Reviving an IBM ThinkPad X31

This is an old repair I did a while ago and posted on thinkpads forums.  Reposting here to put it all under one roof. We have quite a few IBM Thinkpads X31 in the family. They are built like a tank and work well enough to keep using them despite 2003-2004 datecodes. One day two of them started exhibiting identical symptoms- they would just refuse to turn on. No lights, fans or other signs of life. Luckily I stumbled on a schematic on the web, so that was a good starting point.

 Following an excellent service manual from IBM, I took one of the machines apart and started going through power supplies, looking for anything obvious. Finally I found the main DC-DC regulator (MAX1631) that is responsible for generating 3.3V and 5V rails  was producing neither. Control/enable signals looked reasonable, so I figured the chip might have gone bad. Ordered a new one, and swapped it only to discover nothing changed. A few weeks later I was still poking at the motherboard with a scope, trying to see what could possibly be going on. I just happened to look at the regulator output right at the moment I attempted to start the notebook. I saw 5V come up, stay on for a few ms and then go down. Aha, so things are actually trying to start and then shut down. That was promising. I looked at the second output from the chip and saw a few ms of what looked like a 3.3V with a fair amount of ripple on it. That fair amount turned out to be close to 0.4V over the 3.3V baseline. That’s when it all started to make sense. MAX1631 has a built in overvoltage protection comparator, set at around 6% of the nominal output. Since the output was exceeding that, the regulator kept shutting down to protect downstream components. From there it was easy going- I slapped a big electrolytic cap across the output cap on 3.3V (C646) and the system started. I then ordered proper replacements from Digikey and replaced both 3.3V and 5V for a good measure. The laptop booted happily. I then repeated that on the second motherboard with the same results.

Parts I’ve used:  3.3V line (C646): 399-4052-1-ND (Kemet T520D337M006ATE015). 5V line C659 for 399-4045-1-ND (Kemet T520D227M010ATE018). These looked like close enough replacements of the original caps.

After getting many responses on thinkpads forums asking for details I took pictures of the motherboard with the new caps marked

X31 motherboard, 3.3V cap


X31 motherboard, 5V cap

And here is a page from the X31 schematic, showing the power supply circuit described:

MAX1631 based 5V and 3.3V supply


19 thoughts on “Reviving an IBM ThinkPad X31

  1. I first thought that my problem with it not starting was like yours but re-reading your post I notice that you have no lights on. Plugging mine in the charge light shows. Powering the unit on the caps lock and num lock lights flash briefly and I can hear the hard drive start up. But nothing shows on the screen; no bios boot up sequence no response to any of the keys including caps and num locks. Listening closely the hard drive doesn’t sound like it’s doing anything except for a steady hum. Is this a different problem to yours?

    • I have similar sympthoms, although mine starts with just power adapter disconnected. I. e. it starts when on battery power only.

      • My X31 has the same disease.

        The power and battery light goes on, there is just a black screen. It looks like it starts a reset sequence, but nothing appears. Same symptoms as moviedude describes. I have already changed the two capacitors (thanks for sharing the knowledge!) – but nothing changed.

        One more thing. If I leave it on for some time (with the black screen), like for 20 minutes – then I can switch it off and immediatly on again and I have all running normal! Tried this without the power connector, just the half charged battery. Can you guys try it out, please.


  2. Definitely different. Mine looked completely dead- no lights, no response to anything. In your case it actually seems to have power. I’d probably start by going through the usual stuff-unplugging as many things as you can, and maybe looking at memory- sometimes one bad and one good stick would cause something similar, though I do not remember if you’d get a beep from BIOS.

    • Just wondering if my local component supplier doesn’t have the exact part numbers can I buy based on micro Farad and Voltage values?

      • You also have to keep in mind the ESR of the cap- anything too excessive will cause more “fuzz” to appear on the output, possibly tripping OVP

    • Just checked and the cmos has no charge. Would this be it? I’ve never seen these in stores so could I prise off the leads and solder onto a new battery?

        • Managed to prise the tags off the old battery and solder onto the new. Replaced but no change. As the caps are cheap and to put this whole thing to rest I’ll go ahead and order them and replace the two in your photos.

          • Careful with those batteries. I tried various brands and some repell the solder. Some are solderable. But a little to much heat kills the battery, so check if there is still the right voltage after the wire transplant …


  3. I managed to ‘wake up’ my X31 from the ‘power on, fan spinning, black screen’ mode.
    The only lights on when powered were the battery and Z lights.
    I removed the HDD and memory, tried starting it (hearing the error beeps), then reinstalled everything. I powered it up and could see the IBM screen, then it went into windows and the screen went black again. I shined a flashlight on the screen and could see the toolbar and icons, meaning the backlight went out for some reason. So I pressed the key combo ‘Fn + F7’ to switch to external VGA (none was attached), then pressed ‘Fn + F7’ again and the screen came on. Try it and see if it works for you. You might not even have to remove the HDD or memory – I just included it here because it was part of my troubleshooting. Good luck!

  4. Hi
    thx for the howto, you made me try again to repair my X31.
    In may case the caps you exchanged, where still ok ( not so much rippel to trigger the poweroff)
    but th 1R8 and the 2R2 caps have been to weak…
    thus same error different caps.
    regards Willi

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