Dead Laser Pickups - why not replacing the Diode only?

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I played a bit with the dismantled BU-1C shown before.
I ordered some Sharp LT022MC laser diodes as a replacement,
and as expected they were dead. I got a sample of Diodes from,
which raises some questions:
- Could it be that beam divergence of 780nm low
power diodes is almost the same because of the emitting principle?

- the beam of a plain diode looks more like a bar when
being projected on paper. I assume this is the divergence
and it probably wider in the visible spectrum of the diode.
But I wonder: Why is the collimation lens that parallels the beam
before the focus lens - and not much earlier, i.e. after the
diffraction grating?
All the best, Salar
 
Ok, I did start my first attempts to align a BU-1.
As I wrote before, one big obstacle is the diffraction grating in front of the laser.

What did I expect: A diffraction grating divides the beam. The sweet spot for an aligned grating -by turning it- should be one center beam and side beams,
with the side beams close to the center beam being almost equally bright.
Also the optical path should be visible without magnifaction as the beam has not been scaled down by the front lens.

So I did remove the tube with mirrors and collimating lens, (A black grub screw hast to be loosened) the first photograph shows how it looks like.
In the bottom hole is a white part. This is the plastic frame of the diffraction grating. It can be easily turned with a small, flat screwdriver,
Not much, maybe 5° to 10°. This might be good as the grating is probably well prealigned. (Or very bad, as the fraction of a degree might be crucial)

Removing the tube led to another problem:
It blocked while lifting it up. Reason could be the cylindrical lens that leads the returning beam to the photodiodes.
I loosened the silver grub screw that keeps the cylindrical lens in position, the brass part, 2nd photograph, and pushed it out of the way.
But the tube still blocked! Knowing that the cylindrical lens could not be in the way I pulled harder and removed the tube.
No sign of damage. So I think tight tolerances were the reason.
As the grub screws leave marks on the metal, I hope I can realign the tube as well as the cylindrical lens.

I wired the laser to the APC circuit of the player, pointed it to white paper and observed it with a CCD-Camera (CCD/CMOS "see" Infrared light)
Here came the big disappointment:
I did only see one spot! Turning the grating did not change it.
No matter what distance to the paper or angle, the beam even stays relatively sharp.
But only one beam, not three or more. (Next two photographs)
The last photograph shows the beam pointed in the camera. The Douglas Trumbull like rays (Star Trek - The Movie anyone?) might be the image of the grating.

So, if I did not do anything basically wrong, the good news:
There is no use to disassemble the BU-1 to align the diode. I assume it
must be done in Service Mode, with the BU-1 assembled...

BTW, the optical elements are very clean - in a thirty year old player!
 

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Hi,

Last week I succed to replace a weak laser diode by a new one in a BU-1.
I did the adjustement directly in the player but to be able to acces to the diode I had remove the flat cable and I added some wires. Well it is very very tricky because there is obsolutly no logical way to do it, move the diode a little bit in a one direction, try to read the CD, try to correct the position till an RF signal appears.
BU1.jpg

Of course this is not the good methode.
Today I found those two pictures

Laser1.JPG
Laser2.JPG


on this web site
??kSS-190A?????-???


On the first picture we can clearly see the effect of the diffraction grating, it is what Salar try to do in on of his last posts.
On the second one, we can see the 3 beams well positioned on the photodectors array.
Those two pictures seem to be taken by a camera ponting toward the inside of the optical assembly and not on a paper screen. This is many the reason why Salar didnt obtained the expected result.
An other other question is how is taken the seconde picture ? So extra light is directed to the photodectors array from outside I suppose. This allow to see at the same time the 3 beams and the detector surface.
Is it this the good way to do the adjustement of the diffraction grating ?
 
WOW! First of all, congratulations! This fits into the moons landing anniversary!
"A small turn of a screw..."
BTW, where did you buy the diodes? Is the optical output better than before?
Now about the beam reflections:
The second photo is very logical, a dotted line of beams, only the center beam
and the left and right are important. The third / last photo I cannot explain.
Mabe just a transparent disc beneath the microscopes´ optrics and still a part of
the beam is reflected back to the diodes. Or maybe a shift in focus of the camera hides the outer beams?
Thus we see the three beams facing to the camera but not being reflected. I can only guess. Again, congrats!
All the best,
Salar
 
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Yes Salar, I didn't touch the grating.
Concerning the laser diode I bought few months ago a set of 5 SHARP's diodes LT022MC from a chinese seller. All 5 are good but 4 of them seem very weak.
The guy from China who is replacing the laser diodes and sell refurbished KSS151 and other models is using a ROHM laser diode RLD78MYA1.



