Finally got some time to work on my 2 ThinkPad T21s - one is now working correctly (no blink-of-death issue).
What's the blink of death? You press the power button, the power LED blinks, and the machine shuts down. This seems to happen on the Thinkpad A/T2[0-2] series machines equipped with the ADP3421 power regulator chip as they build up years. At first it'll just require more presses on the power button, but eventually any/all button presses will not address the issue and the board must be replaced or re-worked. The root cause was never really determined as some machines with the ADP3421 swap will work reliably again for years to come, while others with replacement ADP3421 chips soldered in...will fail within days of being swapped in, and some conjectured that all DC power electronics will need to be replaced in order to address this. Note that the T23 has a different chip and is not impacted.
I had to strip both machines (800MHz/XGA and 850MHz/SXGA+) down and re-assemble it...looks like swapping the screen from SXGA+ to XGA didn't work - it was the motherboard that needs to be swapped...again. This was rather odd as the BoD issue was first manifest due to me leaving the original machine (850MHz/SXGA+/decent chassis) on AC to charge the battery overnight, and when it was done it refused to boot back up. Bought the spare (800MHz/XGA/crap chassis) cheap on eBay to try to fix the problem, and it arrived working just fine. Swapped the board and both machine shows BoD symptoms...well, the original board with the 850/(now connected to the XGA on the crap chassis) works after a little bit of fiddling, while the spare board on the 800/ SXGA+ on the decent chassis...didn't. Leaving it alone for a week didn't help either. Thought it might be the CPU so it was swapped, which also didn't work. Then tried to swap the screens. Also didn't work. At the end, the decent chassis got the original board with the 850MHz and the SXGA+ screen, and the crap chassis got the spare board 800/XGA back. That somehow did the trick for the original board/850/SXGA+decent chassis machine - - it's now working reliably (well, suspend-to-RAM doesn't work, but that's only relevant in Windows 98).
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The 800/XGA/Crap chassis machine still isn't working, and I am not sure why. Maybe it needs a stripdown/rebuild as well? Not that I really need it, it can be used as a battery charge stooge. Considering that the issue stems from the power controller on the T20/21/22 series dying/wearing out over time, at least the stooge can be used to charge the battery and keep the original working for longer. It's also good to have a spare LCD - those SXGA+ screens tend to have CCFLs that age poorly over time (turning dim and red), and swapping LCD assemblies on the series are a pretty straightforward task.
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Well, after about 3 hours of stripping the machine and swapping parts and waitng weeks for the opportunity to work on the machines, it's certainly good to see this IBM logo again.
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Okay , time for some important stuff to be taken care of, like manhandling the Rebel Alliance...