Nakamichi OMS-5

[skyline889]

Hi Chris,

Thanks for the help! Hopefully I'll be able to find it when I open it up again. About the laser, my dad didn't actually replace it himself but he payed the Nakamichi repair center here on Oahu (When they were still here) to replace the laser. He said it cost him about $500 back in the late 80s so I'm really hoping it hasn't malfunctioned again. The cd doesn't spin excessively fast when reading though, so I'm not too hopeful that it's just a feed belt problem.

-David

[anatech]

Hi David,
Two things I can say from experience.

1. It's probably the belt.

2. $500 is not enough money to replace the head. My cost was over $500 for that head and I was authorized warranty for Nakamichi. This unit takes about 5 hours to set up (and I've done many).

-Chris

[Salar]

Hi Folks,
btw, has anyone an idea, where to find replacement belts for those being used in the Naks OMS-5EII/7EII?
The lenght is 5.4 cm, as far as I remember. I once checked all available catalogues with a technician here in Germany, looks like those lenghts are not produced anymore...
All the best,
Salar

[anatech]

Hi Salar,
I think the best thing to do is walk in to a parts place with an old belt and simply go through them all. I had always used the exact Nakamichi replacement part so I don't know about general ones to replace them.

-Chris

[izmarkie]

Hi,

Sorry to bring back an old thread, but I have an OMS-5A II, I'm having similar symptoms to the ones listed in the first couple posts, and I'm not so sure about whether I want to mess with a CD player on my own. I've built a bunch of speaker boxes, reconed woofers, and redone crossovers and such, but when it comes to anything with a laser, I'd rather trust a pro.

Anatech, or anyone else - can you recommend someone I could send it to in the US, preferably in the southwest? Is it even worth it, or should I just buy something else?

[Salar]

Hi izmarkie
I can mail you the service manual, if you like. (16MB) Then you can judge if you like to do the work...
All the best, Sal

[izmarkie]

Originally posted by Salar
Hi izmarkie
I can mail you the service manual, if you like. (16MB) Then you can judge if you like to do the work...
All the best, Sal

That would be great. I think gmail can handle 16 megs. alexisrael at gmail dot com.

Thanks a lot!

[Salar]

Hi Izmarkie,
you´ve got mail...

And here it comes. My Nakamichi OMS-5EII / 7EII alignement adventure!
But for those, who do not want to read the entire post, one question:
Got my Nak into ball park, but it will only play scratched CD´s perfectly about ONE HOUR OR MORE after being powered up. Any Ideas on that? Some old transistor in the servo circuit? I could have the Nak being powerded up the whole day, but hey, seriously, these are not the times for wasting energy any more...

Now the alignment story:

I have a two test CD´s for this purpose, a perfectly manufactured YEDS-18 test CD from Sony - but in fact, any other well manufactured CD will do the same.
For scratches, I use Digital Recordings "CD-Check".
It has black lines printed on the disc surface, ranging from 0.3mm up to 1,5 mm for testing the error correction, divided into four check levels.

The third CD is an old one from Keith Jarrett. It has a small spot on the aluminium layer, (no scratches on the disc surface) about 0,5mm, maybe a scratch from the label side.

The Nak did not even manage to play errors of about 0.3 mm on CD - Check (which btw delivers a very clean eyepattern, it seem to be very reflective, maybe because it is gold plated). The Nak also hang on the Jarrett recording. The laser is almost brand new, I swapped it two years ago.

But my other player from Sony manages to play scratches on CD-Check of about 1mm without any clicks, as well as the Jarrett recording. Because four lines are printed on CD-Check in an angle of 90 degrees, they in fact sum up to an error/gap of 4mm!!!

A friend of mine is developing a DAC based on the PCM 1704, which i will probably built into my Nak.
Because of this and because i was so frustrated about the Naks bad performance, I decided to give other electronics a try.
Because I have already built a clock from Tentlabs into my Nak, I started to look for a Philips based transport clocked with 16.9344 Mhz.
Got a cheapish player CD 753 with CDM12.1 from ebay and already drew plans for building this transport into the Nak - anatech would kill me for that!!!
But you can find the CDM-12 in almost any philips based player, even in High-End machines, lasers are easy to get, so wanted to check this out.
Unpacked the Phillips, (2 kg of plastic, the Nak weights 10 kg) played CD-Check, again no problems with scratches up to 4mm .
Played the Jarrett recording, (0.5mm on the aluminium layer) and the dammn Philips hang!!!
Now it looked like the Nak was not as rusty as it seemed. Maybe, the Sony´s and Philips optics have another focal lenght, in order to blur out scratches more easily?
I then experimented with the mechanics.
The disc motor could have been worn out. I bought a new one (Mabuchi RF-300T-11400).
But the shaft was too long. I did not want to take the risk in cutting it down, bought a bronce bearing and asked a mechanic to mill an aluminium spacer for the motor to house the bearing and shaft.
In fact, this did not seem to have any effect, but well, this construction should last forever - at least until the brushes of the motor fail...
BTW table height should be 4.5mm, measured from the top of the motor ( a brass spacer is originally fixed on it) to the bottom of the disc table.

Now the electrical alignment:
Alignment according to the service manual is only the first step. The second step is to look at the eyepattern. (Thanks anatech!)
One example:
According to the service manual, you trim focus offset by observing DC level on a certain test pin while playing a disc. You then stop playing and raise or lower the level until there is no change in play/stop mode.
But you will still get a clearer eypattern by turning the trimmer a bit more clockwise. The sweet spot is just before the eyepattern reaches the highest amplitude - and is the clearest. But according to the manual, focus is now far off, but my Nak works better now.
The same with E-F balance. When i try to align according to the service manual, I get a very noisy and bouncing signal in test mode wihich is not easy to observe.
But a workaround is to take a CD with long tracks and simply jump/skip between the tracks while observing E_F. It is now a relatively clear sinewave until the laser has reached the next/previous track and goes back to play mode. This wave should be trimmed while skipping until it crosses 0 V symmetrically.
I then took a CD with small clicks/ errors (i.e the Jarret recording) and fine tuned E-F until the cd was played without errors. In my case, this really meant turning the trimmer in fractions of a millimeter.
Now everything works perfect, I but as I stated, I get annoyed by the fact, that the Nak has to warm up for more than an hour until it performs best and I would like to find the cause for this.
Another backdraw was the additiona bearing for the disc motor . It starts to "sing" at low speeds, sounds almost like a testtone.
I later found out, that the clamper has some play. This is normal, but I fixed the clamper by placing a small brass rivet in the center hole of the clamper. The rivet sits now very tight on the motor shaft when a CD is inserted the clamper is more centered than before. I also removed the rubber cover from the clamper. With this lighter and centered clamper, the bearing runs silent now.

End of story,
all the best,
Sal

[izmarkie]

Thanks Salar!

And... I'm definitely too intimidated to try this myself. Can anyone recommend somewhere to send it for service in the US, or should I just give up and pick up something else?

[anatech]

Hi Salar,
And you now have a technician's touch. That is what separates the good techs from the great techs. The bad ones don't even read the manual.

Glad to hear you have it settled. It does sound like your disc table height is off a little though.

Hi izmarkie,
Look for a well regarded audio service shop or a semi-retired good technician working out of home. You need a patient, careful guy. An ex-Nakamichi tech might be a good bet. TV shops are probably not what you want, they can be destructive. Usually are on audio gear.

-Chris