Micro-disassembly of a DVD player

Armed with a Phillips screwdriver, a pair of tweezers and a 300x digital microscope, I decided to delve into the innards of this three year old Chinese-built DVD player. What follows are some of the interesting things I found inside. If you like, you can skip the regular teardown and go straight to the microdisassembly section at the bottom of this article.

The device

The device: a run-of-the-mill DVD player with the usual bells and whistles. Cheap, Chinese and claimed irreparable by the friendly neighborhood repairman.

Step #1 - under the hood

First look under the hood

The first look under the hood. I'm immediately out of my element: being a software engineer, no amount of the likes of me would traditionally be able to screw in a light bulb. Also, growing up in a family that wasn't exactly swimming in cash, we had to make all our purchases count. Taking something apart to satisfy my curiosity and not being able to put Humpty back together again was not an option for me as a child. And so I developed a hardware phobia. In fact, these teardowns are partly an attempt to cure myself of that fear.

But I do have something going for me: first principles and a good deal of reading. Let's see how far that gets me:

Right off the bat, I can see the drive (1), with a Russell Peters disc I need to liberate in the process of taking it apart. Everything connects to what is obviously the main board (3). Going by how they align with the various bits on the outside, I can identify: the surround sound outputs (2) coming in through it's own little board, as do the microphone/karaoke inputs (4). The USB slot gets its own little connection to the main board (5). Interestingly, the SD card slot seems to connect to a dedicated reader (7). The front panel connects to this (6) board, which I'm guessing houses the buttons, seven-segment displays and whatnot.

Step #2 - the main board

Before we go further, remember: I have virtually no practical electronics knowledge. I'm relying on what I remember from my few waking moments during Analog Electronics 101 fifteen years ago.

The main board, or whatever it is you're supposed to call it

Judging by the heat sink, I'm guessing this (1) is the micro-controller that's in charge of everything from controlling the drive to handling user input to receiving a bitstreams from various sources (such as the disc, USB and SD card reader) and churning out video. There are two more micro-chippy things (2) (3) on board, which I'm not going to bother with. Everything else appear to be input/output from other parts of the device and things that regulate flow of electricity, like the giant capacitor and the relay (4) (5). One day I might return to this board with a magnifying glass and a soldering iron, but right now, I'm not going to spend more time on electronics. When you get to the drive section below, you'll see why. That's where the interesting bits are.

Step #3 - over the sundry electronics in a single bound
A few snaps to summarize the destruction of the various bits of circuit boards, so we can get to the drive itself:

Out comes the cables, starting with the power

Nothing much to see here. Garden variety electronics with microphone inputs at one end and a cable to the main board at the other.

Pulling more cables. This one is the surround sound output.

And the mundane looking cable that connects the USB slot to the main board

And the shin bone's connected to the... USB IN.

Unscrewing the front panel...

...and you get a remarkably simple display, the playback control buttons and the USB slot

... and the connection to the main board

Here's that SD card reader

Step #4 - the drive itself

The drive

The drive seems to have three separate connections to the main board: judging by the type of cable and it's proximity to the video out on the main board, (3) has to be the video data cable. I'm guessing (2) controls the drive's rotation and the laser read head, and (1) controls the motor for the drive tray. Let's see if I'm right once I pull these cables and unscrew the drive off the casing.

Off it comes

The underside of the drive

Yup. The first cable (1) connects to the motor that drives the tray mechanism (2). The other cable (3) has wires running to both the main motor that rotates the disc (4) and another motor (hidden under the gear wheels) (5) that moves the read head (7) radially across the disc's optical surface. And the data cable (6) wraps around the back of the drive and connects to the read head (7).

Two screws later, I liberate Russell Peters from the clutches of the tray

The read head

Well, it seems behind every good Chinese product are a few choice Japanese components. The laser (1) -- the heart of the whole device -- is manufactured by Sony. On the underside of (2) is the drive motor and on the underside of this (3) motor are the plastic gear wheels that comprise the mechanism that drives the read head to and fro.

