Nakamichi OMS-4A issues - diyAudio
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Old 3rd November 2006, 11:32 PM   #1
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Default Nakamichi OMS-4A issues

I have a Nak OMS4 cd player that has stopped working properly it powers up- front panel buttons seem to function properly except play

CD tray opens - but unit will not read any cd's- Ive tried quiet a few different cd's that had work OK in the past. This unit has always been a little finnicky about cd's . I have a service manual and I believe all of the test equip. required. Just very little experience. Guess I need a nudge in the right direction.
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Old 10th December 2006, 01:16 AM   #2
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Default Re: OMS-4A

It is very strange...I JUST fixed my own OMS-4A and completed the repair not 10 minutes ago. It had been one of those 'get around to it' projects sitting for about 2 years. I started searching for info about reclocking this unit when I saw your post.

Does the display read the correct information when you insert a disc (time, number of tracks, etc.)? Does the disc play but you get no sound? Does it recognize the disc, but not do anything when you hit play? I ask, because they are all different issues.

I had no component failures on the board in my unit; however, it went from playing only very intermittently to not at all. Its problems were several - the main ones being it has very dried out and stiff belts and it suffered from very crystallized solder on the main board - mainly around the voltage regulators. I used a rubber-rejuve chemical (forget which brand) and cleaned the belts thoroughly. I also reflowed a LOT of solder joints and removed alot of glue from both sides of the main board. Belts are actually doing their job now - I suspect they will still need to be replaced down the road, but for now they are working fine.

My experience in component repair on older units (I used to be an electronics tech in a used stereo shop) came in handy. Often, that brown glue-crap manufacturers put on boards to hold things down turns into a corrosive and/or a conductor. Best to CAREFULLY remove that glue from the board on both sides, especially around where it hits solder joints. Try to not disturb the lay of the wires too much, because I think manufacturers sometimes use glue to stabilize wire routing - in an attempt to minimize rf/signal and other bleeding. Also, be VERY careful scraping the circuit board. You can cut, tear and break traces which will obviously cause all kinds of catastrophic things - such as rendering your player useless. In truth, I have never had a problem removing the old glue in units like this - even where wires are held down. But I would be careful, nevertheless.

I hope this helps.
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Old 10th December 2006, 01:16 AM   #3
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Sorry - just saw you wrote your unit won't read cd's. Actually, everything I wrote should apply, since that was the problem my unit had. I guess the next question is - does the player spin the disc? If not, I would start with the belts (cleaning/conditioning, or replacing) and go from there. If you are sure the disc fully loads AND the unit doesn't spin the disc, then its possible you have a bad laser. But before I would draw that conclusion I would clean the rails the laser tracks on and CAREFULLY clean the lens on the laser.

BTW, mine is playing beautifully. They do sound nice.
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Old 10th December 2006, 05:03 AM   #4
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I was beginning to think no one had any knowledge of this unit or I pissed off the wrong people.LOL
Anyway thanks for responding. My unit loads disk OK,spins-up not sure if speed is correct then spins down and ejects disk while its spinning up front panel displays [ 1 ] (track #) and [standby ]flashes 8 times,[ load ]also lights up briefly.No other front panel info is displayed. I did have a look at the pcboard under the regulators and I see what you mean about the brown glue and corrosion its not real bad but needs to be addressed. I'll have a look at the belts and sled rails and cleanup the pc board. I,ll keep you posted.
Thanks Dave
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Old 10th December 2006, 09:57 PM   #5
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As far as I can see there is only one belt . And its on the loading assembly. Am I overlooking any? Ideally what type of lube would be used on the sled rails?So far the dried up adhesive has chipped off real easy reveiling some nasty corroded solder joints. Glue either traps or attracts moisture.Some of that glue was placed on some caps topside causing corrosion on the two Elna caps in the photo. At first glance it looked like crusty electrolyte leakage.
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Old 11th December 2006, 04:39 AM   #6
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What I have done to clean the rails is to use q-tips dipped in alcohol, and clean them that way. It seems to dissolve the old lubricant which changes consistency over the years. Be very careful with the q-tips, since they tend to loose lots of fibers as you work with them.