I think that this guy is also on DIYAudio, I found interesting posts here

TOSHIBA DLBA2Z002B laser
 
Today I made some investigations on a weak KSS151A a have in stock for few years.
I removed the upper lens of the KSS and I put my camera directly over the KSS.
First picture, only one beam. Disappointing ?

pic1.jpg


Yes but when I changed the focus of my camera I obtain this picture with
3 beams. It is better !
pic2.jpg

With laser beam turned off and after removing the side plate of the optical unit some lateral light came in and I was able to see the photodetector array.
pic3.jpg

And with the laser turned on the 3 beams are perfectly alligned over the photodetector array.
pic4.jpg

The optical assembly.
pic5.jpg
pic6.jpg


The bad news is that the diffraction grating and the photodetector array are locked with a kind silicone sealant.

pic7.jpg
pic8.jpg


So nothing can be easily adjusted in case of laser diode replacement. The only way is to break the sealant. Not very easy :(
 
Great work!
I think the grating and photodiode array do not have to be adjusted at all.
No sealant to be broken ;)
I misaligned my BU-1 completely by turning the grating and did not find the sweet spot again.
Mysterious because as far as I understand diffraction gratings, just the lightness of the outer beams should change when turning /aligning it.
So the sweet spot should be when center and outer beams are the most brightest.
but the distance between the three beams should not change when aligning it.

But I might be wrong here...

I assume will be the same with the KSS-151 as with the BU-1- replace the laser diode
and move/turn it by fractions of a mm until the eyepattern becomes clear...?
Same mechanics as with the BU-1C, spring and whashers to align the diode?

I do not understand photo 4:
Does the alignment of the split beam in relation the photodiode array
change with the angle of view of the camera? - if not, we might have a reference.

BTW, if you hesitate to play with the KSS-151A, there is another possibility I discovered with the BU-1C
Just raise the gain of the photodiode´s RF-Amp and lower laser power :

https://www.diyaudio.com/forums/dig...er-life-amplifying-current-7.html#post5604526
 
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No, the mechanics around the laser diode on the KSS151 is different from the BU-1.
No springs, no screws, on the KSS151 the laser diode is glued and it is the same for the detector array and for the diffraction grating. Nothing can be adjusted.

Concerning the picure n°4, I took it whit my cellular phone in one hand and the KSS in the other one so the angle is not exactly the same as on picture 3.

I have to check it again but when I rotated the diffraction grating, I found that the line drawn by the side beams was rotating relative to the central spot coresponding to the central beam.

So I think that 2 points must be considered :
- the central laser beam must reach the center of the photodetector array (points A, B ,C and D) this can be done by moving the laser diode or/and the photodetector array.
- once done the 2 lateral beams must reach the E & F cells of the detector. So assuming the central laser beam is the center, the lateral beams will be alligned by rotating the diffraction grating.
 
Does anyone know if a the laser/edge emitter is always centered in the housing? Maybe this is why the diode in the KSS-151S is glued because there was no need any more to adjust centering.
In the BU-1C there was some play, maybe because a perfectly centered
LT-022MC diode was not garuanteed by Sharp.
On the other hand Philips used the same LT-022MC diodes in the CDM-1.
There the diode was sitting tight in an aluminium tube. No way to center it.
But the photodiode array was glued on using epoxy resin, so maybe they did corrections of off-centering with the array...
 
Little bump to this exciting thread. I am wondering where have you been so far?

Hi im new to this and find it very interesting .. but i also like to know the chinese selling new or refurbished KSS-151A laser pickups on ebay i could use on my DENON DCD -1560 ....

What i like to know is if i buy one of those after install do i need to make adjustment? I do not have the necessary equipment to do so so what should i do? ? Is it a waste of money without adjustment with a oscilloscope? ?

Thank you for your advice!!

John
 
I think you have to make an adjustment, BUT: It might be very likely, that the laser will run already without it, but more prone to scratches and errors on the disc. A 15mHz scope is in many cases sufficient for the measurement, 30mHz brings you to the safe side.

Check also this post how far we have come with replacement - not so much but improving

https://www.diyaudio.com/forums/dig...-pickups-sonys-kss-series-11.html#post6027176
 
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