The read head, moved to the outermost position

Notice how I have pushed the drive head to the outer position (1). It slides along a metal rod (2).

Drive tray motor

Now I proceed to pull the guts out of the drive, starting with the motor that drives the tray.

Tray motor

The tray motor (1) is connected to the gear wheel from the last image with a simple pulley with a rubber belt, using power supplied by the main board (3).

Tray motor

And there it is.

Pulling apart the tray.

I was about the break the tray off like the cave man I am, when I noticed that all it took was a little press here to get that tiny little plastic wedge out of the way.

The tray

The tray's off. Now you can see the gear wheel (1) whose teeth are in contact with a complementary linear set along (2). Then there's a little mechanism (3) that gets pushed to a side as the tray closes, which causes another mechanism (4) to slide down, which in turn drags the disc down along with it (5), so that it comes to rest right next to the read head.

The tray, in the raised position

Here it is in the raised position. Notice how the mechanism (3) from the previous image has now moved left (from where I'm looking) and the disc holder thingy (5) has moved up.

The Z-shaped groves above are what convert the sideways motion to an up-down motion.

Next, I'm going to try and see how the read head moves. First, these screws have to go.

Looks like shock-absorbers (1). Most likely to to keep the head steady. The grooves (or track pitch) on a DVD are only about 1/2 micrometer apart. Even a little vibration can cause the read head to miss its groove (so to speak).

The read head assembly (or whatever knowledgeable people would call it)

Now the housing (1) is off and you can clearly see the motors that rotate the disc (2) and move the laser read head (3), and the cable (4) that brings power to both from the main board. And then there's the read head (7) and the data out (6).

The data cable

The data cable doesn't come with anything like a plug. When you pull it out of its socket in the read head, it looks like this.

Laser and tracking mechanism

Here's a closeup of the underside, where you can see the backside of the laser emitter and the reader (1), how the tracking mechanism works (3), the tracking motor (2) (I've since learned that's what it's called) and the drive motor (4) (also the proper term, apparently).

In which the author gets distracted by Safari Snacks

At this point, I realize that continuing the disassembly is going to require finer tools, especially a smaller Phillips screwdriver, tweezers and my digital microscope. While I look for these things, I end up consuming all available Safari Snacks in the house.

Tracking motor

Finally, I manage to pull out the tracking motor. But wait, this looks awfully ordinary for a motor that is supposed to position a laser head with 650 nanometer accuracy. I Google the serial number, and my suspicions are confirmed. It's just a low speed DC motor, not a stepper motor like the ones you get in hard disks (which lets you control exactly how much it turns). So clearly this thing only gets the laser within the general ballpark of the area on the disc that needs reading. Something else must be doing the fine-grained tracking. We'll soon see what that thing is.

Next, I'm going to take apart the tracking mechanism, so I can concentrate on the laser and the read head:

Taking apart the tracking mechanism

Taking apart the tracking mechanism

Taking apart the tracking mechanism

Taking apart the tracking mechanism

As you can see above, now I can concentrate on the actual laser and the read head (1). Let's forget about the drive motor (2). I tried to pull it out, but it won't come off. By the way, this too is a simple DC motor. But here it makes sense. This motor only needs to keep the disc spinning at a given constant speed. The read head will just wait for the relevant sections to pass underneath and read as they pass by. But remember your geometry: that speed remains constant only as long as the read head remains stationary. The further it moves away from the disc center, the faster the surface underneath the reader passes by. So in order to keep the data read speed constant, you have to slow down the motor (and vice versa). This How Stuff Works article has a nice illustration of this (see the second animation).


Step #5 the laser, optics and sensor

Here's the head. Hiding under that lens should be the actual laser emitter and the sensor that reads it as it is reflected off the DVD's optical surface.

First, the plastic piece comes off.

