A few months ago my camera started to develop an annoying fault whereby the first few shots after switching on were completely back (or at least very significantly underexposed). This became more common until it was nearly impossible to take photos. I checked online and quickly discovered that this is a known problem on Pentax cameras made a few years ago (mine is about four years old).
The problem is known as “aperture block” and you can read about it at great length if you wish. The bottom line is that there is a vital electromagnet which controls the lens aperture. A few years ago Pentax switched from the magnet manufacturer they had used for decades (right back to film cameras) and the new ones frequently fail after a few years. I understand that new Pentax cameras use a completely different type of magnet and are not affected, but that doesn’t help me!
Unfortunately, the old reliable magnets are no longer made, but you can buy the new ones from Hong Kong for about £8 each. I bought two and set about following the online instructions to replace the broken one.
The task is not for the faint of heart. You have to remove dozens of tiny screws and as they are different sizes you are best to fix each one to a drawing of the camera as you go along. With the top, bottom and front of the camera removed it was possible to get the the old magnet. It was tricky to remove as the battery housing is in the the way of the tiny screw but it got it out in the end, unsoldered the wires and eventually installed the new one.
I am however delighted to report that the camera now appears to be working reliably. Of course the current magnet also probably only has a life of a few years, but now I know how to replace it and I have another spare in stock.
Update April 2021
Unfortunately, the camera has failed again in exactly the same way!!! I don’t really want to risk taking the camera apart again and fitting the second magnet if they are only going to last for a bit over a year. I’ll have to see if I can find a more permanent solution.