Could also be IC A110, best way to know is to use an LED Logic Probe or an O'Scope and check the signals when trying to access a disk.
When you power on your Atari, does the 810 attempt to seek to a track? If it does, then the write commands are working and would suggest the read circuitry is faulty. If not A110, then maybe LM311 could be at fault. I've had a scratched head before that caused mis-reads, but it did read the sectors...