A closeup of the read head

Here you can see the lens that focuses the laser (1), the data cable (2) and two very thin wires (3) that are part of something else. I'm going to try and figure out what that is. And here's a ballpoint pen to give you a sense of scale.

Lo! a magnet.

Wait, what's a powerful magnet doing here? In fact there are two of them on either side of the lens. They kept sticking to the screwdriver every time I tried to get at those tiny screws. Next, I'm going to try and figure out what they're for.

The magnets on the read head

Here's a closeup. You can see two magnets (1), (2) on either side of the lens/laser and two pairs of tiny cables (3), (4) doing, well, something. I'm guessing their purposes are connected.

Trying to get the plastic (1) out of the way, I run into another (temporary) mystery: where did the laser emitter/sensor assembly go? (2) is clearly just a lens, and from the looks of it, so is (3).

And now, the micro-disassembly everyone's been waiting for

Time has now come to pull out the big gun, namely the microscope. Training it on one of the magnets on either side of the lens (see (2) in the last image), I get this:

Magnets

Fancy, isn't it? Right next to the permanent magnet (1) are two electromagnets (2) and (3). Notice the extremely fine gap between the magnet and the electromagnets. The magnification here is around 200x.

Suspension cables

Here's a closer look at a pair of those fine cables (1) (2), on either side of the lens. You can also see one of the two magnets (3) and one of the four electromagnets (4). Looking closely at this from all sides, I see that the lens is magnetically suspended. It's not touching anything except for those four tiny cables.

And the electromagnets? Following the copper wire lead me to the cable coming from the main board. Conclusion: I think this is the mechanism that compensates for the dumb tracking motor I talked about earlier. The tracking motor gets the read head in the general neighborhood, and small regulated currents to the electromagnets do fine-grained positioning. The suspension system probably also helps absorb vibrations that might cause the head to lose its place. Ingenious.

Inputs to the electromagnets

Here are the inputs to two of the four electromagnets.

The lens

Like I said before, suspended in between the magnets is just a lens. Now I need to go find the actual emitter and sensor.

Looking at the other lens (the one on the plastic housing) I see this:

Another lens

No laser here either. Just another lens glued onto the plastic housing.

That lens, again

Here's a frontal view of that lens, with the tip of a ballpoint pen for comparison. I turn it upside down and I see this:

More optics

On the underside, there's a third lens (1) and what looks like a prism or a reflector (2) directly under lens number two on topside. So backtracking along the path of the beam of light, the laser emitter should be somewhere around (3).

And here it is!

The laser, and/or sensor.

So after stripping away everything else, we find that this tiny little piece is the heart of the entire device. It emits the laser and almost certainly contains the sensor that detects the beam reflected off the DVD too. The sensor can't be far away. The beam displacement caused by the marks on a DVD's surface is very tiny (see the first image on this How Stuff Works article for a schematic version). So let's look at the thing closer:

The heart of the device

There you have it. It's hard to imagine that such a small thing can do such a big job. That black thing to the upper right is a strand of my hair, for comparison. There are 11 inputs/outputs. So I'm guessing it contains both the laser and the sensor. Quite frankly, I couldn't find anything else that could possibly house either.

Here's the other end of those wires:

Pins on the laser/sensor chip

And the data cable:

The I/O cable attached to the laser/sensor

And a closeup of the cable. It's all copper:

Closeup of the I/O cable

Now let's take apart the remaining bits of optics:

Another lens and a mirror

Here are all the pieces of optics:

Optics

A size comparison:

It seems like the laser beam goes out the emitter, through one lens (or a polarizer -- I can't tell), bounces off two reflectors in order to take a 90-degree bend, goes through another pair of lenses, hits the DVD optical surface and comes back along the same path to hit the sensors.

So there you have it, folks. Next time my plan is to take apart a 10-year-old CD player I have lying around the house, to see how far technology has come since then. And finally, if I can find the CD-ROM drive from my old 486 PC, I should be able to give you a look at 20-year-old technology!