going back to basics
the power socket (wall plug) - ok?
kettle lead, fuse - ok?
psu - tested with voltmeter you said? i assume ok? if not you have tried your spare psu?
mobo is new - so unlikely to be an issue (DOA possible, but unlikely)
cpu, gpu, ram all not excluded
don't plug the ssd in yet (extra component to add to the complications)
you have a spare computer - ram is easy to test, as is the gpu
these would be the first items i would test on the working computer
then we can say
mobo ok
ram, gpu ok
then would leave cpu/mobo
the power socket (wall plug) - ok?
kettle lead, fuse - ok?
psu - tested with voltmeter you said? i assume ok? if not you have tried your spare psu?
mobo is new - so unlikely to be an issue (DOA possible, but unlikely)
cpu, gpu, ram all not excluded
don't plug the ssd in yet (extra component to add to the complications)
you have a spare computer - ram is easy to test, as is the gpu
these would be the first items i would test on the working computer
then we can say
mobo ok
ram, gpu ok
then would leave cpu/mobo