486 Parts Testing

thinkpadman

So the 486 I recently picked up on eBay, wasn't getting any joy turning it on.

I decided to rip it out and attempt to use a different board. The old board had a ISA/VESA combination, and this board had ISA/PCI instead and built-in floppy and HDD controllers.

I bought if off eBay with the DX2 66 MHz CPU already in it.



After plugging in a S3 Trio64 card and the RAM that came with the machine, it would boot up. I then discovered the RAM to be 8MB in total.





I then turned it off and tested the original CPU from the machine, a DX4 100 MHz. That worked so I ruled out the CPU was fried.

I then tested a DX4 75 MHz CPU which was no where near as common as the 66 and 100 CPUs. That worked though the BIOS stated 100 MHz. This is probably from running multiplying at 33 MHz instead of 25 MHz though couldn't see the jumper for that, only the voltage. I'm not sure how the 75 MHz chip would go after a longer duration running at 100 MHz with no heatsink.

The motherboard originally in the machine I'm guessing is the culprit, however unfortunately I don't have an ISA or other VESA video card either to test it with.

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Do you have a picture of the original board, specifically around the CMOS battery? If it has leaked, there is a high chance that the acid ate through the solder mask and corroded the copper traces.

I was in a similar situation where the CMOS battery had destroyed three traces on a 286 board. It would POST, but the problem was that the video was dim and distorted, likely to the high resistance caused by the corrosion. I fixed it by soldering wires to bridge the disconnected traces. Works fine now.

I also have a problem right now with a Socket 3 board with corroded traces due to a leaking battery as well. It will POST, but none of my keyboards will work.

I betting that you may have corroded trace(s) that are preventing the board from POSTing.

[EDIT]

If it's any help to you, the board you're using now appears to be a PCChips M918 / Amptron DX9300 with undoubtedly fake cache.

http://motherboards.mbarron.net/models/486pci/m918i.htm

thinkpadman

Below is a closeup of the original board after taking off the battery. Definitely not the worst I've seen.

Thanks for the link. I mainly liked the fact it had a coin battery, PCI slots, and built-in disk controllers. My other 486 has 256KB of real cache.

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That actually looks pretty clean, and most traces appear to be out of the way of danger anyhow. Check to see if any of the tantalum capacitors grenaded. I had a board that would POST and not POST at random. Turned out that one of the tantalum capacitors exploded sometime in its life. Once I replaced it it worked fine.

thinkpadman

Those tantalum capacitors, the yellow ones in the picture, all seem to be fine. All 16 of them. There's only a couple of electrolytic capacitors and they are fine as well. Generally I'm not really seeing anything wrong from a physical sense.

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Do you have a high resolution picture of the board? I'd like to have a look at it if it isn't too much trouble.

calvinb

It's a bit of a misconception that caps are only bad if they look the part. That was poor chemistry. I'd recommend replacing all the caps anyways, considering they have a lifespan of only so long.