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Salar 27th July 2008 01:53 PM

Replacing M5218A in Servo Circuitry
 
Hi,
I would like to tweak the laser servo circuitry in my Nak CD-Player.
The unit (OMS-5II/7II) is now 19 years old and error correction is not good, even though I swapped the laser 3 years ago and aligned it perfectly. 0.3mm is the barrier (scratch width) where one hears clicks and the laser starts to skip.
Any modern unit will easily go up to 1mm.
There are three opamps used in the servo circuitry, M5218A, which were also often used for audio signals.
What could be a replacement? Would a faster slew rate help the circuit to react to scratches much faster?
All the best,
Salar

kevinkr 27th July 2008 09:55 PM

Re: Replacing M5218A in Servo Circuitry
 
Quote:

Originally posted by Salar
Hi,
I would like to tweak the laser servo circuitry in my Nak CD-Player.
The unit (OMS-5II/7II) is now 19 years old and error correction is not good, even though I swapped the laser 3 years ago and aligned it perfectly. 0.3mm is the barrier (scratch width) where one hears clicks and the laser starts to skip.
Any modern unit will easily go up to 1mm.
There are three opamps used in the servo circuitry, M5218A, which were also often used for audio signals.
What could be a replacement? Would a faster slew rate help the circuit to react to scratches much faster?
All the best,
Salar


Unlikely, concealment and error correction are a function of the complexity (cost point) of the player's digital signal processing chipset, not the analog servo circuitry..

Take better care of your disks if possible or find a player that handles the errors so generated..

My Shigaclone is not great in this respect either, but I like its performance otherwise and live with the occasional skip which is actually rather rare - only happening with a few of my wife's disks which have not been well treated.

Salar 28th July 2008 07:40 AM

2 Attachment(s)
Hello Kevin,
no, I am talkung about the analog circuitry which keeps the laser on track and reacts to scratches. Not interpolation. I have attached the circuitry.
All the best,
Salar

Mooly 28th July 2008 12:36 PM

Hi Salar,
I'm afraid it wont help at all, swapping the I/C's. This is one area of performance that has greatly improved since the first generation players.
If I mention Nyquist stability, we think of amplifiers and feedback. The same problems exist here but with the additional complexity of a "mechanical/inductive" component in the loop-- the tracking and focus coils. The characteristics of the "analogue" stage are carefully designed and very critical. Careful setup can help, particularly EF balance if the player has an adjustment for it and focus bias.
You would have to follow the manual, but I think you have done this already.

kevinkr 28th July 2008 01:52 PM

Quote:

Originally posted by Salar
Hello Kevin,
no, I am talkung about the analog circuitry which keeps the laser on track and reacts to scratches. Not interpolation. I have attached the circuitry.
All the best,
Salar


Dealing with read errors is done in dsp. Only in the case where the EFM waveform is incorrect can you make any improvement in performance. Improving servo performance** is not going to make the player magically read the missing data, some players just have much more robust error correction algorithms that can cope with longer data dropouts.. Your player apparently isn't one of them.. (Nor mine if that helps..)


Note:
** It is unlikely that you need to or even can based on the comments of the previous post. The mechanical subsystem is far slower than the existing op-amp..

stephensank 28th July 2008 07:19 PM

If you are having trouble tracking actual cd's, I can add my two cents.
In my experience, it is completely inadequate to align an OMS-5/7 with test discs & the 'by the book' procedures. Works ok for the actual optical 3-axes alignments, but not the servos. Best to do the servo adjustments with the widest variety of cd's(and cd-r's if the laser is healthy enough) you can, and align for the best compromise between worst & best discs. Otherwise, you'll perpetually have tracking or noise problems with some discs.

Highspeed 28th July 2008 08:07 PM

OMS trackability
 
Hi Salar

I have repaired many OMS5/7 II and they have two main problems: the disc motor and the head.

After 19 years, maybe your disc motor is worn: put a disc in play and when the disc is at his max RPM, put the on/off switch at off.
Put your ear near the spinning disc and listen.
If you hear a low frequency rattle, your disc motor is worn: it is loose in his bearings. Replace.

The head: your "new" head is maybe 19 years old. It has been stocked a long time.
Check the head suspension and check if you have side and bottom play: i mean if the head can move on his axe left and right side. Most important. check the down play. Be carefull!!
With a cottonswab, press gently on the lens and see if it can go down a little. If not, it will be unable to read properly some CD's. The lens rubber support is crushed.



