This article takes a deeper dive specifically into the laser diode and photo diode part of the laser assembly, and the RF amplifier controller IC (CXA1081) that drives the laser and amplifies the returned photo diode signal before sending the signals (RF, Focus, Tracking) out to the Servo Signal Processor (CXA1082BQ) that interprets that data.

The laser head contains the laser diode (the laser that shines onto the disc), the photo diode (receiver diode that reads the reflected laser beam back) and all the motors to move the laser head and spin the disc.

The main chip we are interested in is the CXA1081 RF Amplifier and all its signals.

Let's work through the pins in order and what we should see.

This is the same as the RF output only centered around 2.5V instead of 4V.

This is the eye pattern test point where you measure with an oscilloscope the eye pattern. This is the data being read out of the laser, amplified ready to be sent to the Servo Signal Processor.

A well-tuned laser should have an output of around 1 to 1.6Vpp. It is centered around 4V DC but using AC mode to capture is easier to see.

If you add persistence of previous signals and trigger on a rising or falling edge, you should see a clear diamond pattern. This is a sign the data is being read smoothly and in a periodic fashion.

This is a 2.5V DC signal on a well-tuned system.

The Laser Diode P/N switching. You cannot attach flying leads or probe it with an oscilloscope directly without instantly changing its signal and preventing the game loading or seeing anything useful on the oscilloscope.

You would need an active probe that does not interfere with the signal at all to try and read it, including directly probing with a super short ground lead.

Even just touching this pin with your finger is enough to make the disc load fail.

This pin is the output power driver that controls the power being delivered to the laser diode. On a working well tuned system this this should output a flat DC.

When idle it is 4.5V. When a disc is spinning its around 3.6V.

When the laser diode is under-powered, it outputs a pulse.

Tune the laser head potentiometer until you see a smooth DC while spinning.

This mimics the Laser Diode Power signal but is offset at 0V not floating around 3.6V.

Connected to the photo diode pins A and C.

Too sensitive to see with an oscilloscope probe but sits around the DC bias voltage so appears as a flat DC.

Connected to the photo diode pins B and D.

Too sensitive to see with an oscilloscope probe but sits around the DC bias voltage so appears as a flat DC.

Half of the VCC so 2.5V.

Focus input into the focus amplifier, from the F terminal of the photo diode.

Too sensitive to see with an oscilloscope probe but sits around the DC bias voltage so appears as a flat DC. Use the Focus Error pin 19 to observe errors.

Error input into the error amplifier, from the E terminal of the photo diode.

Too sensitive to see with an oscilloscope probe but sits around the DC bias voltage so appears as a flat DC. Use the Error Output pin 12 to observe errors.

The output of the error amplifier.

Sits at 2.5V while idle, and pulls down to about 1.9V offset, with errors shown as ac waves around this bias point when there are errors reading data.

The feedback input of the error amplifier. Used to adjust the error gain.

Shows waves on top of the DC Bias when there are errors reading data.

Should be totally flat DC when reading well. Here is how it looks when it's reading poorly with errors:

Half of the VCC so 2.5V.

The DEFECT bottom hold outputs are capacity-coupled to this input.

Will pulse when it detects errors.

The DEFECT bottom hold output. Will pulse when it detects errors.

Ground.

The Bias pin for the focus error amplifier.

This is set via VR102.

Focus Error amplifier output.

This pulses around the DC Bias every time the focus has an error locking into the disc data.

Here is a perfectly tuned successful game load. Notice you still see focus errors, but they are acceptable and allow the game to load still.

Here is a poorly tuned laser or a disc being held in place to force errors, where the focus errors show a little more aggressively.

Tracking Error amplifier output.

This pulses around the DC Bias every time the track has an error staying inside the track line.

Here is a perfectly tuned successful game load. Notice you still see tracking errors as all discs are not perfectly helical in their shape for data so there will always be corrections.

Here is a poorly tuned laser or a disc being held in place to force errors, where the tracking errors show much more aggressively.

The DEFECT comparator output. When it fails to read data, it pulses high.

If you hold the disc still to prevent it spinning for example this would show multiple pulses to 5V as its constantly failing to read data.

The MIRR comparator output.

This is a long capture of the boot up of the Japanese game Secret Garden.

Close up you can clearly see the data being loaded.

When the game is idling and not needing data it pulses once every 133ms in this particular game.

Has a hold capacitor connected to the MIRR. This is the non-inverting input of the MIRR comparator.

Sits at 1.4V. Jumps to 3.3V when reading disc.

Has a hold capacitor connected to the DEFECT comparator.

Sits at 2.5V. When loading game data it drops to 0.9V.

Digital ground.

Auto asymmerty input. Sits at 2.5V.

EFM comparator output. Shows 5V digital data.

At idle its short random bursts.

When a game is loading, the data is wider.

Focus OK comparator output.

5V when the laser has focus, and ground when it doens't have focus.

Inverting Laser Diode active pin. Low when laser diode is turned on.

5V when the diode is off.

5V supply power rail.