I looked at a bunch of bad backlit portable mobos for Houlton, and all of them had problems with the 5V power regulator, as does my own 5126. Apparently the integrated voltage reference gets flaky with age. Capacitors don't seem to be as much of an issue with backlits as with the original 5120s, nearly all of which have leaky / dried out electrolytics.
I never managed to trace out the circuit or work out a fix for the 5V regulator in backlits, and I think the best approach is to simply replace that section with modern parts. One of these days when enough other projects get cleared off the decks, I'll have at mine. This often produces the spin-up, shut down, spin-up, shut down behavior.
Bad caps in the non-backlits do the same. The problem there is noise on the power supply outputs, which can show up as flickering or strobing of the screen, failure of the floppy to read disks, no sound one or both channels, etc. Replacing a bunch of caps is the answer. There are meters which can measure capacitors and inductors, called LCR meters. They can measure a cap in circuit to tell you whether it needs replacing or not. At this date, the majority of caps are either completely dead, or less than half their rated capacity in the portables I've looked at, so I'd just plan on replacing them all.
Last I checked, my local Batteries Plus store had the cells for Portable batteries for about $35 for three.
I never managed to trace out the circuit or work out a fix for the 5V regulator in backlits, and I think the best approach is to simply replace that section with modern parts. One of these days when enough other projects get cleared off the decks, I'll have at mine. This often produces the spin-up, shut down, spin-up, shut down behavior.
Bad caps in the non-backlits do the same. The problem there is noise on the power supply outputs, which can show up as flickering or strobing of the screen, failure of the floppy to read disks, no sound one or both channels, etc. Replacing a bunch of caps is the answer. There are meters which can measure capacitors and inductors, called LCR meters. They can measure a cap in circuit to tell you whether it needs replacing or not. At this date, the majority of caps are either completely dead, or less than half their rated capacity in the portables I've looked at, so I'd just plan on replacing them all.
Last I checked, my local Batteries Plus store had the cells for Portable batteries for about $35 for three.