Sometimes after heads stack replacement the new heads stack works not stable. That occurs because every heads stack has its unique adaptive parameters. Those parameters are stored in module 47 of ROM.
The adaptive parameters are more different from one batch of drives to another and is connected with date of manufacture. That is why it is recommended to select donor drive for heads stack with the closest date of manufacture possible. Some heads stacks from exactly the same drive as your patients can be totally incompatible due to these parameters.
