When your trying this are you removing the math coprocessor? Pull the math co and then try the dlc chip again. I'd also try changing the j13 jumper to the 20/40 setting.

Are you installing it in the correct orientation? The triangle in the corner should line up with the triangle on the board next to the socket.

Maybe you need to disable the onboard 386dx CPU first, this was discussed here: Re: New 486dlc build problem.
Read the posts after this and you will see pics as well

Hi.
Remove coprocessor, If it still doesn't work try to 'disable' soldered CPU as keropi wrote.
For a test, just short pin 54 to 55 with a small screwdriver....

Locutus wrote on 2023-08-09, 22:01:

Hi.
Remove coprocessor, If it still doesn't work try to 'disable' soldered CPU as keropi wrote.
For a test, just short pin 54 to 55 with a small screwdriver....

The board has jumpers for that, JP1 and JP5..
They're in the center of the board (red jumpers) and set to 386 according to the picture..

Edit: The OP apparently has tried changing them, if I read correctly. 😅

Veeb0rg - I tried with FPU and without + different jumper settings
debs3759 - The 486DLC had the correct orientation . The photo is illustrative , motherboard and CPU
Jo22 - correct 😀
keropi , Locutus - I think it would help 100% but as I wrote earlier. I don't want to desolder or solder anything on the motherboard .
I found a very similar motherboard https://theretroweb.com/motherboards/s/abit-ab-at3
As shown in the picture there is CX486DLC instead 386DX . I need to find another motherboard .
Thanks for the help.

It could be a faulty CPU, as it sounds like you tried everything else.

CPU is ok . I'm testing this CPU on another motherboard now

tomaszdko wrote on 2023-08-09, 14:58:

I have the motherboard shown in the photo . I set up jumpers JP1 and JP5 and run the TX486DLC processor but not working . Screen black and both processors get warm .

Is the mobo working with its soldered 386DX?
Figure out what jumpers JP1 and JP5 do, one (probably JP1) should be connected to the soldered CPU FLT# pin. Not sure what the other one is for, should not be needed and the description printed on the mobo might be incorrect.
Assuming the mobo does run with the soldered 386DX, try running it at 25MHz and then install the DLC CPU and configure jumpers, see what that does.
In fact try running the mobo without DLC CPU but with the jumpers set to DLC/socket - if it still runs then obviously the on-board 386DX is not disabled properly.

Original CPU might get quite warm even when disabled, this is not unusual. So long it's not burning hot it's OK.

Deunan wrote on 2023-08-10, 14:47:

Figure out what jumpers JP1 and JP5 do, one (probably JP1) should be connected to the soldered CPU FLT# pin. Not sure what the other one is for, should not be needed and the description printed on the mobo might be incorrect.

If his mobo is anything like mine was, there is no jumper present for /FLT and JP1/5 are for the DLC-specific signals like /A20M and /FLUSH or BARB (and unnecessary since they could have been connected the whole time since they never touch the 386)

The board is meant to only operate one CPU permanently, not be upgraded or switched at some point. For mine I needed to add a jumper that controlled #FLT, which might not exist at all on his.

Edit: Yep, looking at this, pin 54 is totally unconnected. There is no /FLT jumper and he needs to short pin 54 to 55.

tomaszdko wrote on 2023-08-10, 10:50:

I found a very similar motherboard https://theretroweb.com/motherboards/s/abit-ab-at3

Locutus wrote on 2023-08-09, 22:01:

Hi.
Remove coprocessor, If it still doesn't work try to 'disable' soldered CPU as keropi wrote.
For a test, just short pin 54 to 55 with a small screwdriver....

Sometimes the flt pin is pulled high without a resistor and grounding it will make sparks. You must either test for this or lift it to be safe

I say again to be clear, do not short that pin to gnd without lifting it and isolating it. Unless you are sure it has a resistor between it and vcc

As I wrote earlier. I have a second motherboard just like this one . I fixed 99% the motherboard (does not see the cache sram , I will do later) .I desoldered the CPU and installed the 486DLC , The motherboard start correctly .