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