Symptom: my trusty Pentax K-r camera forgets the time and date every time I swap its battery. Turns out the clock battery is dead. So I ordered the service manual because opening a Pentax camera is not trivial. This is putting it mildly. Hoya used a gazillion types of screws! So, OK, I dive into the manual only to find that the battery is not a replaceble part! With great effort I discover the battery is a rechargeable Lithium cell. No, I don't make this up. Searched my ass off to try to find a primary cell but it isn't. I found a replacement in the Seiko MS621FE, Farnell order code 1614634:
But there were many hurdles that I had to take...
First off, the manual. Specifically the disassembly. While not wrong it causes way too much extra work. Then I discovered that after re-assembly my camera did not work. Nada. Zilch. Nakko. Niente. Turned out a flex print popped loose. So here goes. The authorative guide to replace your Pentax K-r manual. Do not embark onto this trip if you're not comfortable soldering small parts onto a circuit board! Leave it to experienced technicians.
Using the service manual, I found that disassembly should take place in four steps. With each step the corresponding screws must be stored in a small zip-loc bag or some other container. Only the final step's screws can remain in the anti-lose tray that you of course always have handy. The manual also wants you to unsolder the wires to the top part (A301). No need for that. Also, no need to bother with the flash capacitor. You will need a PH00 sized screwdriver and three small zip-loc bags. And a fine tipped soldering iron. Cover the lens opening with the original white protector if you still have it to avoid dust ingress.
Part 1: the flash. Manual page 8. Use the method in the manual to pop it or just turn on the camera and press the button. Remove the two screws and store.
Part 2: bottom cover (A401), page 9. Skip step 1 (battery cover) until step 6. No other problems, store screws and cover. Heed the warning.
Part 3: top cover (A301). Page 10. Do all steps including the 5th. You need an extension for your screwdriver to reach all the way into the battery compartment. Then skip steps 6-8 and only do step 9, lift the top cover, leaving it dangling by the wires and the flex. Store the screws in the third zip-loc bag.
Part 4: Page 11. Skip step 5. Place the body on the lens cover and carefully pry off the back panel. The flex with connector T920 pops off. Now the PCB with the dud backup battery is finally accessible:
The battery really wasn't happy:
It measured exactly 0.00 volts. The replacement cell was about 3 V:
Quicky I replaced the cell, being very careful not to short it:
Reassembly is just following the disassembly in reverse, right? Wrong! The flex (T920) that went into the socket on the left in the above image kept popping out due to the flex being too stiff. I finally solved this by putting some foam tape on top of the plug which held it in place. Unfortunately I don't have any images of that, but the foam was about 1 mm thick.
So, after carefully putting the front and back covers back together again I tested the camera. It took five attempts to figure out T920 had popped loose. Finally I got the orange flashes of the activity indicator when I inserted the SD card into its slot. Phew! Next challenge was figuring out which screws went where. Good thing I saved the screws for each part but still the manual is very unclear if a screw goes into plastic (coarse thread) or metal (fine thread). Using a loupe it was visible, and combined with the size of the screws it wasn't that hard. Still it was quite a puzzle.
When I got my camera assembled again, and actually working, my second body also became forgetful so I immediately performed the above procedure on it. Wow, what a trip: