Nakamichi OMS-7AII - Disc error

[df8cf]

Hi folks

Hope somebody out there who can help me.
My OMS7 (Mark II) doesn't any longer read Discs - here's the things I did already:
Changed the laser - it was too tempting since it's the first maintenance step in the service manual. I didn't browse in this forum before - maybe to invest into a new laser wasn't even necessary. Anyway now I have a replacement part (the old one) - but with the new it didn't start to work.

As a next step I bowsed the web also this formum and found some hints that the feed belt is a weak point in this models. Replacing this one didn't help either - I checked pickup slider - it glides absolutely nice.

Next step was trying to understand how the start-up procedure works and where it fails in my Nak. When closing the tray it does a focus search, starting the spindle motor and try to read in the ToC - correct? My interpretation is that in my case the focus search works, the spindle motor starts, the feed (gear A) rotates little (~30°), then it obviously tries to locate/read the ToC, but the feed motor starts to spin and moves the pick-up to the CD's end. As a consequence after a little while it returns the pickup to it's home track (which is nicely repeatable - I marked the gear A) and it unlsides the CD. It looks like it's not trying "hard enough" to read in the Toc - the ramarkable thing is: if I try to manually dial in the ToC (removing the feed belt and dial the gear A) I have a 100% success-rate - tried it with several CD's. Putting back the belt and pushing the start-buttom though doesn't make it playing - why not - the µC should know everything, shouldn't it?

I also noted by doing this manual ToC dial in I had to rotate the gear A something like 70-90° whereas the initial "jump" by the motor only does a ~30° rotation. Well, does this mean the "home"-position isn't exactly right?
I made an experiment by attaching incremental layers of sticky tape to the rack gear to push the end-switch "earlier". I could see the "home"-position moving "out" more and more - the initial step continued to be a 30° step - and for sure I came into the regin where I have success with my "manual dial in"-procedure. The resolution of sticky tape (0.05mm) is quite nicely translated by the gear, but no success of a ToC read in

As a final step I put some heat-shrink tube on the feed motor's shaft to increase the gear ratio slightly and exlude the belt slipping on the shaft - no success either

This is where I am with my beloved Nak - and tbh. running out of ideas

Does this sound familiar to one of you guys and could anybody successfully repair an OMS with such symptoms ?

I'd love so much having this Nak sitting next to my other ones and doing a good job for a few more years

Thanks for many hints

Gerhard

[Salar]

Hello Gerhard,
before we start to dug in deeply (But I am no technician):
Did you unsolder the bridge on the laser, which prevents electrostatic breakdown? (After installation of course) From which source is the laser anyway?
All the best,
Salar

[Andrew Eckhardt]

Hey Salar comes in with a good suggestion. Most likely that pickup comes with a solder bridge that's still there if you didn't open it. I'm not sure what all would need to be tuned after that but if you have the service manual it's all in there. At least I can say that about Nakamichi CD players from the past. If there's a potentiometer, there's an alignment procedure for it. Sounds like you understand the basic flow of things. If I remember correctly, from the symptoms you mention, that pickup isn't getting Anything as it sits.

[anatech]

Hello Gerhard,

Changed the laser - it was too tempting since it's the first maintenance step in the service manual.

Indeed, there is a short on these type II machines, the original has a nifty looking clip on the new head. To tell you honestly, replacing the head is not the first step you take on any CD player (except maybe Philips types).

So, put the original head back into the CD player and let's begin to troubleshoot. Why would we do this? Well, because there are adjustments to make for each and every head. Some are pretty critical and a little off will cause a read failure. By installing a different head (new or not), you have introduced another variable.

There is no easy way out here. You must follow the service manual exactly in order to get a working machine again.

Okay, so what you need now is an oscilloscope good for 0.5 uS/Div. You need this to align absolutely everything. You will also need a good digital voltmeter. We're talking a Fluke hand held (85 / 87 or equivalent), or a new Agilent hand held (new ones and priced extremely well for their performance - $200 ~ 300). The other solution is a good bench meter, such as a Fluke or preferably Hp / Agilent. Most other meters are almost good enough, but not reliable that their calibration has held. For a test disc, use any good CD.

What should you have done first? New belts for one. Then there is a way to monitor the laser current so you can compare it to the value printed on the head. You are allowed 10% high at the most. If you confirm the laser is okay, go on to the next steps. Connect your oscilloscope to the RF (or eye pattern) test point, set the oscilloscope up per the manual and then play the CD. You are looking for a wave form along with the approximate amplitude, horizontal stability and vertical stability. Make notes or take digital photos of the oscilloscope screen. You can not use a flash, so it's hand held and a long exposure. You can also monitor the focus and tracking servos (different oscilloscope settings).

If you are no able to do these things, or you do not have these basic test instruments, you will not be able to troubleshoot this machine. BTW, it's well worth repairing.

Other issues with these. The original and replacement laser pickups may have a sticky shaft that guide the lens up and down. That means you do need a head. Also watch for a worn spindle motor bearing or shorted poles (monitor with the oscilloscope). It is possible for the wires going to the head to be trapped since the transport was disturbed, or one or more wires may have broken free.

