What is the Model Number of the Time Capsule that you have?
If you are not sure......and your Time Capsule is "flat" like a small white pizza box......the Model number is usually on the bottom of the device embedded in the foam pad base. The model number starts with an "A" followed by 4 numbers.
If your Time Capsule is "tall" like an 8" tower, you have model number A1470.
Regardless of which Time Capsule that you might have, hopefully you are not thinking of replacing the hard drive without also replacing the Time Capsule power supply........correct? We ask because the power supply usually fails before the hard drive fails. The power supply in your Time Capsule is very likely overdue to fail.