The newest kernels don't autoset the correct tuner/card type for cx88xx cards...
1. As sudo/root, do an lspci -v and look for the Connextant card: you want the Subsystem & 4-character device ID (this should tell you the vendor of the card).
2. Search online for that 4-character ID to see what card and/or driver it belongs to.
3. This list has IDs for the card type in the kernel driver.
4a. As sudo/root, nano /etc/modprobe.d/options and add options cx88xx card=# i2c_scan=1 , the # being from the card # found in step 3.
4b. If you know the tuner make/model, look it up on this list, and add tuner=# to the previous command.
5. Reboot and scan for channels with MythTV/tvtime/etc.