Hi Micha:
Tell us your current software version and if possible the IC ID that is bad.
The earlier A810s (some pre 1983 model with LCD display) with older MPU board version (1.810,752.00 or older) will not work with the 25/91 version firmware. The MPU board needs to be replaced to a newer version (1.810.780.00 or younger).
Furthermore, Studer still owns the intellectual properties on A810 software for few more years and the Audiohouse in Switzland sells the set of IC chips. Unfortunately, Audiohouse will only sell the complete set and not single chip out of three. Thus, I would recommend for you to replace the bad chip(s) with same the version of software you already own on your A810 for board compatibility and not to infringe on Studer's IP.
Ki