Repairing a point and shoot camera with a broken focus motor
This post will show you how I was able to repair my SONY DSC-T9 camera.
Last week I enjoyed some nice holiday in france. Just when I arrived at the “Cote d’azur” my camera stopped working. It was really frustrating not being able to capture that nice blue water down there.
What was even more frustrating is that the camera I am talking about was my “Sony DSC-T9″. Now why should I worry so much about a 6 year old point-ant-shoot camera? Well, the camera does an excellent job at taking macro pictures with flash. I newer saw a camera as good as my T9. To give you an example:
This is a mosquito sitting on my jacket. I was outside in a park, took my T9 and photographed it. The result is pretty amazing (click on the picture for seeing the original size)
Also the compactness of the camera is impressive. The optics are placed vertical inside the camera body with a prism at the top (this prism acts as a 90° mirror which allows to place the camera inside in the vertical direction). Altogether a nice camera.
Now back to the problem. The camera suddenly took a long time powering on and then showed the error message E:61:00 along with a very blurred image.
By googling the code I quickly found out that this has to do with the focus mechanic and is not user serviceable. It didn’t say it’s not hacker serviceable though…
So the first thing I did when I got back from holiday was to open the camera. This is what I found:
On the left you see a very densely packed PCB with a lot of brown flex cables going in and out. These flex cables are often used in cameras as connectors and also as flexible PCBs. They are really fragile. So if you start working on your camera, try to bend them as little as possible. Also take care of the connectors of these cables. To remove the cables, you have to lift the black plastic parts which are pressing the flex cable into the connector. These plastic parts are also very sensible.
On the right side of the picture above you see the optical system in black. Luckily it is connected by just two flex cables to the left of it. I opened the connectors and could remove the whole unit. On the following picture you see the back of the unit. Surprise, more flex cables. How I love them! (attention: irony).
Now let us look at more details of the optical system. We find three motors here, zoom, focus and shutter. How did I find out which motor is for what? Simple: I removed the motors and moved the optical system by hand. With the camera turned on I could see in live the reaction on the screen.
Here is a close-up picture of the focus motor:
We see two coils and four pins on the motor side. Most probably this is a bipolar stepper motor. Mounted on the axis is a thread rod. This rod moves the green slide seen in the picture below:
As you see the technique is very simple. But still, this camera (and probably most other ones) is a masterpiece of engineering.
By playing around with the removed motor I noticed that turning a rotor felt a bit “sandy”. Like if there is something stuck in the bearing. As it is not possible to disassemble the motor further, I needed to do a little trial and error. My first guess was that adding a drop of oil would solve the problem. Unfortunately that was not the case. So then I tried bending the metal of the motor a bit to change the relative angle of the two bearings of the axis. With success! Afterwards the motor was turning with no problem. After putting everything back together, the camera worked as it always did :)
Maybe some extra info for the curious on how the focus system of this camera works. One interesting thing to notice was, that there is no “end switch” on the focus slide. So the camera cannot really know where the position of the lens currently is. And in fact, it does not need to know. The camera even does not know how far away the object to focus is. Hm, so the camera knows nothing? Yep, but it can “learn” the right focus point. The principle is known as edge based auto-focusing. The camera takes an image patch (usually the marked rectangle on the screen which you point on the object to focus) and calculated the edge image on this patch. An edge image can be seen more or less as a high pass filtered version of an image. Image areas with strong difference in intensity (eg edges) are getting a high value, whereas constant areas (eg surfaces) get a low value.
Now what effect does an unfocused lens have to an image? It acts more or less as a low pass filter. Sharp edges get blurred. High (spatial) frequencies are filtered away.
So, what needs a camera to do to find the best lens position? It just has to shift the lens until it finds the position where the edge image has the most edges in it. This is exactly the point of focus.
You can now probably explain to yourself why the autofocus is so bad at low light. The reason is, that a dark image has relatively low intensity edges, if any. That’s why most cameras have a LED light to brighten up the image patch under examination.
Another little info on that point: Most cameras offer a button for taking close-up shots (the symbol of that button is often a flower). What this button does is also very simple. It shifts the range of the lens move further away from the image sensor. This lets close objects get inside the focus range. The procedure of finding the focus is then the same as in normal mode. Why isn’t the range of the lens move extended to include close and far objects at the same time? Because then the autofocus would take much more time to finish. That is not really desired for a point and shoot camera.