Good Eye Pattern (CDm4/19)

[Mikelo]

I am learning how to use an oscilloscope and
have identified the eye pattern at the laser
current test point on two CDM4/19 players I
am working on. I have a picture of the eye
pattern in Philips service manuals, but
am not sure how to match picture and reality.
What should I look for on the pattern on the
scope?

Thank you!
Michael

[joensd]

Most important with the eye-pattern-signal is that
- it´s not clipping
- the amplitude has to be high enough.
(about 0,6V-1V depending on CD,DVD etc.)

If you have an old laser unit (too old/defective) the amplitude is too low. Sometimes you can adjust the "diode current" to increase the amplitude of the signal and repair the CDP this way.
(Laser units wear off faster though with "diode current" above nominal.)

Have a look here for a clean signal and a "clipped" one.
(You´ll find nice sites about CDP-repairs/service; don´t have any handy right now)
http://www.project-design.com/Repair...S/Lenscope.htm

[Mikelo]

Wow -- helpful link! I think I can look
for clipping, but am stll unceratin about
how to interpret the amplitude and
voltage/div.
Danke!
Michael

[joensd]

If you have the Philips service manual it should give you a value for the amplitude of the eye-pattern-signal.
Usually they give a peak-peak value.

If you look at the oscilloscope in the link of my last post
and imagine the volt/DIV would be set at 200mV/DIV
you´d get 6 divisions (peak to peak) and so 6DIV*200mV/DIV
which is 1,2V.

You got me thinking about a website I used to visit where they had great tutorials on repairing all kinds of equipment.
Still can´t seem to find the right one but there are dozens.

one example (have a look at the graphic if the "peak-peak-value" is still not clear to you)

[Mikelo]

The p-to-p value is now very clear. The
Goldwasser guide is outstanding, but the
picture of the eye pattern he has is a little
disconcerting because it is so focussed.
MIne looks more like the photo on the
PSX2 site. Also, it looks clipped to me
but I think that's because of the top hits
the horizontal division line.

On another note. Does it make sense to
check electrolytic caps by looking at the
pattern on both sides. Shouldn't they
be the same?

[joensd]

Hi Michael,

because it is so focussed.

if you refer to the drawn eye-pattern-signal in Goldwasser´s guide,
well that´s normal. In reality it´ll never look so nice
and always appear to be somewhat smeared.
The oscilloscope will always catch the movement of the signal being of high frequency.

I think that's because of the top hits
the horizontal division line.

You could "move" the signal by adjusting the "Y-Pos.". (most of time that pot is called like that)

Does it make sense to
check electrolytic caps by looking at the
pattern on both sides. Shouldn't they
be the same?

They should be if they just couple the signal but I don´t think they do in that case. You might have catched a decoupling cap that is connected to ground on it´s second leg (cap for the 5V laser diode supply for example).

Danke!

You´re welcome.

greets

[Mikelo]

I am still obsessing about the eye-pattern. I am
now measuring 1V p-to-p and I would say the
tops are rounded and not clipped. But to get
a pattern, I have to set time/div at 2us or 1us.
The Philips service manual calls for setting the
scope at 5us, but the pattern isn;t differentiated
enough at that setting. Is this a problem?

Also, I am finding
that there is a fairly wide range of the laser
output setting that is compatible with what
seems like a good eye-pattern, and I also
don't see the amplitude change very much as
I change the output pot, although I do see
changes in the quality of the pattern.

Michael

[joensd]

I am still obsessing about the eye-pattern. I am
now measuring 1V p-to-p and I would say the
tops are rounded and not clipped.

Sounds like a (at least partly) perfectly working unit.

I have to set time/div at 2us or 1us.
The Philips service manual calls for setting the
scope at 5us, but the pattern isn;t differentiated
enough at that setting. Is this a problem?

No. Strange but manufacturers not always choose a setting on the scope so that the signal is most "zoomed".

Also, I am finding
that there is a fairly wide range of the laser
output setting that is compatible with what
seems like a good eye-pattern, and I also
don't see the amplitude change very much as
I change the output pot, although I do see
changes in the quality of the pattern.

You probably didn´t catch the pot for "diode current".
There may be lots of pots but I personally wouldn´t turn them all as long as you know what you´re doing.
You´ll maybe find pots for focus and tracking and many more.

Are both units defective BTW or are you doing this "just" for research?

greets

[Mikelo]

Originally posted by joensd

You probably didn´t catch the pot for "diode current".
There may be lots of pots but I personally wouldn´t turn them all as long as you know what you´re doing.
You´ll maybe find pots for focus and tracking and many more.

Are both units defective BTW or are you doing this "just" for research?

greets

I am certain it is the pot for the laser current adjustment. It is easy
to identify from the Service manual and it definitely changes
the mV across the appropriate resistor per Service Manual
instructions.

One unit is defective -- rotel rcd 855. I replaced the CDM 4/19
with a known working one, but I still have problems. First, there
are occasional loud cracks or pops in play whenever it plays.
Second, it is very finnicky on reading CDs, and on too many there
are many background clicks throughout play (not occasional),
like playing a very damaged disc. On some CDs it just stops
playing in the middle. These discs are not pristine, so that is
part of the problem, but they play on all the other CD players
I have, so I figure something is also wrong with the player.

It is just for 'research' that is a hobby. The player was
discarded by a colleague. I have built amps and speakers,
so now I want to learn about CD players.

Michael

[joensd]

I´d just leave the pot as is.
You might not be able to adjust it so that the output drops,
might be necessary to adjust it again when the unit´s getting older.

The defect could be caused by many things and are different from CDP to CDP.

The part that "presses" the CD in position might not do it´s job,
a voltage regulator might oscillate (not very usual here).
Some Philips models have a known fault with focus due to both photo diodes.

Do you happen to know if the Rotel is compatible to some Philips model
or do you have a Philips model number with the CDM4/19?
I don´t know much about Rotel but could check out "standard" faults
with Philips models.

greets