Help Please with Nakamichi OMS-5!

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Captouch

Captouch

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Joined 2012
#1
I just got a Nak OMS-5 in, but it doesn't read discs - tray goes out and in fine (apparently), but disc just spins and then gets spit back out.

I've read the other threads I could find here, and I've seen people point to the belt (motor feed belt is the only one I can see) as a common problem and the need to get a new belt or shrink/tension the old one through a variety of means.

But from what I can tell, the motor that moves the pick-up head doesn't even attempt to move when the disc tray is pulled in. I have the top cover on and removed the bottom cover so I can see what's happening.

I was looking for the motor "spindle" to spin, but I don't see that happening. When I power on the player, it kind of jerks/spasms for a bit, then stops. But when I insert a disc, it doesn't seem to attempt to move.

I did put a piece of shrink wrap on the spindle for better traction, and I could also try to shrink the belt as others have written on. But if the motor spindle isn't even trying to move, then it seems (to me) that the problem lies elsewhere.

Are there mechanical contacts that trigger the pick-up head motor to move to attempt to read the disc? I can't see any, but I don't know what I'm looking for either. If they are there, is there something I can do to enable contact of the necessary switches/contacts to trigger the pick-up head motor to do its thing?

Thanks!
 
#6
Although I spent 20+ years as a Nak warranty servicer, and worked on lots of OMS5/7's, this is not specifically a symptom I have seen before. Next thing to check is if the tracking sled motor is getting any voltage to it once a disc is loaded. Pin 2 of CN203 on main servo pcb is the line to the motor. If you have voltage, motor is either got a problem or wire is broken on the way to it. If no voltage(or very little), then check, on main servo pcb, for cold solder joints on Q123/Q124, then check that they are getting approx. -7V and +7V respectively on their middle pins(if not, go back to supply board & check solder joints again). If ok, try replacing the 4558 opamp chip near Q123/124 which is either U106 or U108(rough scan, not sure which), with any 4558/4559/4560 of whatever prefix or M5218AP. Beyond this, I'd have to have the unit here on my bench to help further. Service manual is available for free at hifi-manuals.com.
 
#7
That's great Stephen, this really gives me some concrete things to check. Appreciate your time in guiding me to some checkpoints. I'll check and post my findings within next couple of days.

Do you still service these items if I can't find anything wrong myself? Spare parts like motor still available?
 
#9
stephensank said:
Although I spent 20+ years as a Nak warranty servicer, and worked on lots of OMS5/7's, this is not specifically a symptom I have seen before. Next thing to check is if the tracking sled motor is getting any voltage to it once a disc is loaded. Pin 2 of CN203 on main servo pcb is the line to the motor. If you have voltage, motor is either got a problem or wire is broken on the way to it. If no voltage(or very little), then check, on main servo pcb, for cold solder joints on Q123/Q124, then check that they are getting approx. -7V and +7V respectively on their middle pins(if not, go back to supply board & check solder joints again). If ok, try replacing the 4558 opamp chip near Q123/124 which is either U106 or U108(rough scan, not sure which), with any 4558/4559/4560 of whatever prefix or M5218AP. Beyond this, I'd have to have the unit here on my bench to help further. Service manual is available for free at hifi-manuals.com.
Click to expand...

OK, I don't see any voltage on CN203 pin 2. I did check Q123/124, and they are getting their approx. -7V and +7V on their middle pins and solder joints look OK.

I wasn't sure if I should I be seeing voltage on pin 2 of CN203 even with the cover off (didn't know whether the interlock cuts off this voltage or not). I was initially doing all checking with cover off to gain access to these pins (I unplugged the cable and probed the pins directly).

But then I checked on the other side (pin 2 of CNM-05 on the Auto Power Control board, this time with the cover on. No voltage there either.

U108 is the 4558, so I can replace that next.

I did take some measurements on U108, with cover off, before and after I inserted a disc. I don't know whether any of these will provide more insight as to whether this opamp is bad, but here are the measurements anyway.

Door Open --> Disc In and Spinning
Pin 1: -30mV --> -150mV
Pin 2: -5mV --> No change
Pin 3: -5mV --> No change
Pin 4: -12V --> No change
Pin 5: -5mV --> No change
Pin 6: -5mV --> No change
Pin 7: -120mV --> -330mV
Pin 8: 11.8V --> No change

Shall I just replace U108 and remeasure voltages on the motor feed power pin?

Thanks!
 
#11
The opamp is not being told to output any voltage to the sled motor, and is pretty certainly fine. I just remembered, and confirmed in the s/m, that this player has a cd-detect led & photodiode pair. Usually only happened on OMS-3 & 4, but could happen on the 5, that the photodiode would get oversensitive, and less than perfectly opaque cd's(and few are anymore) won't block led light enough and cause a no-disc presumption. To diagnose, just put a piece of black tape on the small hole(<1/4") in the disc tray, then tell the tray to close with no disc in & see if the laser moves in to the center. If it does, then try with cd & tape still in place. If cured by this, easy solution is to simply increase the value of R263 on the main board, near CN213, by 50 or 100%.
 
#12
OK, I put the piece of black tape on the small hole and closed the tray with no disc. No laser movement at all. Just for fun, tried it with the CD in as well, but no change.

The only change was that, with the piece of black tape, the player flash the standby indicator and spit out the tray after several seconds even if there's no CD. Whereas, without the tape, if there's no CD inside and the tray is closed, the standby won't flash and it won't spit out the tray.
 
#16
Still in business of working on Nak CD players?

stephensank said:
I still work on them, yes. Parts aren't available, but I probably have any parts I would need on hand.
Click to expand...

Hi Stephen! I currently have a OMS 5 and 7 All that I would like to get service/repaired by someone, like yourself, that has 20 years of experience working on Nak equipment. Finding a tech like you is almost impossible.The OMS 5 seems to have the identical problem as Captouch and the 7 doesn't have the ability to read the CD when it's turned on but works sometimes, after it's been left on for a couple days. Once it starts working, it will continue working fine for hours, as long as I continuely play cds (exercising it). I live in Hawaii and it does get humid.These players are in excellent condition and I have the service manuals; every high end Nak cd Player I've ever had, has had some kind of disc reading problem. I would like to do business with you if you are interested, the question now is how?

Thanks, Kelton
 
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