CDM-12 Problem - Whistling / Oscillation

[poynton]

Hi.

Has anyone come across this problem with a cdm-12 mechanism before?

The cd-p reads toc.
However, when playing track one or two, it emits a high pitch whistle from the motor? the cd-p plays for about 2 seconds and then stops showing toc info on the display not disk or error. If say track 5 was selected, it plays. Hence my thoughts that the problem is in the motor at low revs.

I have fixed the problem temporarily by adding a small weight(2 coins) to the tpo of the disk clamp. It now works OK but I would like a better fix if any is known.

Thanks
Andy

 

[Extreme_Boky]

Very interesting!!!

Connect the CRO to motor windings and observe the analog voltage that controls the motor speed. It should be clean DC level of between 2, or 3 volts to close to 5 volts DC. Do the frequency sweep and check if there are any high frequency oscillations superimposed on top of this DC level.

If everything looks clean, you have a motor / mechanical problem.... somewhere....

Cheers,
Extyreme_Boky

 

[anatech]

I'm thinking that the focus servo may be oscillating. The tracking servo does output a sinewave. That's the way a single beam system works. It's possible you can hear that.

-Chris

 

[poynton]

HI.
Thanks for the advice.

To eliminate possibilities, I replaced the cdm with a spare.
Result - it still whistles but less than before. ( the design of this cdm is slightly different - the centre has more mass so it would be like one of my coins)

So, I think that the problem definately lies in the electronics.
The 'fault' goes if the cd player is left on for more than 20 mins.

Question for anatech - You mention focus servo and tracking servo.
What should the signal be at the focus servo?
Like wise for the tracking servo?

Thanks
Andy

 

[anatech]

Hi Andy,
To troubleshoot, I would need a schematic. Hang an oscilloscope (CRO) on the focus servo output and tracking servo output. This will be on the flex cable side. You can look at the focus and tracking error signals too.

On some of these transports, you have a tracking gain and a focus gain. If they are too high, the servo may oscillate. It may simply not read discs as well as it should. I was trained to use a pair of filters and adjust a lissajou pattern for an equal propeller shape. Different filter depending on model.

A service manual would be in order.

-Chris