Interesting Motherboard Repair That You Should Not Miss

By on October 12, 2018
486dx motherboard repair












This repair is about my good old 486DX-2 mainboard of which the RTC clock chip goes bad after the internal lithium cell after many years fails to back up the setup settings. And the computer no longer boots. Resulting in a start-up blocking “CMOS Battery LOW” error.

Why I still use my old but trusted 486 PC you may ask? That is because I use external IC testers, GAL-/Eprom-/PAL- & Universal programmers that simply do not work on newer boards anymore.

For one thing because the LPT port on every newer mainboard has changed so that it uses changed internal port addresses and the LPT port no longer is directly controlled by its original port address. Newer boards changed because they now have the option to set it to ECP,EPP,Bi-directional or other settings.

Therefore none of my old LPT controlled devices do work on those mainboards. And the second reason is because newer boards no longer support internal ISA or VLB slot cards. And my Taskit Eprop Eprommer and my ELV IC tester ISA cards do not work on PCI or any newer expansion slot.

486dx motheboard repair

Above my first MB1433 mainboard with 486 DX-2 33MHz processor and 4MB RAM with extra mainboard Cache Ram memory. 3x VLB/8-16bit ISA expansion slots, 3x 8-16bit ISA expansion slots and 1x 8 bit ISA expansion slot.

I already must have repaired at least 4 of those 24 pins (not all pins connected!) RTC chip modules by replacing the internal lithium battery. When the integrated 3V lithium is completely discharged after up to 10 years or more, the PC no longer operates and the RTC will need this repair! And setup will give the error message “CMOS BATTERY LOW!” and will not continue its startup. Next photos show

the now empty TH6887A RTC chip top and solder pin view. This RTC chip also exists with other names but same function. And I have not yet had any pin incompatible RTC. Every repaired chip functioned!

rtc th6887a

Below the saw I used to open the RTC with. First I placed the RTC in a experimenter’s board covered with paper to prevent the contacts getting dirty. This way protecting the RTC pins from breaking off while sawing.

rtc th6887a ic

Above photo on the right shows that I almost am about to discover the internal now empty 3V lithium cell. Below photo shows the – pole of the empty cell which also is attached to pin 12 of the RTC. I placed the quality (machine-) socket for the RTC module after I had removed the bad module.

bad rtc motherboard clock

bad rtc motherboard clock th6887a

After I had removed the useless 3V lithium cell I inserted it in the 24 pins socket and tied it with a tie-wrap to prevent it from coming loose which would render my PC useless if that happened.

repair th6887a

The bad removed lithium was a small CR1220 cell. I will replace it by a much larger 3V lithium cell but attached to 2 flexible wires. This way I also keep the height of the RTC chip within its maximum intended space to prevent the chip from blocking the inserted ISA slot cards. Next photo shows the old CR1220 cell. And the next photo the result after I attached it to a new bigger lithium cell.

bigger lithium battery

It now was time to give the 486DX-2 mainboard with the repaired RTC chip a test run. On next photo the setup screen of my fixed mainboard is working again and the “CMOS BATTERY LOW” is gone!

cmos low

old vga card

Previous photo shows the mainboard under test with ISA VGA video card and the VLB/ISA 8-16 bit I/O card inserted. After this RTC module repair my good old 486DX-2 66MHz and also my other 486DX-2 33MHz type MB1433 work again like they had worked the past 20 or more years! And they probably will for many more years now.


Next photo my MB1433 DX-2 66MHz computer.

