Osborne 1a Not Booting

I have been attempting to restore an Osborne 1a to working condition. The problem I am currently having is that the machine will not boot. When I turn it on, there is no beeping noise, the monitor does not power on, and neither do either of the LEDs which mark the A and B drives. I recently replaced three Rifa capacitors on the power supply board after having one of them fail when I first turned the machine on. I am fairly certain that the power supply is now working as intended because I used a multimeter to measure the voltages of the wires making up the cable that connects the power supply to the motherboard and found them to be carrying voltages of 0, 5, and 12. However, when I attach this cable to the motherboard, I wind up with a reading of 0 volts on all of the wires. I did not observe any obvious damage to the motherboard or any of the components it houses. I would greatly appreciate any advice on how I might be able to pinpoint the cause of this issue and what steps I ought to take from here.

Thanks for reading,

C.C.