictated by the oscillator.

In any case, you don't need another CPU to run @20MHz, just change the oscillator and run the DX-33 at 20MHz.

TheMobRules wrote on 2021-09-07, 19:20:

A picture of the board would help, but usually for boards of that era there is an oscillator that determines the CPU frequency, in some cases it's 1 to 1 (e.g. 33MHz oscillator to run the CPU at 33MHz) but sometimes there's a divider (66MHz oscillator for 33MHz CPU). So if you want to change the CPU speed you need to swap the oscillator accordingly, the rating of the CPU itself doesn't determine anything, it's more like an upper bound, it will just run (or try to run) at the clock frequency dictated by the oscillator.

In any case, you don't need another CPU to run @20MHz, just change the oscillator and run the DX-33 at 20MHz.

Thanks for the information.