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Old 28th September 2008, 10:27 PM   #1
auric is offline auric  Canada
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Default Servicing CDM 12 - Distortion

Hi:

Could someone with experience on Philips CDM 12 offer some advice?

I have a Bang & Olufsen CD player that uses the CDM 12 -IND mechanism.

The CD player stopped playing. With the help of posts to this forum, I learned that this optical pick can be replaced.

I purchased a pick up and swapped it into the mechanism but this did not fix the problem. The fault was finally sourced to a defective servo board. Happily I was able to find a spare one.

With the exchange the unit was working for about 2 months.

Recently however it has problem playing many CDRs. The unit has since become progressively worse. It will play a cd for several minutes and then begin to distort and cut out. The disc will spin down and then up to try to compensate.

I tried swapping the DAC decoder board but the results are the same. This points to the mechanism.

I also tried to turn up the laser current on the pickup but this did not make any difference.

Any ideas?

Thanks,

Derek
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Old 29th September 2008, 05:51 PM   #2
Mooly is offline Mooly  United Kingdom
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Hi Derek,
Is it a CDM 12.1 not the CDM12.4 which has the RF amp mounted on the pickup itself.
Is it just with CDR's that there is a problem ? Unfortunately with any problem like this it's best to actually "view" the RF signal from the pickup on a 'scope. That's the only way to be sure what is happening. It will show conclusively whether the signal is "marginal" at best, even with pre recorded discs or whether there is a problem with the CDR media itself, possibly not being burned properly.
It's possible the replacement pickup isn't 100%. Certainly as far as the CDM12.4 goes the ones I have had experience of lately are rubbish. The original manufacturer parts dried up a few years ago here. If the fault was elsewhere it's possible that the original pickup could be better than the replacement.
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Old 29th September 2008, 06:27 PM   #3
auric is offline auric  Canada
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Hi Mooly:

It is a CDM 12.4. This is the model with the IC chip mounted on the pick up, not the one with the metalic shielding.

The mech started out being picky about CDRs but played most CDs properly.

Then things got progressively worse. Now it plays fine for about 30 seconds and then distorts and cuts out.

On a working CDM 12.4 the distortion can be reproduced by gently rubbing the side of a CD while in play. It's as if it bit stream is dropping out.

I was told that the CDM 12.4 -ind is no longer available. The only way to salvage it was to swap out the pickup keeping the servo electronics and aluminum casting. The original pickup appears to be dead.

Unfortunately, I don't have access to a scope a the moment to view the RF signal as I am working abroad at the moment.

Any suggestions on pinpointing the fault?

Thanks again for your reply!
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Old 29th September 2008, 06:43 PM   #4
Mooly is offline Mooly  United Kingdom
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Hi,
I have never had anything other than pickup trouble. You have to scope it to see if it's OK on a good disc. The quality of the signal tells all, and where any problem may be. It's vital !
I did this a while back,
Stripdown/Clean of CDM12.4
The CDM 12.4 suffers from "grease" problems as you probably know.
When you touch the side of the CD the disc speed varies enough to cause the phase lock loop of the digital signal processing stages to "lose lock" giving the effect you describe. However poor RF quality can give the same symptoms.
Worth popping the sled motor out and seeing if the pickup is "free" to slide, that said they never feel silky smooth but any obvious sticking is worth investigating. The teeth on the blue cog can get burrs on them too.
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Old 29th September 2008, 06:57 PM   #5
auric is offline auric  Canada
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Hi Mooly:

Thanks for the link.

Yes I'm aware of the "grease" issue. I also read that the gear ratio on the sled motor is a bit high causing problems as well.

It looks like the best thing to do is to swap out the pickup for another one.

Is CDM Pro 2 a better mech? The manufacturer is offering a replacement kit. It will cost much more though. People I know in the Juke Box business say the 12.4 is much more reliable.


Thoughts?

Derek
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Old 29th September 2008, 07:17 PM   #6
Mooly is offline Mooly  United Kingdom
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If you can get hold of a 'scope check first. It proves beyond any doubt the signal integrity. We are just guessing at the moment.
Other than that see if you can get hold of a new pickup only, not the Chinese reclaimed rubbish I have seen. The last one I ordered was made in Singapore.
There is nothing wrong with the CDM12.4 ( when it's working ) I think they are great, and if I get more than one or two skips in a year, and that's with CDR as well then there is something wrong.
The "Pro" kit sounds like it's going to be expensive, and is it better, I don't know.
Another problem I have seen with some is that the turntable isn't true, causing the disc to wobble and again the scope shows this up as a "bounce" in the RF.
I wish I had stocked up with a couple now but there you go. At least the Singapore head units seem OK and the mech is pretty serviceable even down to fitting new spindle motors etc if needs be.
Good luck with it, and if you have any questions just ask
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Old 29th September 2008, 08:02 PM   #7
auric is offline auric  Canada
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Mooly:

Thanks so much for your answers and offer of help.

How can I tell where these pick ups are made? My only source is ebay or small online companies. Most of the vendors do not know themselves where these things come from.

Derek
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Old 30th September 2008, 07:48 AM   #8
Mooly is offline Mooly  United Kingdom
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Hi,
Donberg
http://www.donberg.ie/search/search....&group=&page=0
the last time I asked said they had genuine Philips stock. Worth an email perhaps.
Maybe it's worth you starting a new thread in the components section asking for sources of supply in your area, or whether anyone has had any bad experiences of any.
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