If your head has no down play, you can play with the focus offset ( check the eye pattern and adjust for best signal ) to "correct" this situation but keep in mind that the servo circuitry will work harder. Adjust the EF Balance with a 1mm scotch tape on a test disc. Listen to a tone and adjust for the least error play.

Replacing the M5218A will do nothing for your problem.

Nakamichi OMS-5/7 are completely differents units: they have a 1 beam laser and a completely different electronics circuitry.

Good luck

Salar 28th July 2008 11:32 PM

2 Attachment(s)
Hello to all,
thanks for your advices!
The laser is three years in use old, I bought it ten years ago, when it became clear, that Nakamichi would vanish from the european market. The lens moves freely.
I also swapped the disc motor (RF-310TA-11400). Because the shaft of the present motors is too long, i had an aluminium spacer milled which holds a brass bearing. It was little tricky to fix it on the motor without adding any additional load to the axis. I also added a brass tube in the disc clamper to make it fit tighter on the motors axis to prevent imbalance. (which was there before, the clamper had a little play horizontally when it presses a CD on the disc table)

So I guess the Nak is mechanically better than it was before my mods.
But the weak error correction has remained.
I also have a second-generation Toshiba XR-Z70, the Tosh is a bit better than the Nak in error correction.
The disc with simulated defects is Digital Recordings "CD-Check". The error size ranges from 0.375 to 1.5 mm. As I wrote, the Nak fails at 0.375 already, the Tosh passes with maybe two dropouts in 10 seconds. The defects are not one black line or dot, but four lines in a 90 degree angle. So with one turn, 4X0.375 have to be compensated.
Another player of mine, a CDP-X5000 from the mid nineties can easily compensate up to 1.2mm.

Highspeed, do you still remember setting the E-F balance of the OMS?
I am using the YEDS-18 Test disc for this. But the E-F Signal does not look as clean in real live as it is shown in the service manual.
In test mode with the spinning YEDS-18, it E-F resembles a sine wave, which has to be adjusted to swing equally around 0VDC.
But no matter how correct I adjust it, (or how much it is out of adjustment) it changes frequency and amplitude randomly very fast (but can somehow be adjusted around 0V)
Thus effect was also before any mods and with the old laser, so this is normal behaviour?

The eyepattern does not look very clean, also no matter what laser. But I do not know wether I should expect a clean eyepattern because it is not documented in the service manual. Strange, because this is the first thing one should look at.
The lines are not sharp, the outer edges are blurred and noisy. Checked it on two scopes.
To make the nak safely play CDRs, focus offset has to be raised a bit, this also raises the amplitude of the eyepattern.
I spent many days with adjustment, also with tracking/servo gain and Dropout detection circuit, but 0.3mm is the barrier...
I attached a photo of E-F balance from the service manual. Maybe they used an external trigger to make the waveform look stable?
All the best,
Salar

Mooly 29th July 2008 07:11 AM

Hi,
When using scope make sure you use the correct grounds as this can make the also RF look noisy. The eye pattern should ( not all players achieve this ) be very sharp and well defined. The diamond shape should be "steady" , any slow vertical bounce in the waveform indicates that the turntable is not true and that there is an up/down motion as the disc rotates. Check by looking at the outer edge of disc and turn slowly by hand.
The EF balance will be correct if it is symetrical about zero. Can be useful to adjust/check while repeatedly using track search and slow search I use to find on some players. Try it ? Can you feel any play in the spindle motor bearing.
Ultimately I think you will find it's just the way it is, my first CDP, a Sony CDP101 was not good on error correction and playing less than perfect discs (from the library etc ) but a cheap and cheerful Philips CD150 found it no problem.
If it's any help Philips Test disc SBC 444 A states that any player should be able to play without audible errors a 500 micrometer interuption in the recorded layer, also a 300 micrometer black dot as a single one off defect. Now this isn't very severe but that's the "standard" minimum as it were.


Salar 29th July 2008 08:22 AM

Hi Mooly,
the test point for GND is not defined. I used common ground of PCB/Chassis for this.
What did you mean with The EF balance will be correct if it is symetrical about zero?
That it swings around zero (equally positive/negative), or that the the bottom swing of the signal "sits" on the zero line?.
I have two disc tables for the machine, both will result in a vertical swing. So nothing to do about that.
All the best,
Sal


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