Nikon D100 aperture control unit repair

A couple of months ago I managed to find a used DSLR body for a very good price at a local flea market. I had been thinking of getting a proper DSLR for some time now (so far I’ve been using a “bridge” camera, the Fujifilm S9500), so for only £30 it was worth the risk. What I got was a Nikon D100. This is a fairly old, 6 megapixel DSLR, which cost about $2,000 when it came out in 2002.  I primarily got the camera to use it together with my telescope for astrophotography, and for that it didn’t need to do much. However, now that I had the camera body, I also wanted a lens. With Nikon being one of the two big DSLR manufacturers (Canon being the other one), it didn’t take long to find an inexpensive used lens on ebay. When I tested the lens with my camera body, it became clear that something didn’t work properly. While autofocus worked fine with the lens, whenever I wanted to use even a slightly smaller aperture (with F-numbers as low as 5 or 6) the camera displayed an error, and depending on how high an F number I had dialed in, the image tended to over-expose. I decided to investigate.

The mechanical link between camera body and lens

The way Nikon SLR cameras set the aperture is via a mechanical link between lens and camera body. On the inside of the bayonet mounting ring is a little lever that can pivot up and down. The lens itself has a similar, spring-mounted metal “hook” that also goes up and down and opens and closes the aperture respectively. The aperture lever in the camera body is normally in an up position, whereas the lens hook is normally pushed down by its spring. Therefore, when the lens is mounted on the camera, the aperture lever pushes the lens hook up thereby opening the aperture and letting the most amount of light through the lens. In order to achieve smaller apertures, the lens aperture lever has to move down to allow the hook on the lens to move down as well. This happens either just before the picture is taken, or manually when an appropriate “preview” button on the camera body is pressed. Nikon calls this button the “depth-of-field preview button” and it sits (I suspect typically) on the right side of the lens so it can be easily reached by either the index or middle finger of the right hand.

Since the lens mechanism seemed to work perfectly fine when I moved it by hand, I suspected that the aperture lever on the camera body wasn’t doing its job properly. Several posts on various webpages and forums also suggested that the aperture lever could be the source of similar problems, e.g. that it might be bent. As the camera body I got had clearly seen some “action” (e.g. there is a fairly visible dent in the bayonet ring just above where the lever sits), problems with this lever wouldn’t be at all surprising. Having found a copy of the official service manual of the camera, I decided to take a closer look.

Don’t turn it on – take it apart!

From looking at the service manual it was immediately clear that this was going to be a major undertaking. Since the aperture lever is buried deep within the camera body, I had to take the camera apart almost completely. This not only involved a large number of very small screws of various sizes, but also disconnecting flat flex cables from pretty tiny connectors, de-soldering various wires and even de-soldering a few flat flex cables. In order to keep track of the various screws, I taped a white sheet of paper on my work surface and drew little boxes on it to put the screws in. The service manual had the different screws individually labelled (e.g. #636) so all I had to do is write the number into my little “boxes” as I went along. I also put only those screws into individual boxes that came out in each “step”. That way I could easily  keep track of what needed to be screwed back together at the end. After a few hours of disassembly my camera looked like this:

A quick overview of the different pieces

The first piece to come off was the bottom cover. This was the least interesting part of the camera as it is only a mechanical base, but it its own way it was also one of the most difficult ones. There are a total of 9 screws in three different sizes. Unfortunately I managed to strip the heads of two of them – for one I actually had to drill out the head and then remove the rest of the screw with pliers once the bottom cover was already off. I will have to get some replacement screws eventually. The bottom cover revealed two stacked circuit boards. One is relatively sparsely populated – it has the mini-USB connector on it and what I suspect are two RAM chips. The second one was clearly the power supply board. If was full of passive devices, had connections to the main capacitor (Edit: make sure to discharge it as soon as possible) and also held the backup battery. The back cover housed the LCD and all the buttons around it. The side and front covers came off easily. The board assembly with the CCD sensor was itself a two-board construction and was covered by metal shielding. Taking off the top cover was one of the more involved steps, as I had to de-solder several wires and make sure not to tear any of the flat flex cable connections. Last but not least, the centre part of the camera with most of the mechanical pieces had to come out of the structural frame. This centre part alone is an incredible piece of engineering with several layers of flat flex circuit boards wrapped around mechanical structures. Luckily I didn’t have to take it apart much further.

The aperture control unit

To get to the aperture control unit I only had to take of one of the motor assemblies, which was easy enough. A few more screws and a bit of de-soldering, and I had the aperture control unit apart from the rest of the camera. It is a relatively simple mechanism consisting of a housing, a few gears, springs, and a few metal pieces, one of which was the aperture lever that protrudes out of the lens mount. Electronically there were only two small devices, which however are critical for the correct operation of the unit. Connected to one of the gears was an optical encoder disc similar to the ones that used to be in old computer mice. Together with its sensor it must serve to provide feedback about the amount of rotation. The other electric device was what I suspect to be a very tiny solenoid. This solenoid could lower and raise a locking plate which allowed the rotation of the gears to be stopped at a specific point. The aperture lever itself was effectively the last element in the chain of gears. Therefore, the optical encoder and the solenoid locking mechanism are used to determine how much the aperture lever pivots down and thus closes the aperture. The service manual stated that, with the solenoid fully retracted, one should be able to pull the aperture lever down, and that the spring mechanism should pull it back up into its starting position. This wasn’t happening on my unit – either the solenoid was stuck and not in its fully retracted position, or it had shifted its position. Given the age and abuse the camera had apparently seen either one of those were possible. The solenoid seemed to move easily enough, so I simply (re)positioned it as indicated in the service manual: there should be a gap of between 0.2 and 0.35 mm between the locking plate and the gear. By folding aluminium foil a few times until it had the desired thickness I created an improvised gauge to set this distance.

Putting everything back together

It took me a few more hours to put everything back together. While de-soldering flat flex cables was very easy, soldering them back to the PCBs was a bit more of a challenge. Eventually the camera was whole again and it was time to turn it on. Much to my delight it turned on fine and seemed to function perfectly. Most importantly I was now able to set the aperture to any of the available values. Initially I thought that the autofocus no longer worked, perhaps because one of my soldering connections wasn’t good enough. However, this turned out to be a simple setting that wasn’t set as I expected (AE/AF lock button instead of shutter release button halfway). Since it was already dark when I was finished, I couldn’t test it out. However, I took plenty of pictures the next day. Here’s a random one that had a relatively small aperture (F/11, 1/100s, ISO 200).

Tynemouth Priory from Longsands during sunset.

Conclusion

Having now taken apart a DSLR camera I have gained an enormous amount of additional respect for the engineering that goes into producing such a device. Of course I knew that my camera was a fairly complex device, but seeing and experiencing it for myself increased my appreciation for these devices tremendously. It is no wonder that these cameras are not very cheap, and it is almost shocking that I was able to pick one up used for only a tiny percentage of its original sale price after only a little more than 10 years of its life.

Would I take apart another camera like this? Absolutely!