Nakamichi OMS-5EII/7EII weak error correction

[Salar]

Hi,
my Nak already clicks with errors of about 0.375 mm.
Another gear of mine, a Sony CDP-X5000 plays fine with errors of about 1.125mm (Got this measurements with Digital recording´s "CD-Check".)
I swapped the laser two years ago , made any adjustment possible according to the service manual. (I am again playing around with the trimmers in the moment)
I also swapped all caps in the electronics, with no improvement.
I even swapped the discmotor to the same model but with a longer shaft, so I took the occasion to build an aluminium spacer with an additional bearing to keep vibrations from the disc motor low. The bearing works perfect, but has no influence on the Nak´s poor error correction.
And no, the pole piece of the laser is not sticky.
Listening to both units through my Parasound DAC, I have the impression that the Nak sounds more grainy, maybe because of small interpolated readout errors.
I still have spare parts, for example two laser heads to give the Nak a long life, so is there anything I could improve electronically in the servo circuit? (i.e swapping opamps)
All the best, Salar

[Nanook]

try to find the diode bias for the laser and adjust it . I don't have a manual , but perhaps some of the techie types might.

[Salar]

Hi Nanook,
thanks for the reply, but the laser is only two years old.
I also HAVE the service manual.
Trimming the laser diode bias is the last thing one should do.
If you´ll have to, the laser will probably fail anyway in the near future.
Some tech once stated to me, that lasers are more durable than expected. If they are not working correct, it is very often not the dirty lens but the dirty mirror which splits the beam.
No, I guess, the problem lies somewhere in the servo circuit - because the range of trimming, until you hear an audible effect, is v e r y wide, almost 3/4 of a turn on any trimpot.
The eyepattern on my scope is always a bit noisy. The diamond shapes are relatively clear, but the the upper/lower edges of the eyepattern are noisy.
Trimming the focus offset will have the most visible effect on the eyepattern.
The amplitude rises - this is okay-, but if you go too far, it will become too noisy. I guess, this is also normal.
But trimming E-F balance has no visible effect on the eyepattern.
Trimming E-F according to the service manual (You have to observe the E-F signal in service mode,) always delivers a very noisy E-F, which bounces in it´s peaks and amplitude duration. This is very hard to observe for right adjustment. If you trim off too far, the disc motor will suddenly speed up to maybe ten times ore more. Don´t know, wether this is normal.
My workaround is to use a CD with two very long tracks and simply adjust E-F while skipping between the tracks.
But cklicking with errors of only 0.3 mm does not seem normal to me.
For E-F, I now use a CD which has three very definable but small errors/scratches (Keith Jarrett, "Tokyo Concert" from the "Sun Bear Concerts" btw.).
I sometimes get the E-F right, and there are no audible clicks - and i really try to trim in fractions of a millimeter - and the next day, the CD will click again...
I still have the issue of a german magazine called "Audio" from May 1987, where the Revox B226, Denon DCD-3300, Marantz CD-94, Kenwood DP-3300, Meridian 207 and Nakamichi OMS-5/7EII were reviewed - all of them state of the art in their times. All players, using less Philips and Sony electronics, at least the Denon and Nak sharing the same laser (KSS-123A) , got all the best possible notes for error correction...
I will swap the laser today, maybe I simply swapped a weak original laser to a mechanically misaligned replacement...
But when I designed the spacer and additional bearing for the disc motor, with the help of a mechanic who specialized himself in repairing bearings of turntables, we found out, that the mechanics have more tolerances than expected. (i.e. the housing of the disc-motor has tolerances, also the angle and thickness of the shaft, and therefor the disc tabe might be slightly off).
So I still guess, something in the servo circuit has become weak...
All the best, Salar

[Salar]

O.K.,
I found the sweet spot.I am in a hurry in the moment, so I will post details later.
BUT:
The player will only correct/play heavy scratches one to two hours after being powered up !!!!!! The service manual states, the electronics should settle within three minutes
Any ideas about that? Maybe a defective transistor?
All the best, Salar

[QSerraTico_Tico]

Originally posted by Salar
O.K.,
I found the sweet spot.I am in a hurry in the moment, so I will post details later.
BUT:
The player will only correct/play heavy scratches one to two hours after being powered up !!!!!! The service manual states, the electronics should settle within three minutes
Any ideas about that? Maybe a defective transistor?
All the best, Salar

Adjust focus gain for optimum results with black stripe, 1 mm wide, on a CD.

[Salar]

Hi Tico,
I have such a CD, but on my old scope, dropout and recovery are not easy to judge.
You can read further details here:
http://www.diyaudio.com/forums/showt...26#post1376726
All the best,
Salar