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Identify where to plug external battery on 486 motherboard

vbug

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Jan 2, 2021
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France
Hi, I got that old 486 motherboard, it seems working (at least it turns on) but I got a Low Battery error at launch. And actually there is no battery on the motherboard, and no place for an internal/soldered one. I assume I should plug an external one to it, but I'm unable to find where ! Most of the time there is a 4 pin connector, but I can't find one on that motherboard. At the bottom of the keyboard connector there is a J11 with 5 pins, there is 5V between pin 3 and 4. I suspect it could be that connector, I tried to plug a 3.6v battery on pin 3-4, and another try on pin 1-4, no results. Does someone have an idea ? Thanks.
 

T-R-A

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See the part in front of the memory with the sticker on top. That's your RTC module.
 

maxtherabbit

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That IC next to the RAM and keyboard controller labelled "QC-2-OK" is a sealed RTC module (aka Dallas). If it is in a socket you can simply replace it with a brand new one, if it's not you have other options
 

modem7

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Hi, I got that old 486 motherboard, ...
The motherboard is quite close to a BCM model LX400A/P VLB. See [here].

At the bottom of the keyboard connector there is a J11 with 5 pins, there is 5V between pin 3 and 4. I suspect it could be that connector, I tried to plug a 3.6v battery on pin 3-4, and another try on pin 1-4, no results.
You may have damaged the motherboard by doing that.

That IC next to the RAM and keyboard controller labelled "QC-2-OK" is a sealed RTC module (aka Dallas).
The 'QC-2-OK' sticker is a quality control (QC) one, probably for the entire motherboard.
The part number of the RTC module is under the sticker.
What is that number?
The part number may lead to information that indicates whether or not the module is compatible with a commonly used Dallas one.

If it is in a socket you can simply replace it with a brand new one, ...
Noting that there is a risk that the battery in an acquired replacement module (same or compatible) is bad too, the module having sat in storage for many years.
 

maxtherabbit

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The 'QC-2-OK' sticker is a quality control (QC) one, probably for the entire motherboard.
Obviously, I didn't say otherwise just a handy way for OP to know what I was referring to
Noting that there is a risk that the battery in an acquired replacement module (same or compatible) is bad too, the module having sat in storage for many years.
Thus why I said "brand new" - they are still being manufactured
 

Chuck(G)

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If the battery is soldered in, and this was my board, I'd repair it using the Dremel+coin-cell holder method. Given where the RTC is, the risk of damaging fine traces (going to the DRAM array) is too great for my taste. Besides, the Dremel approach pretty much assures that there won't be compatibility issues.
 

vbug

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France
Hi everyone, I checked the reference on it : TH6887a. I'm wondering if all thoses RTC modules are compatibles between them because I found some of them on my closets ! I would try to add an external battery on but are all thoses modules physically the same internally or should I find the right position where I should cut the shell to solder the battery ? Thanks :)

Edit : I found http://greyghost.mooo.com/th6887a/ showing where I should cut the shell for contacts. I removed the module on the motherboard and below to it I can read "DS1287". The old ones I found on my closets are "DALLAS DS12887A" and "ODIN OEC12C887", I'm wondering if I can test them on that motherboard, according to the link I found there are clones, then should be compatible, right ?
 
Last edited:

oldpcguy

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Sep 23, 2021
Messages
183
Hi everyone, I checked the reference on it : TH6887a. I'm wondering if all thoses RTC modules are compatibles between them because I found some of them on my closets ! I would try to add an external battery on but are all thoses modules physically the same internally or should I find the right position where I should cut the shell to solder the battery ? Thanks :)

Edit : I found http://greyghost.mooo.com/th6887a/ showing where I should cut the shell for contacts. I removed the module on the motherboard and below to it I can read "DS1287". The old ones I found on my closets are "DALLAS DS12887A" and "ODIN OEC12C887", I'm wondering if I can test them on that motherboard, according to the link I found there are clones, then should be compatible, right ?

According to the article you referenced the TH6887A is a Twinhead branded Dallas DS1287/12887 thus they're interchangeable. Knowing that and that you were already removed the module why not replace it with a new one or maybe a drop in replacement which uses a CR2032 such as the following:

DS12887 RTC Drop-In Replacement (fwscart.com)

Or if you want to spend the money on the real thing:

DS1287 by DALLAS INSTRUMENTS - Buy or Repair at Radwell - Radwell.com

NOTE: I have no affiliation with either manufacturer / seller, I just found these while looking for replacement Dallas RTC modules after having read your post.
 

vbug

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Jan 2, 2021
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France
I soldered an external battery on the original chip, it is not too much crappy and works perfectly now :D Thanks for the links for replacement pieces, I'll keep in mind that exist in case I need one. Thanks everyone for your advices and information :)
PC180173.JPG
 

Chuck(G)

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There you go! I prefer to mount a 2032 coin cell holder right on the chip; looks good and no dangling wires. Like this guy did:
8565_clock_seite.jpg
 
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