Have you tried connecting a speaker to the 4-pin "SPK" connector (under the bottom-left corner of the CPU in your photo)? You could use that to listen for any beep codes. An even better option to know if it's running BIOS code is to get one of those cheap POST cards that display the status codes.

There seem to be a couple of DIP chips missing next to the cache, not sure if those are necessary or not.

Also, can you post the dumped BIOS? I can check it for any identifying strings and verify that the checksum is correct.

EDIT: these 3 seem to be pretty close matches, did you try the jumper settings on your board?

https://theretroweb.com/motherboards/s/impres … esa-motherboard
https://theretroweb.com/motherboards/s/impres … cio-vesa-ver.-2
https://theretroweb.com/motherboards/s/atroni … na-master-rev.b

Altstar wrote on 2024-07-17, 08:48:

Does anyone have the manual for this board or suggestions on what to check next? I have an oscilloscope and a logic analyzer but little experience with x86 hardware.

Any activity on BIOS ROM pins? If you post the BIOS image I can try to verify the checksum.

In general unless you're quite lucky and it's something simple like misplaced jumper you will need a POST card and/or scope (preferably both) to debug such conditions. Since you have a scope, start by probing the BIOS chip and perhaps ISA slots. For the BIOS you want to see /CE (or /CS, one of them might be tied to GND permanently) toggling at the very least. Probing the address and data lines might offer some hints as well although that is already LA teritorry and frankly it's usually easier with a POST card. But if you do already have an LA then if the scope doesn't reveal anything obvious it would be time to use it.

POST card shows no CPU activity and no beeps on the speaker. I have attached the BIOS dump.

BIOS checksum is correct, so I would discard a corrupted ROM chip as the source of the problem. The BIOS identification string is:

40-1000-001707-00101111-060692-USA-9204

Your board seems to be a clone/variant of the 3 boards I linked from TheRetroWeb in my previous post. It has manufacturer code 1707 (Chaining Computer and Communication Co.) while the ones on TheRetroWeb have 1805 (unknown), so maybe yours is the original one and those 3 are the clones 😆, you never know with these early '90s Taiwanese boards.

Luckily there seems to be an actual manual in TheRetroWeb for one of those, maybe you can use it to review the jumper settings. It also seems to have a page that lists all of the pins of the USA chipset in case you want to attempt more advanced troubleshooting:

I have ordered a 486DX33 just to rule the CPU out. I'll post and update once it has arrived.

I'm pretty sure I've seen the manual for this board floating around.
I'm also pretty sure at some point I downloaded it. I'll have a look.

History repeats

Try to fix old 786 ACIO VESA 386/486 Motherboard

Try swapping out the KB controller.
I can't read the writing in it very well. Is it an AMI "MEGA KB" by chance? I'm not sure why, but this variant often goes bad. A dead KB controller can make a motherboard appear 'dead'.

Anonymous Coward wrote on 2024-07-24, 01:10:

Try swapping out the KB controller.
I can't read the writing in it very well. Is it an AMI "MEGA KB" by chance? I'm not sure why, but this variant often goes bad. A dead KB controller can make a motherboard appear 'dead'.

It should still give POST codes if the KBC is bad or missing though.

Looking at the manual and comparing the picture, the JP5 is set correctly as open but the CPU speed / frequency jumper settings on JP9 look wrong, that jumper that's half on should be reinstalled. ": : | |" should give 33mhz.

If your multimeter has the capability, or with an oscilloscope, it would be worth checking the CPU frequency pin - like Necroware/Scorp does here: https://youtu.be/lAl5XJNSnKE?t=598

Quick update. I have tried it with a known good 486DX33 but with the same result. Machine does not POST. CPU clock and VCC look good, CPU gets warm so the board is not completely dead.

Altstar wrote on 2024-08-13, 12:45:

Quick update. I have tried it with a known good 486DX33 but with the same result. Machine does not POST. CPU clock and VCC look good, CPU gets warm so the board is not completely dead.

Did you check for activity on the BIOS ROM /CE and /OE pins with the scope? You should also check the address lines. If there is nothing then check the address lines on the CPU - either from the bottom of the PCB, or perhaps using 386 socket if those are shared (and probably are).

Thermalwrong wrote on 2024-07-24, 13:05:

Anonymous Coward wrote on 2024-07-24, 01:10:

Try swapping out the KB controller.
I can't read the writing in it very well. Is it an AMI "MEGA KB" by chance? I'm not sure why, but this variant often goes bad. A dead KB controller can make a motherboard appear 'dead'.

It should still give POST codes if the KBC is bad or missing though.

I've had some of these go bad in such a way that no beeps or POST codes are generated. Swapping it out is easy and worth a try.

Can I just swap that one with one from another 486 board?

Had a look at the address and data bus on the EPROM. There is activity on the address bus but the data bus is stuck high. Does a 486 access the BIOS with full CPU speed? My 100 MHz scope had issues with some of the address lines...

Altstar wrote on 2024-08-14, 11:15:

Can I just swap that one with one from another 486 board?

I'd say for boards made after 1991 it's usually not much of an issue.
Sometimes the OEM assigns the turbo function pins of the KB controller differently. If you have access to MODBIN or AMIBCP it's possible to correct this.
In any case, it doesn't really hurt to try.

Altstar wrote on 2024-08-18, 07:04:

Had a look at the address and data bus on the EPROM. There is activity on the address bus but the data bus is stuck high. Does a 486 access the BIOS with full CPU speed? My 100 MHz scope had issues with some of the address lines...

That would suggest the ROM is not being selected. Did you check /OE and /CS for activity as well? It could be that one of these lines is broken (usually one is tied to GND permanently and the other goes to chipset) or the ROM itself is dead.

Mobo chipset is doing the address decoding and once it detects BIOS ROM access it will activate /OE or /CS, and introduce wait states to make sure the ROM has enough time to settle its outputs. These parts are typically 150ns no matter what mobo it is (386 or 486, or even Pentiums which do not store PnP data in Flash). So any proper access to the BIOS ROM should be plenty slow for a 100MHz scope to pick up without any issues.

Test the CPU socket to chipset connections, the address lines for now. A31 in particular should be connected.