You have done one of the following, I trust:
1. Video hack to allow CC video operation in 640x480, which the replacement logic board demands (which I do not recommend as it effectively destroys a CC); or:
2. ResEdit hack to allow the 575 board to boot in the CC's native 512x whatever resolution (which I do recommend, as you then have a ready way to get back to your stock CC)?
A 575 board in a CC won't work without one or the other. If you need info, google it as the procedures are available "out there."
I personally think a 550 board a far better match for the CC than a 575, the latter machine being better preserved than butchered for yet another CC "upgrade," as it is a gem
in itself. A 550, on the other hand, is far more dispensable by comparison.
Unlike the 575 logic board, a 550 logic board in a CC works without hacks, it is generally cheaper to source, it turns your CC into an über-rare CCII (incl. in how the system reports itself), your newly-minted CCII will boot from standard HD utility disks (which a "Mystic" simply will not without yet more work — which I think is undocumented), it will then be the fastest of the genuine Compacts as it makes the machine more than twice as fast as an SE30, it will take up to 128MB of RAM, it will take a maths co-processor (68882), and all of this is just plain meant for any lowly CC, which in its stock configuration is, sadly, basically unusable. Plus, the 575 board will strain the CC's power supply.
I have had both and much prefer the 550/CCII configuration.