If it were me, would try a DX2-66 5v cpu and set jumpers proper including the cpu jumpers as in the other topic about the board. Re: Mustek 486 motherboard settings help
As PC Hoarder mentioned, the files cannot be found, they were on ftp and long gone.

I thought the same.

I don't have a matching CPU but I do have a Cyrix DX2 v66GP which works on 3.45v/3.6V/4.0V.

I tried copying the jumper configuration and setting the voltage to 3.45V but there was no difference in the behaviour of the board. It still does not POST.

Hi byte_76,

If you have an (E)EPROM Programmer, can you post a dump of the BIOS chip contents here?
Then I should be able to tell you if the BIOS is corrupted or valid and correct for your board with SMC I/O.
And it will be nice to have for TheRetroWeb as well. 😀

Cheers, Jan

Try reseating the BIOS chip a few times, and/or cleaning with Isoproyl alcohol & a toothbrush. As well as the RAM slots.

MikeSG wrote on 2025-03-15, 10:03:

Try reseating the BIOS chip a few times, and/or cleaning with Isoproyl alcohol & a toothbrush. As well as the RAM slots.

It was worth a try but unfortunately no luck.

The power supply turns on but the board does not POST, with or without the analyser card installed.

Just three lines on the display of the analyser card and the LED's for the +12, -12 and +5V are lit up. The CLK LED flashes at power on and then glows dimly.
I compared it to another working 486 board and the only difference is that the working board gives post codes on the analyser card but otherwise all the same LED's are glowing.

If this is not a BIOS issue, then I have no idea what's wrong with it or what else to check.

Does the Reset LED on the analyzer flash briefly ? Many old boards seem to get the reset line messed up.

Yes, it flashes briefly when I turn on the power and then it just glows dimly while the power is on. (Which seems to be the same with my other working 486 board)

What do you mean that the reset line gets messed up? How does that happen?

This board appears physically in excellent condition and it’s the only SiS chipset 486 board that I have so I’d really like to save it if I can.

I don’t have an oscilloscope but I have a multimeter and I’m quite capable of doing some tests (and component replacements) but I don’t know much about electronics, only the basics, so I need some guidance on what to check.

byte_76 wrote on 2025-03-15, 18:57:

Yes, it flashes briefly when I turn on the power and then it just glows dimly while the power is on. (Which seems to be the same with my other working 486 board)

What do you mean that the reset line gets messed up? How does that happen?

Good that is what it supposed to do, flash and stay off. Some boards it gets stuck on so no boot, some it never flashes so cpu is not initialized....
search.php?keywords=%22reset+led%22

Yes, you’re correct. I checked again now, the reset LED flashes at power on and then turns off until I turn off the power and it flashes briefly again.

Pics of the LED’s while powered on and another pic to see the labels of the LED’s on my card.

Pic of the under side

It's worth to check pins of booth Sis 85C496 and 85C497. Maybe some lost contact with pads due to cracked solder joints. You could try to press both chips with fingers to potentially allow loose pins to get in contact with pads.
On the other hand which type of SIMM memory have you used? I suggest to use just FPM for tests

I have checked the legs of the SiS chips and the SMC chip. They're all well soldered.

I don't have a lot of sim modules in my collection and the modules that I do have, don't have labels on them, so I'm not sure of the type.

I assume that the board would give some post codes without the memory installed or with unsupported memory installed.

Is it possible that a BIOS from another board might work?

Some boards might not display anything on the post card without RAM installed.

The board should work with FP and EDO DRAM 72pin.

Do any of the chips get too warm or too cold compared to others?

Correct voltage at the power connectors 12v and 5v lines? Correct voltage at the CPU VCC pins?

If I can suggest something, I suggest you use only one RAM at a time, instead of two, try it in all four banks, then if it doesn't work, use the other RAM (of that pair), and repeat the tests, if you get results, one of the two RAMs may be defective, once you find the right combination, you can do other tests, but you have to be sure that those banks work in that PC, it is not said that if they work in another PC, they must work in this one.

The Cyrix DX2 66 remains cold but the Intel DX4 100 gets slightly warm over time.

The Cyrix supports a range of voltages and I’ve tried 3.45v and 4.0v.

The Intel lists 3.0V on the processor but the board doesn’t have an option like that printed on the silkscreen. The lowest option is 3.3V so that’s what I set.

I’ve tried a single memory module in each slot. No change in symptoms.

I’ll need to read up on the pins to measure VCC at the CPU socket.

The post analyzer does indicate +12, -12 and +5V.

Does it matter that the socket is socket 2 and not socket 3?

Chkcpu wrote on 2025-03-15, 08:59:

Hi byte_76, […]
Show full quote

Hi byte_76,

If you have an (E)EPROM Programmer, can you post a dump of the BIOS chip contents here?
Then I should be able to tell you if the BIOS is corrupted or valid and correct for your board with SMC I/O.
And it will be nice to have for TheRetroWeb as well. 😀

Cheers, Jan

I've ordered an EPROM programmer and will take a dump of the BIOS once I've receive the programmer. (Expecting shipping to take at least a month)

How likely is it that the voltage regulator on this board does not support CPU’s below 5V?

Also, maybe the voltage regulator is faulty?

I cannot seem to get a 3.3V reading on the legs of the regulator regardless of the jumper settings.
Voltages are around 4v and upwards.

Can anyone advise which pin of the socket should be used to measure CPU voltage?

(It’s socket 2 in case that matters)

zami555 wrote on 2025-03-15, 21:52:

It's worth to check pins of booth Sis 85C496 and 85C497. Maybe some lost contact with pads due to cracked solder joints. You could try to press both chips with fingers to potentially allow loose pins to get in contact with pads.
On the other hand which type of SIMM memory have you used? I suggest to use just FPM for tests

Indeed, you should use FPM memory to troubleshoot, if I remember SiS boards with that chipset will only work with EDO if the northbridge is "PR" revision (comparatively rare), which you do not have. In my experience with several similar boards they would not post at all with edo, nor with modules with higher capacity than 32mb (which is certainly not the case here)