A friend has an obsolete Japanese CNC metal-working machine which is built around an NEC FC-9801V "factory computer". The FC-9801V is a V30-based machine similar to the very popular (in Japan) PC-9801UV of around the same (1984) era.
The FC-9801V computer has developed a rather bizarre fault - if left unpowered for >28 hours it will boot (apparently) normally, but it will not re-boot if switched off and powered on again within around 28 hours of switch-off, and it will not reboot if the reset button is used while powered on. When it fails to boot it generates a sequence of three "beeps" - presumably a POST error indication - and the processor READY signal is then found to be held low.
We have carefully gone over all boards and replaced any capacitors that look suspect, but to no avail. The phenomena is unaffected by temperature, was unaffected by baking the board-set at +40C for 24 hours (so unlikely to be a board leakage problem), and is unaffected by the presence/absence of the RAM/RTC holdup batteries. The time constant also seems unaffected by attempts to provide relatively low-resistance discharge paths by holding the solder side of boards in contact with a sheet of black conductive (anti ESD) foam with the power off. The power supply seems fine with no obvious difference in output voltages or power-on voltage rise-times when the computer boots as compared with when it doesn't. Both I and my friend are experienced electrical engineers and have spent considerable time trying to find the problem - so far to no avail.
My guess is that the problem could be of an electro-chemical nature (possibly due to ingress of contaminants into an IC) with perhaps electrical bias driving a chemical process in one direction which then reverses slowly over time when bias is not applied. It's a pretty "left-field" theory but I can't think of anything else that would explain such a long time constant independent of the things we have tried to influence it.
Does anyone out there have technical info on the FC-9801V or its desktop PC-9801 cousins. Even a description of the POST "beep codes" could be a help. Better still a technical hardware manual (an english lamguage translation is known to exist) and/or a circuit schematic.