-Chris

[Salar]

Another idea:
One of my very first mods was to provide the oms ´s tray with LEDs. Not a good idea, it blew a fuse for the power supplies, those fuses look like resistors and are close to the power supply on the main board, ie the rectifier and caps.
With the positive voltage missing, the focus was pumping, but no turning gears at all. I also purchased a broken Nak, long before ebay. It´s repair was declared as being too expensive, but it also had a blown, resistolike fuse.
So, maybe, you should simply check the voltages as they are derived from positive/negative supplies, and should have the same values, i.e +5v/-5V...
Again, a fault there is not very likely, but...
All the best,
Sal

[df8cf]

Hi Salar

Thanks very much for the hint.
Yes the ESD-bridge is opened - I dare to say the Layer pickup works correctly (maybe the original one would still have done the job). The proof is that it works by "manually" dialing in to the ToC.

Kind regards
Gerhard

[df8cf]

Hi Chris

Thanks very much for your reply.
I'll undergo the alignement procedure again.
Instrument-wise I'm fine - can borrogh any Digital scope of the Tektronix sereies - currently I'm working with a TDS 2116, and I exactly own a Fluke 87 True RMS DVM.
Laser current is cecked: <1mA deviation form the pronted value.

Well, "...set the oscilloscope up per the manual and then play the CD." this is exactly the problem since I can't get that far. It doesn't read the ToC, subsequntly doesn't start to play.

Did I read that these type of machines are reluctant to play with open cover?
The bottom one (where the anloge staff is sitting) or the upper cover (where the digital pcb with the 5 RV's is sitting)? I can swear I had in operating in closed mode as well (this is how it all begun) - but I'll reeat this as well.

Thanks for all the good hints and the remark that it's really worth while repairing it - won't give up trying that fast ;-)

Thanks so much
Gerhard

[anatech]

Hi Gerhard,

Instrument-wise I'm fine - can borrogh any Digital scope of the Tektronix sereies

Hey, that's wonderful! You wouldn't believe how happy I am to hear most of that response. Why most and not all?

A DSO is a poor choice for this type of work. Believe it or not, you actually need either an extremely expensive Tek DSO type scope (we're talking probably 500 Ms to a Gigabit one), and setup will not be easy. Failing that, use something older like an analog 2235 or 2213A - you get the idea. I did have a chance to test drive a number of DSOs, and the easiest ones for this type of work are the Agilent and LeCroy. They set up much easier and even an Agilent DSO6000 series, 500 Ms or better will give you a very analog-like display. Given enough times, I could probably figure out how to get a good display on a Tek, but it's nowhere as easy to use. So if you have access to some good analog Tek scopes, latch onto one. I once had a 350 MHz, 4 channel one that I liked. Went with my shop, but that would be an excellent 'scope if you could get a-hold of one coming out of inventory (cheap). Right now, I have a 2235 that was destroyed in a move. I can't tell you how much I miss it.

I exactly own a Fluke 87 True RMS DVM.

Perfect!
Now, you need to figure out how your meter measures noise. Use your oscilloscope to confirm a noisy signal has an average DC level of 0 mV. Measure the same signal with your meter to ensure it also measures 0mV (or close). If it doesn't, don't feel bad. Many true RMS meters seem to have a problem with this. Just use your 'scope and meter so you know what you are measuring for sure. Most DSO's are only 8 bit vertical resolution and a few may have a 12 bit mode on one channel. This will determine how heavily you weight the reading from the 'scope. I just switch between AC and DC coupling and watch for a shift in the display. Your '87 did give you an accurate reading for the laser current though. So we know the laser in the old head is perfectly fine.

There is another test you can perform on the head assembly. As I mentioned earlier, there is a post in the center of the housing that the head rides and pivots on. These tend to get sticky. This is not normal for most heads, but this particular series of Sony head does have this problem. Of course, not a peep from Sony - as usual. Test both the new and old heads, there is no such thing as a new head in that series. They are all old and they may all suffer from this fault. So, on to the tests.

I can't remember exactly the resistance of each coil (focus and tracking), but they are relatively low impedance. Expect one around 8 ohms and the other less than 20 ohms I think. The value isn't important as you'll see. You need to be able to use a variable power supply or two in order to run controlled current through each coil. So, stick a resistor is series with each coil connection to a power supply. Now, vary the current smoothly up and down while observing the motion of the lens. You will need to do this across both polarities. One of those HP bipolar power supplies would be nice to have right about now. The only thing to watch out for is that you don't run too much current through the coil. All you need to do is go near the ends of travel (60 ~ 80% tops). Like a loudspeaker, these normally see an AC signal - but with a DC offset (unlike a speaker). I would expect less than a volt across the coil proper. The observed motion should be smooth and relatively constant in sensitivity. Expect this problem to affect the focus more than the tracking. A sticky guide will cause the focus servo to overdrive the coil and also prevent accurate tracking of the CD surface -even though it does move. That's why the motion vs applied voltage is so important.

