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-   -   Laser Diode current for KSS 151A? (http://www.diyaudio.com/forums/digital-source/89000-laser-diode-current-kss-151a.html)

beauty_divine 25th October 2006 11:40 AM

Laser Diode current for KSS 151A?
 
I need to know the value of the laser Diode current for KSS 151A, used in Sony, Denon etc. CDPs.
Please, help me with this info.
Thanks!

Guido Tent 25th October 2006 12:34 PM

Re: Laser Diode current for KSS 151A?
 
Quote:

Originally posted by beauty_divine
I need to know the value of the laser Diode current for KSS 151A, used in Sony, Denon etc. CDPs.
Please, help me with this info.
Thanks!


Hi

Expect something between 40 and 60mA

best

beauty_divine 25th October 2006 12:44 PM

Re: Re: Laser Diode current for KSS 151A?
 
Quote:

Originally posted by Guido Tent



Hi

Expect something between 40 and 60mA

best

Me too but this difference is somewhat like between 30% to 50% of the full scale :)
I'm really surprised that in the service manuall Sonny have ommited to describe this initial setting with exact figures...

Guido Tent 25th October 2006 12:51 PM

Re: Re: Re: Laser Diode current for KSS 151A?
 
Quote:

Originally posted by beauty_divine

Me too but this difference is somewhat like between 30% to 50% of the full scale :)
I'm really surprised that in the service manuall Sonny have ommited to describe this initial setting with exact figures...


These figures represent real world data, I worked for Philips, designed OPU's and we ran production in Shanghai. The currents (among others) were all logged. Semiconductor lasers deviate......

best

Guido

beauty_divine 25th October 2006 01:16 PM

Shall I estimate that my LD is still OK if it works fine and is well around the 60mA current range?
And is there test setup requirements when measuring the LD current through the serial resistor?

anatech 25th October 2006 02:02 PM

Hi beauty_divine,
The laser current is marked on the white label on the laser head. You need to put a decimal point before the last number. If Iop exceeds that value by 10 % the head is judged to be beyond it's useful life. Do not adjust the laser power, ever!

Most makes of laser heads operate in this way, except Philips.

-Chris

Edit: Just play a good, normal CD and make your measurement. Philips test disc 5 is normally specified, or some other standard test disc. Never align with burned CDs.

beauty_divine 25th October 2006 02:04 PM

Quote:

Originally posted by anatech
Do not adjust the laser power, ever!

And why is this? Slight increase proved to work fine in and old CDP I used to have...

anatech 25th October 2006 02:49 PM

Hi beauty_divine,
This is because it drastically shortens the life of the head. If you want to do it to your own, fine. Just never do this to someone else's, or recommend it be done. It's just plain wrong.

Why? Because other problems can be the reason for mistracking or failure to play. It's very easy to destroy a perfectly good head. Increasing the signal may only mask the fault, not fix it. So you needlessly ruin a good head.

A properly trained technician (most guys are not) can tell the difference and correct the trouble. A hack will cost you more money down the road. I know this because I've serviced CD players since they came out. I've seen what people can do to these machines.

Another hint. Guys who repair TV's are normally terrible with audio. I'm not very good with TV's (so the converse is true). It's just that most jockies will not admit it.

-Chris

beauty_divine 25th October 2006 02:56 PM

Quote:

Originally posted by anatech
This is because it drastically shortens the life of the head. It's very easy to destroy a perfectly good head.
Hi Chris,
In general you are perfectly right, but what I had in mind was that I've got pretty old CDP and a new pick-up on hand. That is why I allowed myself to slightly touch the adj trimmer-just to squeeze little more life from the tired head.
What I still do not get after the discussion is how it is possible to have such great tolerances in the LD current and at the same time slight increase of it could ruin the laser? It seems controversal to me.

anatech 25th October 2006 03:31 PM

Hi beauty_divine,
Each head is set at the factory to output a specified beam strength. The current is then printed on a label after the locking compound is put on the trimmer.

As you can imagine, current creates heat which reduces the efficiency of the laser diode to some extent. That is one reason a metal plate is used for the laser diode on the head (heatsink).

Now, the preamp chip in your CD player also contains an APC circuit for the laser diode. (APC = Automatic Power Control). It works by sensing the current through the monitor diode in the head. You are adjusting the sensitivity of the pickup diode. The APC has a limited range. What this means is that as the laser diode ages, the APC will demand more current to keep the output in a range. The limit allows the output to fall as the diode becomes less efficient. This protects everything in the laser drive circuits from outputting too much current and damaging a component. This may instantly destroy a new head (if the pass element shorted).

The last thing to consider is that a laser diode only outputs a laser beam over a range of current. Too high and the diode output falls, then you get into destruction soon after at slightly higher currents. An APC circuit would guarantee this if you set it in this range. Most people, technicians included, don't understand that.

-Chris


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