If the deck starts the spin-up, then your belts aren't the problem. Or rather, belt, as I seem to remember you're right about this having only one belt. I don't keep track of such things, I just 'do the belts' when that is the next step on my mental checklist. And the good news is that your laser is working - at least to a degree. Lasers don't generally allow even a spin-up unless they can focus on something to read such as the cd.

That's the good news...the bad news - for me, at least, is that after several hours of playing flawlessly, my deck reverted back to its old habits. Just like yours, it starts the spin-up, then the display largely goes blank, and the drawer opens.

I still think it is likely a power supply issue. The other thing I will be looking at is the two relays on the board near the output jacks. I have not yet re-flowed the solder around them, but I suspect they may be involved.

Please keep me informed - who knows, maybe you will find something I missed. I do NOT think it is a faulty component in this case, unless it is a ps cap leaking or the relays as mentioned that have corroded contacts. I don't have a schematic for this unit, so I am largely fumbling (or peeing into a headwind, to use my description of working without a service manual). If you have a service manual, I would look at the voltages on the regulators, as well as the functions and voltages/signals on the relays. That would be my next step, anyway...

Good luck!
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Old 11th December 2006, 04:39 AM   #7
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Oh, yeah. Lubricating the rails is tricky. To be honest, I generally don't relubricate them, since the alcohol seems to change the consistency of the grease I can't get off to something closer to original. If you feel you want to lube them, use a fairly thin white grease - and use it VERY sparingly. My guess is the head won't hang, though, if you don't use any grease at all. Just don't be too thorough about cleaning the rails beyond removing any dust, hair, etc. that may have accumulated.
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Old 11th December 2006, 06:06 AM   #8
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I checked the PS voltages at the regulators both +15vdc and both -15vdc and the +5vdc are still good . If you would like I could scan the schematic for you.I shorted TP105 and TP106 this overrides the laser safety interlock and with a optical power meter I measured 0.31mw according to the service manual 0.15-0.4mw is OK. As far as the rails go the sled travels real smooth.I'll keep plugging away at it.....Isn't this stuff fun...Later Dave
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Old 11th December 2006, 08:52 AM   #9
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I would appreciate the schematic. If I may ask, could you send it either in a large enough pdf or other file so I can read it? I have so often found a schematic after endless looking - only to find it compressed to the point where it is virtually useless.

It might be worthwhile to monitor the supply voltages at places downstream from where the regulators are soldered. Also might be worthwhile to monitor the voltages when the unit is cycling through its functions - that way, you might see a sag in voltage with current draw - which could account for the unit shutting down. Something might be drawing down the reserve that is only apparent when the demand on the particular voltage source is high. That might be a poor solder connection, or a leaky cap in the PS itself - or downstream, for that matter. Its always interesting to look at what the actual output looks like on a scope, too, if you have one. You might see some wierdities that shouldn't be there...

BTW, thanks for the pic. Keeping in mind the old glue could be acting as a resistor (it can be somewhat conductive), imagine the effect the glue could be having on the visible components that are swimming in it...I would recommend carefully removing all of it that is around and across the components. I doubt that is the problem, but I can't imagine it doing any good for the signal path. I get dental pics from the local dollar store that work great for removing the glue. I don't know if the Dollar Store phenomenon has swept over your neck of the woods, but I would suspect it has...
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Old 11th December 2006, 11:06 PM   #10
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I'll see what I can do for the schematic or e-mail me with your snail mail address and I will make a copy of the entire service manual....I think tonight I will probe for Vcc on all the chips and power distribution to there destinations. Ive got to button up a Yamaha C-4 preamp first. Had a problem with left channel on phono 1 found and repaired a cracked solder connection on the RCA input/output PC board. Fun Fun Fun..


Yep, I have a scope Tektronix 2445 4-channel 150mhz /with isolation transformer .

Dave
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