If a hard disk is not powering up, there is basically only one option to troubleshoot it.
Find/Buy/Acquire an absolutely identical model drive.
Swap controller boards.
If the drive powers up, it was the drive controller board. If it does not, it is likely the fault is in the electronics somewhere inside the drive.
Basically, there is no realistic way you can independently troubleshoot the drive. The controller for a hard drive is both enormously complex, and entirely proprietary. It's not like you can call up Segate or Western Digital and ask for the schematic.
If you're really, really desperate to recover the data on the drive, look into professional data-recovery services. They have the specialized facilities (a cleanroom, among other things) required to open the drive, and move the drive platters into a working drive.
Anyways, to answer your question, rather then what you are trying to do:
The controller board in a hard drive does a number of things.
- Handles analyzing and parsing the raw data-stream from the HDD heads (analog front-end is likely at least partially in the drive casing itself. I'm not sure where the analog-digital conversion itself actually occurs).
- Error correction on the stored data stream.
- Sector addressing, bad-sector remapping.
- SATA interface (this runs at either 1.5, 3.0, or 6.0 Ghz!)
- Command-queue parsing and reordering in the case of NCQ support.
- Data cache management.
- Translation of logical partitions to physical location on the platters.
- Analog servo for the head positioning actuator.
- Three-phase motor driver for the spindle motor.
And probably a lot more I can't think of off the top of my head.
A common hard-drive has a enormous amount of R&D and design effort involved in its design and production. It has embedded firmware, significant non-volatile memory, high-speed data-interfaces, multi-gigabit-per-second data streams, and complex analog processing. There is really no way you're going to have any luck doing anything but the most superficial reverse-engineering without a multi-million-dollar budget.