Hi,
I have an Osborne 1 which I recently fixed up. I previously used it about 9 years ago, so before powering it up I replaced some capacitors on the power supply.
Here's how to get to the PSU board. Place Osborne top side down on a soft surface. Remove 5 bolts from what is now the top half of the case. Remove two video knobs (just pull off), the video pass-through plug, and unplug the keyboard. From the front bezel, remove the top three bolts, and loosen the bottom three. This is so you can pull the top of the bezel away from the top half of the case, which should lift straight out. Remove four bolts from metal brackets used to fix main PCB to chassis. Should now be able to waggle PCB away from bezel to reveal the base of the PSU, which has a plastic sheet covering it (see photo). Remove two screws holding PSU in, wires should all unplug.
As per Tezza's article there were three old Rifa X capacitors, one 0.1uF and two 0.01uF. The 0.1uF was starting to crack, the smaller ones were probably fine but I replaced all three anyway.
I also checked the electrolytic caps in the PSU with my LCR meter. The two mains smoothing caps (250V 100uF) were high ESR (1.0 ohm @ 1kHz) and one was slightly bulged so I replaced them with the same value but 105C (United Chemi-con KME series), the new ones had ESR of 0.3 ohm (at 1kHz) so should last a while. All the low voltage ones were fine (I think my Osborne has low running hours).
When I plugged it all back together, before powering on I turned the floppy drive spindles by hand (were a bit stiff), powered on and all was well. Booted from floppy on 2nd attempt (disk wasn't centred the first time), loaded CP/M and running fine, thankfully no issues. BTW my screen is black & white rather than green.
Regards,
John