 MB1433 DX-2 66MHz computer

I will show now why I need to keep my trusted PC system in good working order. My not cheap but about 20 year old Xeltek Superpro L universal programmer would be useless without my PC:

superpro universal programmer

My Superpro L or short SP/L is capable to read PAL PLD chips and is also able to write PAL-CE IC’s. Which can’t be done with most universal programmers today!

xeltek superpro universal programmer

The Xeltek SP/L works on the LPT port. Also my perfectly working Elektor GAL programmer (1993) in above photo only works on the LPT port of my good old 486 PC. Following photo shows my Taskit

(published in German C’t magazine) Eprop eprommer device that uses its own ISA controller card and therefore works only on my good old 486 computers.

xeltek superpro universal programmer adapter

xeltek superpro universal programmer connector

Above my ELV IC tester that also uses an ISA controller card. It is capable in testing TTL/CMOS IC’s up to 20 pins. But not existing test algorithms can also be programmed and added to its library of

components. I also have the stand-alone Elektor IC tester (3/’98) and that is sometimes even better than the TL866 because it also tests certain TTL/CMOS types that the TL866 can’t.

Elektor IC tester

Elektor IC tester machine

In recent years I’ve added a few testers and digital meters to my equipment but since they all are different and none of them is perfect, together they come close!

Next schematic I’ve drawn after I previously assumed that my mainboard was defect somehow because everything worked but not my 4 external devices.

After replacing the hard disk by a bigger one, replacing the 720KB FDD by a 1.44MB, and replacing the CD-ROM player by a better DVD player it probably was the changed port settings that were wrong. And they only can be changed in some of the programs used. Anyway, my PC works again after I inserted another VLB I/O card and checked and changed the port settings again. Following schematic could be of help if defects are found. I didn’t find anything wrong with my mainboard afterwards but made the schematic by measuring on the 74LS244 chip beneath the JETkey 5.0 keyboard BIOS chip .

Nothing can be found on the internet, at least no schematics or user-manual, about this MB1433 mainboard. Which is rather remarkable because these still do work perfectly and were of very good quality! So I hope that this schematic contributes in filling this gap. The controller chips on my mainboard in this article were BIOTEK’s 100 pins type 82C3493 and a 208 pins 82C3491. I hope I made no error but if I did please don’t hold me accountable for any mistakes made. So please recheck my findings.

mb1433 diagram

*Do not get confused by the A from the ISA bus with the A from the schematic address lines. It was not intended and neither avoidable.

I hope that this article will be of good use to those who also still need to keep these old mainboards for their old but expensive devices up and running!


Albert van Bemmelen, Weert, The Netherlands.


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Note: You can read his previous repair article in the below link:




  1. A.R.Balasubramanian

    October 12, 2018 at 3:47 pm

    Thanks for useful Article
    How ever I would like to know how to repair Philip's DVDR 3570h mother board
    Otherwise unit replacement spares , where available. I'm from India

  2. Albert van Bemmelen

    October 12, 2018 at 6:32 pm

    I guess you have an internal HDD boot problem? Maybe have a look here:
    You know that without matching firmware most modern devices won't work? It often is a boot problem if it is not a hardware related fault on your mainboard.

  3. Parasuraman S

    October 12, 2018 at 11:20 pm

    My head is reeling! You are a real expert and true professional in your field! I do not wish to try these out! Too much of techie stuff! Hat's off!

    • Albert van Bemmelen

      October 13, 2018 at 3:46 pm

      Thank you dear Parasuraman, but I have no doubt that you've had to accomplish many more difficult repair tasks in the past which would likely have made this mainboard repair just another easy job for you!

  4. Robert Calk Jr.

    October 13, 2018 at 6:40 am

    Nice work, Albert!

    • Albert van Bemmelen

      October 13, 2018 at 4:15 pm

      Thanks Robert. In case you were wondering all external devices were DIY made projects, except for of course the Xeltek SuperPro L universal programmer.
      And the chips my 3/'98 Elektor TTL/CMOS tester is able to test that the TL866 can't were for instance the cmos types 4060,4066,4099 and many more!

