Removing the top cover on my pentax 645n - Pentax 645N Medium Format Camera

You're posted in Instant Cameras. More likely you need help with film camera.

Many possible causes. First is that the battery is in upside down. Next you should remove the bottom cover 3 screws and the bottom cover. You can then remove the battery compartment and check for battery leakage corrosion on the wire on the bottom. If the corrosion is too bad it will wick up the wire to the top circuit board. You would have to remove the front and top covers to replace the wire.

the least likely cause would be the failure of the photocell that turns on the meter.

finally, if the camera has been impacted the meter may be broken. Without a battery the needle should rest In the middle of the scale (half way between top and bottom, in the cut out). If, when you put in the battery and the needle goes either to the top or bottom and stays there, and stays there no matter what setting, it is a sign the meter is broken.

there are three different possibilities. remove three screws, take off bottom cover. remove three screws of battery holder, lift to see if the battery wire has corroded or broken off. solve by extending wire and resoldering. the next involves removing the top cover to see if the meter coil is dead,if this is the case it needs to be replaced, or there is a broken lead somewhere. usually it is cause no. 1.

K1000: Remove counter cover (3 setscrews around perimeter), remove counter disk (screw is right threaded - turn right to remove!!!!!), washer if not glued to bottom of disk, round nut with slots (also right threaded), lift off counter base, three advance lever spring screws, turn spring until loose, remove advance lever, remove washer underneath

K2: using a rubber stopper, and downward pressure on the cap/ cover screw, turn to right (this cap screw is right threaded!!!)If you try and turn it like you would a normal screw you will break it off. If the camera has corrosion under the cap screw it may stick - try heating with a soldering iron (careful the screw cap will be very hot). In extreme cases I have drilled two holes (one on either side) and used a pair of round nose fine tip pliers to turn it.

Unsure of which model you have as both are mentioned in post

Remove the battery for 20mins, replace the battery then go to the camera's menu to set the Date & Time.

It's not free but you can order it on CD from
http://uscamera.com/pen645nsm.htm

Some batteries can emit caustic fumes when discharging heavily that can attack battery contacts in the camera. Try using a pencil eraser to scrub any metallic contacts you can reach in the camera and the battery terminals too.

By opening, do you mean the back cover or the top cover?
after rewinding the film opening the back door will reset the counter. if it does not reset, try firing the shutter with the back door opened a few times. this counter is mostly plastic.
with the back door opened try taping the top cover over the counter to free the counter gear.
if it still wont reset the top cover will have to be removed and the counter repaired.
there are no new parts available for this model Pentax so a good used counter could be expensive.
if you take the top cover off, usually the pointer plate on the right side of the counter is rubbing on the counter preventing it from resetting.

You can download a pdf copy (in 2 parts) of the instruction manual from http://www.butkus.org/chinon/pentax/pentax_645n/pentax_645n.htm

this tipe of electrical sound can be a electrolitic condenser that is defective.
i can repair it if you are in europe.