Audio Systems: NAD 514, nad 514, laser diode

Question
QUESTION: Dear Sir,
I have a CD player, NAD 514,bought it one year ago as a used  one. After few months, problem showed up.  The problem is, when I put CD inside, pick up lens started to go up and down ( I opened CD player so I could see this), there was a cracking sound while lens was going up and down. Sometimes, CD could play for 20-30s, and repeat the same thing. What could be a solution for this problem, since there is no NAD service in my country. Thanks in advance
Radan

ANSWER: Does it do this with all cds?  Does it do this at the very beginning or after it starts to play the disc?  Are you getting any music out at all?

Based on the not so complete explanation of your symptoms it would appear either the laser diode is going (gone bad) or the servo drive system (including the IC controller) is malfunctioning.  If this is so, the repair costs are generally more expensive than replacing the entire unit.

It is also possible that due to vibration and a strong mechanical shock to the player the tracking system has been knocked too far out of adjustment and the optical beam cannot focus in to read the track pits on the disc - hence it is moving up and down trying to focus in.

These are the possibilities.  Hope it helps in your analysis.

C



---------- FOLLOW-UP ----------

QUESTION: Dear Sir,
first of all, thank You for such a fast respond.
My NAD does the same with all CDs, sometimes it's able to read data regarding numbers and duration of tracks and sometimes not even this. I will use this opportunity to mention that I have the same problem with my old SONY CD101 (I think first commercial CD player ever produced), by Sony start this after few songs played (15min of playing). Since I'm an electrical engineer and have some electronic knowledge and practice, do You believe that,if I manage to find laser assemblies for these players, replacing lasers would solve problems with these players? Or I could try to adjust the tracking system?
Thanks in advance and
Best regards
Radan

Answer
When a disc is first set to play mode the instruction set tells the system to read the TOC (Table of Contents).  The TOC is always recorded on the first tracks at the center of the disc surface.  The system also tells the optical pickup to null to the best focus so the reflected beam can be read the strongest.  So, what is happening in your system is the beam cannot focus nor get a good strong, reliable signal.  This is likely a badly aligned tracking system and/or weak laser diode output as previously stated.  

If you could find the optical replacement (a new diode is comes inside the pickup head) you could install it and find out what happens.  The tracking adjustment requires a test disc and oscilloscope to adjust the eye pattern to the best shape.   The service manual has pictures of what the eye pattern should be adjusted to in most cases - or at least some criteria for nulling it to the best signal strength.  OR, with some trial and error you can sometimes set it by playing a series of good and bad discs, getting it somewhere in the middle for acceptable performance.  If the tracking is close, but not right on it overloads the error correction circuits and gives break-up or dropouts.  If it is too far out of adjustment it will not read back anything at all.

So, maybe this will help you decide on what to do and how to do it.

So far as where to obtain the optical pickup, I cannot say.  NAD probably uses the Canon pickup - as most of them do - but I am not sure.  If you can remove it you may be able to see the markings on it and order the part on line or through the likes of partstore.com or other such resources.

Hope this helps.

C

PS:  Laser diodes come out of the factory on a gaussian distribution; some are really good and last for over 5,000 hours, but some of them are on the short side and only give a few hundred hours.  And there is no way to tell until they begin to fail.