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An Idea for saving Laser Life - amplifying the current?
An Idea for saving Laser Life - amplifying the current?
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Old 5th November 2018, 11:29 AM   #11
Salar is offline Salar  Germany
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Hello Mooly, thanks for your detaild reply but this was not the basic question of this thread and i never intended to play around with the existing RF-Amp.

The question was:

How - this means by which circuitry- can I raise the current of the photodiodes before they are fed into the RF Amp?

I assume some low noise transistors could work...?

All the best,
Salar
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Old 5th November 2018, 12:02 PM   #12
Mooly is offline Mooly  United Kingdom
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An Idea for saving Laser Life - amplifying the current?
The current the diodes produce is fixed and proportional to the illumination they receive.

If you try and amplify the diode output (its voltage output) by transistors then you are going to run into noise issues. You would then have to mix the signals in the same way the chip does. You could try making an opamp I/V convertor with higher gain which would in practice be the same as in the chip, just with a higher feedback resistor wrapped around each opamp.

To try and make a transistor discrete I/V convertor for this would almost certainly be very inferior to using dedicated opamps.

R102 on your circuit looks to set the voltage gain of the final RF stage. You could alter that to increase the RF level to compensate for a lower LD current but signal to noise ratio of the eye pattern will deteriorate. You also have to make sure the side spot detectors generate the correct amplitude tracking error signal so you would need to add gain to the TE signal.
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Old 5th November 2018, 06:53 PM   #13
martin clark is offline martin clark  Europe
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Be very careful. The photodiodes are the most static-sensitive element in a CD head; playing with interposing an amplification stage is highly likely to kill an otherwise working, if aged, CD pickup.

If the player is old enough to have tracking, focus and laser power adjustment potentiometers, find a manual and re-set to the recommended settings while monitoring the eye pattern.

If it is newer than that, all the above are actually 'servo controlled' by the relevant chip/dataslicer (e.g. for Philips mechs CDM9 or newer / SAA7310 and later). Nothing to play with there....
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Old 6th November 2018, 04:54 PM   #14
Salar is offline Salar  Germany
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Hello Martin,
you do not mistake the laserdiode for the photodiode, do you?
Because when a new laser head was /is shipped, the laserdiode is the
part that is short circuitred with a blob of solder. Never saw this
extra ESD protection with the monitordiode or the photodiodes.
Please take also into consideration that this thread is not about the usual
aligning process. It is only about the technical possibility to amplify the current
from photodiodes that get lesser light from a laser diode that deliberately runs
with less power in order to lenghten itīs life.
The current does not need to be amplified that much btw, just doubled…

Probably this could have been a part of the aligning proces if the designers
of the RF-Amps would have allowed the I/V conversion to be adujusted externally…

Maybe they did this in an alternate universe or early laboratory designs

What About R102, R104, R105, R108 and R109 in the schematics posted the page before?
Can the output of the opamps be raised? Same result…?

Last edited by Salar; 6th November 2018 at 05:11 PM.
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Old 6th November 2018, 06:37 PM   #15
Mooly is offline Mooly  United Kingdom
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An Idea for saving Laser Life - amplifying the current?
Those resistors would alter the gains and it may well work and play but it can't be optimal. (R109 isn't a gain setting resistor)

Its like saying that if we cut vinyl discs with less groove modulation then we can make up the difference with a higher gain RIAA preamp. The downside is reduced signal to noise ratio.
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Old 6th November 2018, 09:13 PM   #16
Salar is offline Salar  Germany
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Well, who says that S/N is that poor that there is no headroom left?
Remenber the bad reflection from CDR, sometimes down to 60%
and still playable.
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Old 7th November 2018, 01:00 PM   #17
Mooly is offline Mooly  United Kingdom
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An Idea for saving Laser Life - amplifying the current?
Oh absolutely... and I'm not saying don't experiment, just realise that tweaking things like this isn't an improvement as such, or that you are correcting some oversight the designers got wrong.

Try it, enjoy and have fun
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Old 9th November 2018, 06:14 PM   #18
Salar is offline Salar  Germany
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I checked some CX20109 RF-amp circuits today.
A BU-1C, a Sony CD-40 with an ordinary worm drive and a belt driven Nakamichi OMS-5II.
The (feedback) resistors of the Sony CX20109 circuits are the same, in the Nakamichi they differ.
The Nak uses a Sony KSS-123A Head, the Sony CD-40 uses a KSS-120.
So it looks like the CX20109 has been been built for versality.
I am a layman with opamps, but tried to change the values -theoretically-
of the resistors surrounding the opamps, mostly feedback resistors.
The idea is to raise the output power of the opamps in the CX20109 by one third.

I have a BU-1C where the amplitude of the eyepattern is down to 0.8V p-p instead of 1.2Vp-p
So this BU-1 with an aged laser diode would be a candidate to see if the mod may work.
But before staring soldering, someone could take a look on the values
I altered in the attachment below?

As far as I know, there are no schematics of the BU-1C available from Sony,
where the resistor values are noted. I do not know why the values are missing in the service manuals of the CDP-102/103/302/502

But I found the service manual of a Luxman D109, where the values are noted.

So the first attachment (BU-1C Schematic) shows the BU-1C circuit unaltered, some people might need it for other purposes.

In the second attachment (BU-1C Schematic_Altered_Values) I added the altered values in red. Maybe someone might be so kind to take a look and give a comment.
I am confused how to deal with the circuitry around in 14 (FE+) I assume I can simply raise the offset voltage at VR102 about 1/3 without altering R109 / R110?

All the best and thanks,
Salar
Attached Images
File Type: jpg BU-1C Schematic.jpg (866.9 KB, 81 views)
File Type: jpg BU-1C-Schematic_Altered_Values.jpg (870.7 KB, 83 views)
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Old 10th November 2018, 08:29 AM   #19
Mooly is offline Mooly  United Kingdom
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An Idea for saving Laser Life - amplifying the current?
Its very much a case of trial and error. If you bring the gains of the front end up then I wouldn't have thought you need alter things after that point because the levels should be back to original values.

The CX20109 was as I recall used in the first original Sony Discman MK1.
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Old 10th November 2018, 09:42 AM   #20
Salar is offline Salar  Germany
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The CX20109 was used everywhere between 1984 to 1988 as long as Sony supplied the electronics. From Sony's own Discman (called D-5 respectively D50 btw) to High End (Accuphase - DP 80)
What about R109 / R110? A voltage is led to the positive input of the focus error amp,
I assume they do not need to be changed?

Last edited by Salar; 10th November 2018 at 09:49 AM.
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