I have an E error its mainly E3 but I have gotten E1

basically it can be hard to open or eject as the lens mechanism sometimes needs to be helped along though I was able to play some discs and it seem to play error free though it might be noise than my other players but can't get to them at the moment.

What does it seem like a belt?

Sounds like m-holder could be at fault, does it play side B ok?

I agree M-Holder

what does the m holder look like? Also is the m holder universal or would it be specific to the CLD-D504?

It is the plastic housing for the gears on the laser pickup, look where the little motor on the laser is, it is to the front of it.

Here is link with pictures, your's will be grey instead of black, which is replacement:http://www.donberg.co.uk/catalogue/audio_spares/audio_motors/piac_199.html

I think you can still buy it there but it is very expensive, possible cheaper option would be to look for a DVL type player that is broken and salavage the holders from it.

How would this fail? So this holder affects the movement of the lens mech to move inward or outward of the discs? Sometimes the clamp doesn't let go of the disc and not move in the position to elect the drawer.

It is one of the most common failure of LD player parts besides the grip ring.

I guess I don't really understand how it completely works and the looking inside the player kind of confuses me.

I think its due to the material being different than the upgraded one.
That's all I can help with as I don't have any players with the faulty part.

I once had a CED and it wouldn't play, the belts looked fine but it wouldn't play. Once I replaced the belts it worked fine with no issues even with the old belts being fine.

It's kind of like a little transmission or axle, the motor runs the sequence of gears that move along the track, if the holder breaks then the gears won't mesh together right, so the laser can't run the track properly or at all. You would have to check to make sure it's the broken though, could be other things, it's just the first thing to check on that player.