Maybe the external battery connector wasn't making good contact. And the activity of replacing the EPROM may have wiggled it a bit. Then you attributed the EPROM change as the solution.
If you apply, say, 10 cycles of remove/re-connect the external battery, then put back the old EPROM and update the configuration settings and time, you may find that it will retain the configuration settings.
The external batteries have always worked perfectly. This is about what happens when you disconnect the external batteries with the PC turned off.
- With the original factory-installed EPROM, BIOS settings and date/time instantly reverted to defaults (nothing configured and 12 AM, 1/1/1980 for the time and date) and it wouldn't boot again until I configured the BIOS. For example, if you disconnected the batteries and pressed the power button on the PC to turn it completely off, then waited exactly 1 second, and pressed the power button again to turn it back on, all the BIOS settings were completely and utterly gone, every single time, no exceptions whatsoever.
- With the new EPROM that I programmed using a copy from the original EPROM, BIOS settings are retained for at least 2 hours while the PC is turned off and the batteries are disconnected, and the time of day is only off by the same amount of time that the PC had no power available to it.
But did you try returning an original EPROM? Maybe it's not the EPROM at all that made an improvement but you cleaning the board for example. It could be a current leakage from a backup circuit into the rest of the circuit, and you replacing M27C512 with a different one somehow reduced that leakage. Also more than a single die version of this IC was produced, so not all of them are exactly the same. Not to say if you bought the replacement from China, they very often relabel EPROMs into another similar part.
No, I hadn't tried putting the original EPROM back in there. It's kind of a pain to get to and I had figured it would just go back to the way it was before, but I just tried it and it didn't go back to the way it was before. Now it's retaining the BIOS settings even when turned off and the batteries disconnected, with the original EPROM in there, just like it first started doing when I put the new EPROM in there the other day.
I don't know what to make of that. I'd already had that original EPROM in and out of its socket several times about two weeks ago when I was trying to figure out why the PC was always freezing after being on for about 10 minutes or less. It turned out to be a heat issue. I put a heatsink + fan on the CPU and it's been 100% stable ever since, even when running nonstop all day. I didn't have a heat sink or fan for the CPU before because most people don't put one on an AMD Am486DX2-66, and it's not printed on the CPU itself that it needs it (back then, the ones that needed a heat sink / fan had it written on them; like on
this AMD Am486DX4-100 for example). But it was during a heat wave here and I guess the 95-degree F ambient temperatures were more than it could handle without any cooling.
That board has tantalum capacitors though.
Yes, it does. I was thinking that tantalum capacitors were a type of film capacitor, but Google says they're a type of electrolytic capacitor.
I still have no idea why it can suddenly retain BIOS settings for at least 2 hours with no active power source. I've had it since 2002 and it has never been able to retain BIOS settings for even 1 second without an active power source.