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Pentium board boot problem

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First post, by naujoks

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I have an ASUS board with a Pentium 1ooMHz.
It had a DS12887A with a dead battery. With and without it the board doesn't boot.
I bought 5 replacemt ICs from two different sellers. With one of those the board boots and I can chnage the BIOS settings, but the board complains about low CMOS battery. It says press F1 to continue with default settings, but when I press F1 there's continuous beeping to be heard and the board hangs. BIOS settings don't get saved.
I drilled into the IC, found the two contacts and by measuring found out that the battery only reads 0.6V.
I then soldered a new 3V coin battery to it, but with it the board doesn't boot at all, the boot code remains at 0000.

Anything else I could try? To find a DS21887A with a fully charged battery seems difficult. I also donn't understand why the replacement of the battery hasn't worked.

Reply 1 of 5, by dionb

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Where exactly did you try to get those DS12887A chips? Anywhere on eBay and you are most likely getting a >25 year old chip as dead as what you are trying to replace, frequently with the original label sanded off and replaced by a new one making it look like the chip is new. Only place you will get good reliable ones is from reputable electronics stores. I occasionally buy some from reichelt.de, for example.

Reply 3 of 5, by Gmlb256

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The mod to add the coin battery usually works. An alternative would be getting the Glitch Works RTC replacement module.

BTW, I bought one of these DS12887 batteries on eBay years ago and it worked. The seller from where I got it wasn't from China, but don't consider it as a source of reliability.

VIA C3 Nehemiah 1.2A @ 1.46 GHz | ASUS P2-99 | 256 MB PC133 SDRAM | GeForce3 Ti 200 64 MB | Voodoo2 12 MB | SBLive! | AWE64 | SBPro2 | GUS

Reply 4 of 5, by Horun

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naujoks wrote on 2023-11-12, 22:27:

Admittedly I got the chips from Aliexpress.
Nonetheless, shouldn't the drill & solder battery to it method work?

Yes, have done the mod many times, very easy to do with a dremil. Just becareful and make sure you follow the instructions carefully.
Also before you cut the one int. battery lead you should test the voltage with good Digi voltmeter once you uncover the two pin points, if above 3.1v is good for a few years or more w/o the mod.
Only buy those things from reputable sources such as Quest Components, Digikey or Mouser.
some info on testing: Dallas battery replacement ?

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

Reply 5 of 5, by naujoks

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But I DID drill into it, and I did measure the battery (0.6V) and yet it doesn't work.
Worse still, without the 3V battery inserted, the board at least boots into BIOS, but when I have the 3V battery inserted, when I make changes to the BIOS and wanted to Save and Reset, the board hangs, and when switching power off and on again, the board doesn't even boot into BIOS anymore. I have to remove the battery and then it boots again.

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