My old DVD reader is not reading DVD's, it reads CDs though.
I want to adjust its laser diode's power by changing the potentiometer behind the laser. There are two of them , one for CD, one for DVD.

What I need is the optimal working range of resistance the DVD laser diode.

How can I reach that data? I do not want to burn my DVD rom's laser. I searched by entering the DVD rom's model and found DVD's manuals. But, manuals includes general conditions of working not laser diode specifications etc..

What info I need inside DVD(i.e pcb card number ???) rom and where can I get it? I hope someone has a knowledge about it.
Thanks.

Resistance is not a parameter for diodes, current is. Resistance is non-linear and measuring it won't give you anything. I have 5 or 6 DVD readers in my locker, ask around, someone will give you one for free.

I found that every computer part have its service manual according to its model number. When I found "Service Manual" of my DVD rom my question will be answered. In service manual there should be circuit schematics, electronic parts list etc. So I think that info helps all people who want to fix something in his/her own!
At the end, my question found an answer, nobody can know which specs my DVD rom laser diode potentiometer has, only Service Manual has that info. Solved.

The diode is only a part of the circuit - a narrow light. This light shines on the CD/DVD and is reflected onto another device, which detects what is written - high or low. Those two are less than 1% of the circuitry of any cheap CD/DVD player. Admittedly dirt there is the biggest cause of trouble. But don't think that's a sure-fire cure. I repaired Electronic hardware for 17 years straight, and I decided what got fixed, and what did not. Apart from cleaning things, I never tried to fix CD/DVD player. It's just not worth it.

Thought there were two colors that were used on DVD and CD??

But the laser power output is pretty much the same across brands and player models.

If your player is failing to read DVDs because the laser diode has gone a bit dim (which is a quite common scenario), adjusting the power might get you another few years of usage.

You can make a primitive laser power meter using a CdS cell in a suitable cardbord or plastic enclosure (to keep ambient light out) and any multimeter set to measure resistance. Simply measure the resistance while the sensor is exposed to the laser light of both your player and a known good player, and compare the readings. If the defective one reads a bit low, simply adjust the potentiometer until you get the same result as on the good one.

@jefro

From a quick trawl of t'internet I get:

CD laser, 780nM, infrared
DVD laser, 650nM, red
BluRay, 405nM, blue/violet.

my

Play Bonny!

Please, metric system is case sensitive, the unit is 'nm'. Nanometers, not nano-mega (absurd unit).

It pains me to see you guys wandering around here.

The OP seems out of his depth to me. He's a home user wanting the right to repair, which is fine. But at that level, he needs to be an Electronics engineer/technician with data on the device in question. He definitely needs a high spec oscilloscope, and the knowledge to use it.

Even then, he should save effort and replace, because it's not worth the time or effort if the replacement cost is low, which it currently is.

The reason the drives might read one only was the color of the diode issue. One of the diodes would fail.

Not really. If it's just the laser power that's the issue, and it very well might be, turning up the power slightly may be all that's needed.
If he wanted to replace the laser diode (not the entire pickup), then an oscilloscope would be pretty much required for adjusting the mirror. It could be a really low-spec one, though, as the frequencies involved are all well below the 10 MHz mark.

Replacing the pickup might involve adjusting the tracking and focus offsets, and then a scope or a voltage meter capable of measuring HF AC is needed for accurate adjustments. But even then, you can get quite far with just trial and error. And for the focus setting, you could just listen to the hiss-like noise coming from the pickup mechanism as a disc is played (less noise = good).
This is unfortunately true, which greatly contributes to the ever-growing mountain of electronic waste.

You do you, but a new DVD-ROM drive is going to cost you less than the diode.

Last winter I bought some nice second-hand complete collection boxes of original Star Trek, the Next Generation and Voyager... yes, laugh away you ignorants!

Despite being promised by seller they should be in mint condition, I had trouble reading some discs. Tried cleaning discs, but still... sigh.

Then I wanted to see Futurama again, weeee! Found a brand new complete collection box - and still found reading errors!

Decided it was the trusted old reader giving up, found new DVD player and - Voila! Even lightly scratched discs reads without any issues. Probably way better aligned head/mirror and correction circuits?

Price? Some $35...

Conclusion? $5 per year of playing is way less than I let go up in smoke each day....

Suggestion? Just get a new player and let the old one go into recycling.

Many thanks Ser Olmy, that is a real guide. I am trying to do what you are saying and in my opinion no need to become an engineer to adjust the laser power. I do not know that I need a good osciloscope to adjust its power. I just have an home type

All the effort I put is to not to garbage the whole DVD rom while there is a room to save it. My attitute which is my guiding principle is that "when broken, first try to repair it".

I saw that, the topic's point biased mainly towards "is is worth to replace/repair". In my opinion, people should gave always second changes to every machine human produced(if it is not produce-send and forget type machines like satellites etc.).
In my opinion it is always worth it to repair old DVD-CD roms, electrical-electronical machines. Sometimes a simple liquid polarized capacitor replacement can save your monitor/tv monitor etc. Why not try it? Learning is a good thing.

I am following a good guide in ;

https://www.repairfaq.org/sam/cdfaq.htm

now. It also says that, you need service manual to do such adjustments Which is not quite easy however. Also there is a "Laser power adjustment" title which is very helpful to me.

I learned lots of info from Ser Olmy's posts and the above website.
I did some small adjustments on potentiometer but, when I am doing it, I noticed that the "rail mechanism" has its own planar adjustment screws with springs, I am thinking that is another important factor. I think it makes laser to closer or farther from the media so adjust focus. Better to mark as I did the original position with CD markers. When you noticed that CD stops turning in a second after you put it into the reader, then you can raise the rails a little by little when read fails. Since there is a three(i.e to make it planar adjustment) screws, it is not easy to adjust it. However, in about two years ago, in my other old DVD player, I did some just "rail adjustment" so that it starts reading both CD and DVD perfectly!