Pushing the 8 button to clear all will return the unit to factory defaults, so you will have to manually re-establish the previous settings.
I'm a little puzzled by the "Bad checksum in presets" message, as this means there is a problem with the presets in internal flash memory. If you have the last update, you could try re-updating the unit. We no longer have updates for these old units. You could also take the top off, and wiggle the flash memory board to clean the contacts, but there is a risk of breaking something. If all the presets are OK, I recommend that you ignore this message, rather than doing the above.
The battery is actually a combined sram and battery, BQ4013YMA-120. This is available from Texas Instruments suppliers, but given the political situation, you may find this hard to get.
I would suggest that you change the battery in the card, as this can corrupt the data on the card and sonetimes crash the unit if not in good shape.
