Philips CDM-1 swing arm alignment - diyAudio
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Old 23rd July 2014, 01:09 PM   #1
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Default Philips CDM-1 swing arm alignment

Hello everyone,

I am kindly asking for advice in mechanical alignment of CDM-1 swing arm. I disassembled spindle motor and swing arm for cleaning and lubricating. After re-assembling, the unit works well. However, the focus voltage is not constant from center to the edge of disc, which probably means that the laser does not move parallel to the disc surface.
Has anyone performed a successful mechanical alignment as described in the service manual? If so, what accessories do you use instead of special mirror and a glass CD? Also, instead of optical alignment, would it be possible to align the swing arm by measuring focus voltage ad beginning and end of disc and bringing them close enough?
One last question: with an obviously misaligned transport as mine, what should be the permissible focus voltage range and should I expect weaker tracking and slower track skipping speed at the end of CD?
Thank you in advance.
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Old 23rd July 2014, 01:44 PM   #2
Mooly is offline Mooly  United Kingdom
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I've never used the official method either and tbh have never looked at equalising the focus voltage but yes, doing that should be an accurate way of ensuring the arm is parallel to the disc. It has to be, the DC bias determines the position of the lens (focus coil). I've no idea off hand what the typical average DC focus voltage would be, only to say that it will have a wide range of values that will work. The platter height will be the main factor with the swing arm at the start position so if that has not been disturbed then I would take that as a "normal" value.

I suppose tracking ability does vary at disc edge due to the combined mechanical/electrical characteristics of the complete servo being different at disc start and disc end. A good test would be the official Philips playability discs with simulated errors.

Edit... what I would definitely look at is the quality of the RF at start and finish and make sure that there is no change in quality or amplitude.
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Old 23rd July 2014, 06:08 PM   #3
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Hello Mooly and thanks for your opinion.
Today I have attempted the optical alignment procedure with a transparent CD and looking at the lens at an angle which actually gives reflection, as I don't have an appropriate mirror to put on. Spanning a string across a light above the lens will cast a thin but sharp shadow.
I am now confused because the distance between the two reflections which should be brought to a single line (from CD and lens) actually varies with the distance of string from the transport.
Any advice?
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Old 23rd July 2014, 06:18 PM   #4
Mooly is offline Mooly  United Kingdom
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I'm afraid I've nothing useful to add really. The string and transparent disc (where do you get a transparent disc from ? A blank from a pack of CDR/DVDR etc ) is one of those things you would have to see in operation to get a feel for what its telling you. I can't quite visualise it tbh, and I'm thinking that as the arm traverses an arc then it will deviate from a line cast by the shadow of a taught string... but I guess you don't mean that.
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Old 23rd July 2014, 06:21 PM   #5
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You are correct, it's a plastic CD from a pack which can be used instead. :-)
And you are right, I didn't mean the deviation along the arc path of laser pick up.
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Old 23rd July 2014, 06:37 PM   #6
Mooly is offline Mooly  United Kingdom
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Why not stick with your average DC voltage method. That sounds a very workable idea to me.
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Old 23rd July 2014, 07:58 PM   #7
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Simply put, because it's a PITA.
Axial and radial adjustment play are only a few mm so I don't see how is ANY adjustment supposed to be done here.
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Old 24th July 2014, 05:57 AM   #8
Mooly is offline Mooly  United Kingdom
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Yep.

Philips were often fanatical in some of their alignment procedures (TV/ video to) and technicians always worked around them or devised their own workarounds.

Unless you have a problem with playability or can see the RF quality as non consistent between start and finish on the disc then I wouldn't worry.
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Old 2nd August 2014, 12:07 PM   #9
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Here is my contribution to the knowledge base on how to align the swing arm assembly within satisfactory tolerances using the method of minimizing the focus voltage offset across the CD. No reference discs or tools are required.
It is absolutely not necessary to make this adjustment if the factory alignment has not been disturbed. In my case, alignment was required because I had disassembled, cleaned and lubricated swing arm and disc motor and voltage offsets were exceeding 1V afterwards.
I tried the optical alignment described in Philips' service manual. However, it is simply too sensitive and transparent plastic CDs from a CD-R pack, suggested by several sources as a replacement for reference glass disc, are not suitable as they are wavy and can give erroneous parallaxes from different angles.
I used 10 different pressed CDs from my collection (I did not use CD-Rs). They need to be as flat as possible and you can observe if they are wavy by looking at the reflection on their top sides near the edge. For every disc, I measured the offset voltage value at beginning and at 50:00 minute mark (thus measuring the latter at the same distance from the disc center) and put the data in a small Excel table. I calculated the average offset for the series which was around 800mV. As a side note, you will discover that there are factory CDs of all kinds: with small offsets, huge offsets, falling and rising offsets across (therefore reflective surfaces at different angles), and so on, but this has no importance, whatsoever.
Next step was to pick the CD with biggest offset from the series and adjust the swing arm to minimize the offset as much as possible. To do this, unfasten the two Torx screws which hold the shin so that it can move but do not loosen them completely. Position the player horizontally and allow access to the transport form bottom side - I used two chairs set apart a little. The swing arm shin has only a few mm play on X and Y axis and has to be moved very gently and gradually. By trial and error, adjustment will also cause the laser to completely fall out from focus and reproduction but in the end I was able to greatly reduce the offset on that CD.
Next step was to measure offsets for other CDs and compare new values with old ones - note that here the start and 50:00 values will have changed but they are not important at this step, the goal is to see that offset differences on all discs are reduced after the alignment. By measuring all CDs and adding the data to the Excel table, this is likely the result you will have attained. In my case, the attained average offset went down from 800mV to 197mV. This is a satisfactory result and the screws can be fastened, taking great care that the shin remains in adjusted place and is not moved. Make sure that you did not disturb the alignment by measuring offset on one CD again.
It is very important to check that swing arm moves absolutely freely after the alignment: when flicked with finger, it should bounce back and forth a few times. Any stiffer than this and you might have tracking difficulty and problems in operation. Even the Philips service manual clearly states that the swing arm alignment is a compromise between optical adjustment and easily moving assembly. Lubrication will not improve movement of a misaligned swing arm. Also, use neutral oil to lubricate in order to preserve the small MBR O-rings which enclose ball bearings on the axis. A worthwhile check of swing arm movement is to put the player in service mode and check its deflection on various steps as described in player's own service manual. This can be compensated by adjusting radial motor offset in +/-100mV range.
As a final step, height of the disc motor should be re-adjusted by turning the bottom spindle bearing. I went back to the Excel table and calculated the value which would ensure equal average offset against zero value. That is, a theoretical CD which would be the average of chosen 10 CDs, should have equalized offset in + and - range. By doing so, you will avoid that the player operates mostly in + or - offset range. To illustrate in concrete numbers:

VALUES AFTER ALIGNMENT
Average offset 00:00 = -200mV
Average offset 50:00 = -400mV
Average offset difference = -200mV
Therefore, the offset difference should move between +100mV and -100mV. Therefore:
New average offset 00:00 = 100mV
New average offset 50:00 = -100mV
Average offset difference remains the same.
The difference between old and new offset is +300mV. Select any of chosen 10 CDs, add 300mV to the measured 00:00 offset and adjust height of the disc motor to that value. Secure the spindle bearing with lacquer and the procedure is over.

I hope this helps. Please feel free to comment or correct if you find any error in described procedure.
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