Laser Diode current for KSS 151A?

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

beauty_divine said:

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
 
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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.
 
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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
 
anatech said:
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.
 
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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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Hi beauty_divine,
I forgot to ask, what machine do you have?

The KSS-151A is a very nice head, a linear tracking motor design. Normally these go into some very nice CD players. Well worth repairing.

-Chris
 

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Thanks for the nice explanation:)
My CDP is Sony 338ESD.
May I conclude that the APC system with the ageing of the LD has to increase the Iop this way I can directly measure this increased current in order to judge the LD remaining life???
that is a bit stunning conclussion for me if I am right :att'n:
Could you pass me a bit more info - what is written on a test CD that makes it being a test CD?
I tried to follow adj instruct. on an old harman/kardon incl. focus and tracking gain/bias with a good matrix CD and successfully completed all of them.
Or can I make a test CD by myself recording test tones in different tracks on CD-R or so?:)
 
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Hi beauty_divine,
Unfortunetly, you can not make your own test CD, they must be purchased. What makes a test CD is the pitch, eccentricity, reflectivity and pit shape. There are also manufactured defects on some involving all of the above attributes.
May I conclude that the APC system with the ageing of the LD has to increase the Iop this way I can directly measure this increased current in order to judge the LD remaining life???
Yes, that is how it's done. LD life may vary, all you can tell is whether it's past it's useful life or not.

When adjusting focus and tracking gain, the proper test CD is very important. There are "ball park" methods that seem to work well. You are probably fine. The offsets and E-F balance adjustments are much more critical.

-Chris
 
beauty_divine said:

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.


Hi

A laser hbehaves like a diode. the first say 50 mA run, but do not generate light. Than, at the bending of the power - current curve, the next mA generates about 1 mW of power. OPU's have a servo loop to control the power, as the bending of the curve also dependson the temperature.

best

Guido
 
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Hi beauty_divine,
Try setting your Ilaser to Iop + maybe 5%. With a precorded music CD you should be getting a sharp, clear eye pattern. I don't know what the expected amplitude is for your machine. Your laser current may even be close to the Iop value marked on the head. If you have the manual, they usually include a scope shot of the RF pattern. You can put the eye pattern close to that if your scope is in calibration, but less than the Iop + 10 % figure.

Try different discs and choose the one with the highest and clearest eye pattern to work with. You have a higher chance of needing a disc motor than a laser pickup.

Hi Guido,
A laser hbehaves like a diode. the first say 50 mA run, but do not generate light. Than, at the bending of the power - current curve, the next mA generates about 1 mW of power. OPU's have a servo loop to control the power, as the bending of the curve also dependson the temperature.
That's a good explaination. Thanks, it may be clearer to some for sure.

-Chris
 
Hi Chris, Guido!
1. I checked the Iop to find out that touching the adj trimmer has not created a disaster :) I measured 51,7mA vs 50,7mA printed on the paper label.
2. As I do not have a test CD to measure the eye pattern I did put an orignal matrix one to see that adjusting the corresponding trimmer could not increase the Vp-p above 1V. What I saw on the scope was that this amplitude depends on the disk type - pirated matrix copy or CD-R produce less amplitude.
In the service manual they recommend 1,3Vp-p +-0,3V. It seems I'm at the bottom line - even below. Could this be a sign for exhausted LD, or it is just because I do not use this test CD?
So far with these adj levels my 338ESD plays all matrix and CD-R CDs I have.
Thanks a lot for your valuable help and for the provided knowledge!:)
 
BTW,
http://www.tohri.co.jp seems to be a good supplier for older laser units.
They are not specialised in that but still have some old units in stock.
Just checked for KSS-151A, they still got about 100 of them.
I bought three KSS-123A from Tohri three years ago, (I bought the last ones I guess) as far as I can remember, it was about €40 each, without tax.
A supplier here in germany charges €129...
All the best,
Salar
 
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