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Another 486 no-POST

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First post, by quicknick

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Hi folks,

Got this mistery board that has an odd expansion slot (OPTi local bus, maybe?); it does't want to POST no matter what I tried. Diagnostic card displays only dashes, no beep from the speaker.

SiS85C360 board.jpg
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SiS85C360 board.jpg
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- Modded the Dallas chip, CPU gets hot, tried three processors (all Intel, DX and SX/33, DX2/66), known good RAM (and no RAM).
- Reflowed the SiS 85C360 chipset to be sure there's no broken solder joint.
- Tried another keyboard controller, another (un-modded) Dallas chip, tried a BIOS from another SiS 471 board.
- Oscillators on the board are working, even replaced the 66MHz with a 50MHz one for testing, no change.
- Cache sockets were populated when I got the board, I removed the chips to make sure they are good; tested OK on my TL866 but I'm not putting them back until I get the board going.

Scoured the net for another BIOS for this SiS 360 chipset, couldn't find any. Same for the chipset datasheet.
There's this string to be found inside the BIOS image, based on which the board seems to be manufactured by Flexus (never heard of them):
40-0000-001470-00101111-060692-SIS3486

As usual, looking for any advice or suggestion.

Here's the BIOS if someone is interested:

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Unknown 486 SiS 85C360 Local Bus.rar
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38.93 KiB
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56 downloads
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Fair use/fair dealing exception

Reply 1 of 6, by aaronkatrini

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Could you please try the Tantalum capacitors near the AT power connector. Those are known to fail after some time. Maybe you need to check the ESR value.

I might be mistaken, but the connector on the bottom of the board might be a riser of some kind? Cannot tell for sure...

Reply 2 of 6, by evasive

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Flexus=FreeTech and from the ASY 01-00193- marking it is a 486F19 for which we have no documentation, information etc yet.

Indeed, check for shorts in the tantalums and check if both oscillators are still alive. If you have a POST card, use that to see how far it tries to boot.

Reply 4 of 6, by jakethompson1

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No idea on the hardware side but here's what those BIOS screens look like if it helps any.

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  • Filename
    bios1.png
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    Public domain

Reply 6 of 6, by quicknick

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Fiddled a little with the oscilloscope around the board today. Noticed that after some resets there was activity on the EPROM data/address lines, all except A2. Traced the signal to a nearby LS245, then to another one, then it split three ways: A2 pin on the CPU, pin 92 on the chipset and some pin in the local bus slot.

This line was almost shorted to ground (2 ohms resistance). I hoped the second LS245 was the culprit, but after removing it the short persisted. I made a small cut in the trace going to the chipset, short disappeared, so dead chipset 🙁

Jumpered the cut trace and the short was there again, for it to disappear for good when I removed the chipset...

RiP Flexus/FreeTech board.

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