Optical Laser unit defective after replacement?

Dear Technician

In the repair of everyday life, it‘s sometimes not so easy to see whether the cause is a defective laser or the problem is in a different location. Even after a new laser has been replaced and desired result is not 100%

The following "Checklist" to help you:

It’s recommended to examine the spindle and sled motor, as it’s not always a defective optical.

For the following reasons it can look like a defective laser unit, although it is not always the case:

• Troubled RF signal with all his false behaviors

• Hooks and jumps

• The CD player is sensitive when playback or vibrations

Our tip

Run the DC motor with an external power supply, E.g. 1 minute with highest permitted voltage in both directions. The collectors will clean themselves in most cases. Or replace, always a good alternative.

Once this has been done it’s time to replace the laser head.

The checklist

In some cases, pairing of the laser head is required! Please refer to the respective equipment service manual.

When adjustments are required?

• Disc will not run

• If no RF signal, but only a DC voltage pumps at the HF-(EYE Pattern) test point

• When the device plays but sounds (such as grinding sounds) Audible.

• That is too large focus reinforcement.

Focus gain control / comparison

Setting gain focus intensifies the focus error signal so far, until the focus control loop can close, so results focus OK. The disc running  at nominal speed.

In many devices, the gain is set so that complete focus control loop (the RF signal is produced and the disc runs). The gain will be too high because the 2-axis element in the laser unit can start over to vibration. This can cause Laser unit to be defective

Many manufacturers advise as a "Running off and overshoot" between the representation using the “Lissajous” character or similar.

Focus offset control / comparison:

In many cases, the focus offset must be readjusted and optimized the HF (eye pattern) amplitude.

Tracking control / adjustment

Is the envelope of the RF signal (in the 2mSek. Range resolved) similar to broken sinus, arise like the typical tracking error noise. Here, the laser rotating disc tries to find his trail. This does not succeed however and the tracking lens is "wild" back and forth controlled. The typical chirping sounds are created. This is a clear sign of an affiliate.

Most will proceed as follows:

Tracking gain is minimized (or the tracking control loop is interrupted as per manufacturer's instructions), thus deliberately no tracking is possible and there is a large tracking error signal.

Now such as tracking offset or tracking is adjusted balance, set this tracking error signal on symmetry (symmetric about 0 V or a reference voltage in some devices).

Then the tracking control loop closed and reinforced with tracking gain so far until the device works also shock resistant (sensitive).

Grating is a mechanical rotation of a lens in the laser unit. This is the laser light, generated beam divided into the actual laser into the main beam to track scanning and an e and f. This e and F radiance are the producing of the tracking error signal and must now palpate the own spur(!). By moving a slot - directly on the laser unit (which the grating setting) this lens is now moved. Thus, this e- and f rays across 5 tracks can be moved. Scan the wrong track this e and F signals will cause the unit to an affiliate.

The match is usually as follows:

• The tracking control loop is interrupted (see Service Manual)

• On the tracking error test point is measured

• The device may be taken into the service mode in operation (CD is playing without tracking feed)

Now run through the grating setting

• This will signal greater the tracking error when e and f scans a track - The service mode and test

• Is tracking critical, can all CD read (BB´s), then repeat the process grating, further move and adjust the next highlight of the tracking error signal

• As long as this repeat until the e and f-scanning on the own trail takes place and the device can optimally render all CD BB´s.


  1. 1. Remove available solder bridges, jumpers and anti-static locks (transport protection) before you replace the laser unit.
  2. 2. Avoid damage caused by your own static charge. Ground yourself and take attention to protect ESD components.
  3. 3. Adjust the device with an oscilloscope if laser unit is not working well. See adjustment instructions in the Service Manual.
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