How do I repair a corrupt Admin Password?
I'd reset the password from the Recovery HD and then change it in the Keychain afterwards.

Have a look at Reset Forgotten Admin Password OSX 10.10 Yosemite, which is also applicable for 10.8 and later.


Reboot (Apple menu > Restart) to the Recovery HD holding down Command-R during startup.
Go to the Utilities menu and select: Terminal
In the Terminal type resetpassword and press Enter.
In the Reset Password screen follow the directions.
Go to the Apple menu and click: Restart 


Then reset the password in the Keychain after booting back normally to the Desktop.