To-do list:
- copy bios and attach file
- remove all socketed chips
- desolder keyboard connector and check traces
- add more vinegar and scrub
- wash
- rebuild traces
- test

Guess that's all for now.

I desoldered the keyboard connector first and it took some time.
There is a lot to do in my opinion, many traces aren't doing that well.
I'll probably have to remove the power connector to work with ease.

Current situation is:

Today it's about rebuilding traces.

I'm going to scrub with a fiberglass pen. Then tin the traces and put some copper wire to rebuild traces.
If everything is working I'll put new masking on the area.

The crystal oscillator is 33mhz, it should work anyway with the 386 dx-40 @ 16mhz. It just needs to boot.

I repaired all the broken traces.
Slow boring job but yielded the desired result. Board woks perfectly.

Among the broken traces were BIOS connections, if I had tried a fast power on it would have done nothing.

I still need to put solder mask to avoid shorts and resolder the keyboard connector.
To test the keyboard I used dupont wires to connect the motherboard to the keyboard. I got inventive to avoid desoldering.

Screen pics.
BIOS is the usual purple/violet + orange etc.
Is there a newer version of this BIOS if generic or maybe it's specific to this board?

I'll test it in the coming days.

First tests aren't promising,
- ram issues: 2 chips isn't well supported; doom bench crashed at sprites issue
- booting from floppy hangs at loading ms-dos

I put different sticks and things work fine now.
- doom bench is close to 386 results: max 2134 gametics and 17642 realtics / slow 4291 realtics
- speedtest detects 486 dx2-66 but score is on par with a 386dx-0 at 9.77 points

Is there anything I could try to make this 386 with 486 support go faster?
Do I have to solder the disable pins of the pqfp anyway? even if jumpers are correctly set?

Cache is detected correctly to its amount of 128kb.

I'm starting to think that those 386 to 486 upgrade motherboards were a bit of a scam.

I don't see how much faster it can go compared to a 386 if all the other components aren't 486 class too.

Landmark System speed test 6.00
Cpu 17.9, even below dx-33

I have to take back everything on performance:

in BIOS there is an option for TURBO switch to be either enabled or disabled. I thought it was an always on or off option. It makes no difference at all as it's always working and has to be open or close to do anything.
Actually you have to put a jumper on the header or have it always closed to perform as a 486 at full power.

Benchmarks' results are as expected for a dx-2 66. I didn't push it farther than the usual Phil's pack.

I didn't bother checking turbo but I made a mistake. Lesson learnt.
It was indeed a nice upgrade from a 386, having to buy a new cpu instead of a whole system.

** Quake finally finishes at 6.0 fps 360x480