Okay, let's say you discover that both heads exhibit similar faults. What to do? Do you have steady hands? If so, remove the dust cover from the head, there is a catch (or two) near the narrowed part of the dust cover. Just lightly push the catch back while gently pulling up near the front. It should release easily. There are no adhesives used here. Also, if you see a small felt pad on the dust cover, remove it. It collects dust and debris. The original purpose of the pad was to damp out the servo sounds from the servos. It served no other purpose, so get rid of it if there is one there.

Okay, you can now see the connections for the two coils. Take a couple digital pictures. Then, carefully heat the connections and use a copper paper clip or fine plastic or wood tooth pick to lift the leads out of the molten solder. There are strong magnets in the head and you don't want to fight with a metallic, magnetic object. Try to work quickly releasing the coil leads, but don't rush things. Once the four leads are free, there is only the pliable suspension holding the coil / lens assembly in. Release one end of the suspension carefully and allow it to flip over to one side. This is not the time to sneeze over the head since the optics are now exposed.

At this point you should be able to see the guide post and metallic sleeve on the coil / lens assembly. This is now the point where you need to concentrate. Using some cotton swabs (with a paper or wood shaft, not plastic!), you're going to clean both the sleeve and guide post. Moisten some pulled off cotton with methyl hydrate. Not wet, just moist! Feed / draw the cotton through the sleeve several times. Think cleaning a gun barrel if you know how. Follow up with some dry cotton. You can also use a residue-free cleaner like "Electrowash", just make sure it's completely safe on plastics. The lens is plastic. Now it's time for the post. Pull some cotton off the swab and moisten the remaining material on the swab. Create a hole in the cotton and feed the post into that hole. It should be a tight fit for the post without being too large to fit in the well where the coil / lens assembly normally sits. Work on cleaning the post for a while, then put it aside. Cover the head(s) so that dust doesn't contaminate the optics. You may as well remove the old solder from the pads and clean the flux off.

Once everything has completely "dried", try fitting the coil / lens assembly back over the post and confirm that the motion is now smooth without any sticking. If not, repeat the cleaning process. If everything move freely and smoothly, reassemble the head assembly in reverse order. Refer to your pictures to make sure it's all together right. Apply a small dab of silicon adhesive over each solder joint to prevent them from breaking off later on. Before you install the dust cover, install the original head and confirm proper operation. Once you know the head is working properly, you can install the dust cover. Don't try to use the new head yet if you had to clean it out.

I should mention that the coil / lens assembly sleeve bearing and the post are to be absolutely dry. Do not attempt to use any type of lubricant there, that'll jam the head up but good!

Well, "...set the oscilloscope up per the manual and then play the CD." this is exactly the problem since I can't get that far. It doesn't read the ToC, subsequntly doesn't start to play.

True, but you will still see the RF pattern and can take note of any unusual effects. For instance, excessive jitter (horizontal movement of the trace) may point to a bad disc motor. A low amplitude may indicate a focus problem, or a disc table that has settled down lower on the disc motor shaft. This would most probably be the result of previous service attempts elsewhere. So, you get the idea. An attempt to read the TOC will still allow you to assess the system.

I'll undergo the alignement procedure again.

Normally, that would be the last thing you should try. Don't forget that the CD player worked fine for years with the existing alignment. Attempting an alignment only throws another variable into the mix. Since you've already changed the alignment, try and get it close to the way you found it.

Did I read that these type of machines are reluctant to play with open cover?

Yes.
On many of these machines, they use an infrared LED and detector to sense if there is a CD loaded. Ambient light when the top cover is removed may cause the machine to ignore the CD. Later CD players either didn't use this method, or they refined how they implemented the feature. 5 disc carousels represent a later machine that still uses LED detectors. Nakamichi never did produce any of these type. They went straight into a stacking type machine.

So I guess I've talked your ear off, figuratively speaking. Please let us know where you're at with this repair.

-Chris

[Salar]

Anatech,
one question about the KSS-123A:
Out of curiosity I bought two KSS-123A players on ebay, Denon DCD1700 (almost the same as Denon DCD-3300) and one "cheap" Sony, the latter has the same chipset for servo control as my OMS-5EII.
I use CD-Check for error observance, all three players fail already at 0.375mm error width. (four black arrows on the cd, starting at 0.375 each in the inner tracks and ending at 1,5mm at the outer tracks.)
So I guess, the KSS-123A and the servo control can not go further?
Another cheapo from Sony with KSS-240 will play 4 x 1mm errors without any clicks...
All the best, Salar

[df8cf]

Hi Chris

Thanks so much for your comprehensive reply. I'll study in detail on weekend, try to learn and understand more about the system and decide on next steps.
Got your message 'bout analogue scopes vs. DSO's - I do believe I can even dig out an excellent analogue one which I understand is the recommended type. I'm definitely more the "retro-tech" type of guy - just on weekend I fixed a problem on a 50's tube amp (a Leslie amp for my Hammond organ).

Keep you posted, Gerhard