Could be the capacitors on the motherboard failing and causing instability?

heat sink? fan?

The motherboard is on my test bench with no case, however there is indeed no fan nor sink on the 486DX2. Is it required for such CPU? Another point: I was wondering if the HDD could in a way or another interfere so I unplugged it and used only the floppy and a DOS disk boot to access DOS. Got the A:/ prompt but had the same freeze problem one minute later. So indeed it looks like it is a hardware issue from the MB. As far as the capacitors, they don't look swollen but I'm really not an expert. If there is a failed capacitor, would the setup work fine for some time, or would I get an issue at post?

how about you put a finger on the cpu? you need a heatsink or hs+fan, depending on cpu brand.

Indeed the CPU is quite hot. I am trying to find a cooler that fits with the socket 3 - not easy for such an old model. A couple are available on eBay but at an outrageous price or coming from the US (I am based in France) with a significant shipping cost + potential tax. Will keep you aware

what is quite hot? can you keep a finger for more than couple of seconds?
any old socket 7/a radiator + metal saw + thermal conductive glue will do

Iirc all DX2-66 requires at least a heatsink.

Choux69 wrote on 2022-06-07, 13:59:

Hello all. I am testing a 486 setup based on a PC Chips M912 v1.7 motherboard + 486DX2 + 16Mb RAM. I installed also a VLB Graphi […]
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Hello all. I am testing a 486 setup based on a PC Chips M912 v1.7 motherboard + 486DX2 + 16Mb RAM. I installed also a VLB Graphic Card (Cirrus Logic 5426) and and 16-bit ISA multi I/O card to which I hooked up a 3.5 floppy and a Quantum HDD.
DOS 6.2 was installed on this HDD using another computer (on a 2GB FAT16 partition) before connecting to the PC Chips MB.
So when I power on the setup things look fine and I can get the c:/ dos prompt. Floppy is accessible, DOS operates properly. But after a certain time (from a few seconds to a couple of minutes) the system suddenly freezes: I can no longer type anything - the screen stays frozen without any warning message of whatsoever: the cursor blinks but the system is completely nonreactive. No other choice than power off.
These are the things I tried without any success to fix the issue:
- Work with one of the 2 RAM chips (one or the other) only
- use other RAM chips
- use another I/O card
- Move video card to another VLB slot
- Move I/O clard to another ISA slot
- Setup BIOS under the preset 'fail free' mode
I am stuck and really can't figure out why the system works fine for a while and then freezes (assuming if there were an issue with any component I could not have a working system for some time at start). Thanks for your suggestions

So you really are running a DX2-66 without a radiator? Just the ceramic chip?

I confirm the issue was the lack of cooler. After testing with one, the system no longer freezes. I really thought this CPU could work without any heatsink/cooler. Thanks for putting me in the right direction!

Basically:
DX-33 and DX-40 can run without heatsink.
DX2-50 needs heatsink.
DX2-66 needs heatsink + cooler.

False. Back in '93 at college, Tried to run my new AMD 486 40 at 40 without heatsink, no luck and tried using a passive heatsink used for TO-3 transistor, no luck. Had to go and get correct heatsink with fan on visiting computer show, back in the day when 386 and 486 was a king. Once done, tried overclocking to 50MHz with success. Stayed this way ever since.

33MHz and below is borderline. DX 40 and up needs heatsink for any 486.

Cheers,

Choux69 wrote on 2022-06-09, 20:36:

I confirm the issue was the lack of cooler. After testing with one, the system no longer freezes. I really thought this CPU could work without any heatsink/cooler. Thanks for putting me in the right direction!

It can be a bit of a heated (pun) subject. In this thread we tried to pore through the datasheets and try to figure out exactly what Intel/AMD specified rather than a word of mouth approach: 486DX2 heat sink debate