      • Albert van Bemmelen

        October 13, 2018 at 10:49 pm

        PS: I yesterday also did add another about 20 year old but still perfectly working device, a TEAC PD-518 PD writer/CD player to my minitower 486DX-2 66 Mhz PC shown in the article. This TEAC uses special PD boxed in rewritable PD CD 650MB optical discs that can be rewritten without any problem 500.000 times! (of course only as long as the laser keeps working). And one should know that there also exists a Windows 7+ driver that still enables this special player/burner to work on newer PC's. But I haven't tested this, only noticed the new available driver. So it could be an interesting fact for music studios or other music lovers if these players show up on eBay for a small price.

        • Robert Calk Jr.

          October 14, 2018 at 10:23 am

          Hi Albert,

          Most of that stuff is over my head. I didn't even start surfing the Net until about 7 or 8 years ago. I have never used Win7. Right now I use Win10. If I can't use something with W10, then I don't use it.

  5. Justice

    October 13, 2018 at 6:44 pm

    Hi Albert. You have stepped up the pace with this article which is a motivating force to keep up with the pacemakers. Thanks for such complex article and so informative/ educational.

    • Albert van Bemmelen

      October 13, 2018 at 11:05 pm

      No problem Justice. Glad you liked it! Maintaining such an old computer by fixing the RTC prolongs the system to work for many more years to come without any problem. Adding a working Network connection on this old Windows 95/98 OS to connect to Windows XP or Windows 7 computers is however a real challenge!
      But is mandatory if one also wants to backup his Windows 95/98 image (with Ghost software or other) onto a Rescue CD to be able to restore it in case of boot problems. Mainly because these old Windows computers can't boot from CD or run Setup to install Windows directly from disc. A Floppy Disk is the only way to DOS boot run and activate a CD/DVD player which makes running the standard Windows Setup command impossible. (Which gives the "Setup cannot be run from DOS" message!).

  6. Henry

    October 14, 2018 at 10:49 am

    That is impressive. Highly specialised field, requiring a real understanding of the computer functions at a chip level. Way beyond my comprehension.
    I have never heard of or seen pictures of a chip being treated by a humble hand saw. Yet, it worked nicely. Hats off Albert.

    • Albert van Bemmelen

      October 15, 2018 at 3:13 pm

      Thanks Henry. The humble hand saw and some other not so sofisticated tools helped me out in fixing more complex devices more than once! Like just recently when I had to use a small about 1.2 mm drill bit to fix my larger Hot Melt pistol in which the Hot Melt Glue got completely stuck in the metal opening. After drilling out the old plastic Hot Melt Glue inside the 240VAC Hot Melt pistol it was ready to be used again!

  7. Albert van Bemmelen

    October 14, 2018 at 3:44 pm

    This is given the fact that these good old 486DX-2 mainboards do not support directly booting from CD or DVD in their BIOS system yet. Only booting from floppydisk A: and HDD C: is supported. Something that newer mainboards today support. Why you also need a floppy MSDOS bootdisk with also the CD drivers already on it installed. Which you only can create with the help of a ready working Windows computer. (Which can be a from Windows 95/98 or ME floppy created bootdisk). Also keep in mind that you are asked to insert proof of having acquired a previous Microsoft Windows product (Windows 3.0 or 3.1 on 3.5 inch floppydisk) if you are going to install Windows 95 on a new uninstalled computer. After that disk is read and checked the installation of Windows 95 will continue.

  8. Albert van Bemmelen

    October 15, 2018 at 3:50 pm

    I have Windows 10 installed on a refurbished Lenovo T430S that originally had Windows 7 Pro installed but apparently no longer was supported by Microsoft why I had to reluctantly upgrade. The problem with Microsoft products always is that they make new versions and ugrades with more problems being added at the same time. Of which new versions often are incompatible with previous releases. Like being unable to transfer a Ghost image Windows 95 file over to a Windows XP computer with a simple FastLink F3R program and a parallel cable connection because both Windows's are incompatible! And I have read about the many upgrade problems that Microsoft unintentionally created by distributing rather evil upgrades that corrupted many systems afterwards! Why I rather stick to proven old faster and safe versions! Especially when Microsoft corrupts those newer versions.


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