Rebalancing CDM4 Radial Arm? - diyAudio
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Old 25th September 2008, 05:39 PM   #1
auric is offline auric  Canada
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Default Rebalancing CDM4 Radial Arm?

Does anyone know how to re balance the radial arm on the CDM4?

I notice that on one of my CDM4 the arm floats in the middle of the arc path. I assume this is the normal rest position.

On another, it floats to the outer path of the arc (ie further away from the the center where the turntable motor is.) What happens
then is that at the end of a CD or when I stop the CD during play the arm will disengage and fall away from the turntable motor and hit the
housing of the mechanism making a "click" sound.

If you gently lift the optical pick up with your finger and move it to the center of the mechanism (ie as far as it can go towards the turntable motor) and then let go, it should return to the center without striking the body of the mechanism on its way down.

Any ideas?

Thanks in advanced,


Derek
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Old 26th September 2008, 10:17 PM   #2
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Hi Derek

The restposition of the laserarm depends on the forces put on to it by the so called flexprints.
The ballbearings in the center of the arm cannot really be adjusted. The force is reguleted by the springplate on the underside of the arm. Perhaps you can take this apart, be careful not to lose the balls!!!
If the laser is balanced well, you should be able to turn the CDM while the laserarm remains in position.

Succes!

Steven.
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Old 26th September 2008, 10:52 PM   #3
auric is offline auric  Canada
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Steven:

Many thanks for your reply!

Am I correct in understanding that that the spring plate (which holds the bottom bearing in place) only puts vertical pressure on the radial arm?

The only lateral force is with the flex print?

I have one of those Bang and Olufsen CD players that sits up vertically.

If what you say is true I suspect that over time (because the mech is placed vertically, not flat) its been weighing on the flex print thus over time it no longer centers.

Now every time the radial arm disengages, it slams against the outer extreme of the arc path before bouncing to rest position.

Perhaps I can add some weight on the other end of the arm as a counter balance?

Any ideas?

Derek
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Old 27th September 2008, 04:19 PM   #4
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Hi Derek,

The counterweight is already mounted on the other side of the arm. This is done to allow the servo cicuit to move the arm without having to lift or descend it.
The bouncing you mention is caused by the servo system putting some kind of output signal to the coils on the laserarm. To me it seems the laser loses the track and the servo is unable to correct.
This can have many causes but as you are having your CDM mounted vertically, it is possible that the angle between the disc and the laserbeam is out of limit.
It is possible to readjust this by losening the screws of the springplate and move it in the right position.
According to Philips this should be done with a mirror positioned on the lens and a transparent disc.
I'll see if i can find the servicedoc.
Is the problem worse on the outer edge of the disc?

Steven
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Old 28th September 2008, 09:04 PM   #5
auric is offline auric  Canada
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Hi Steven:

The "bounce" is not electrical.

I have a new CDM 4. If you put it flat on a table, the pickup naturally rests in the center of the arc.

The one out of my B&O CD player rests closer to the outer edge of the arc.

On the new CDM 4: When you use your finger to move the pickup towards the center of the mechanism (ie towards the spindle motor) and then release, it travels past the center of the arc towards the other extreme end before returning to the center position.

On the old CDM 4: the pick up will slam against the mechanism wall on the outer edge of the arc before returning to center.

I suspect that this is a design flaw. The CDM 4 was not designed to be mounted vertically. Over time gravity may have altered the characteristics of the flex print causing the arm to droop away from center.


Now whenever the CD player is stopped during play the optical pickup hits the mechanism wall before coming to rest. I don't think the shock is good for the laser. I've tried introducing more weight to the counter balance but this introduces other problems.

What I've done for now is adhered a thing rubber pad at where the collision takes place to cushion the impact.

Thoughts?

Seperately, do you have any experience with repairing CDM 12?

Thanks!

Derek
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Old 2nd October 2008, 08:07 PM   #6
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Hi Derek,

You might be just right by stating the CDM4 is not designed for vertical mounting, however I would guess that a manufacterer of CD-players knows how to mount a mechanism.
Have you already taken the springplate off to readjust it?

I do have some experience with CDM12, but as the laser/photodiode assembly is one part, it is very difficult to replace.
CDM12'ves however are very cheap and easy to find. What is your problem?
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Old 2nd October 2008, 09:40 PM   #7
Cobra2 is offline Cobra2  Norway
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Quote:
Originally posted by Steven505
Hi Derek,

You might be just right by stating the CDM4 is not designed for vertical mounting, however I would guess that a manufacterer of CD-players knows how to mount a mechanism.
....
And how long did they expect it to last? (and how old is it now? )

Arne K
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