Maker Pro
Maker Pro

CD player troubleshooting

M

meowbus

Jan 1, 1970
0
I am fixing my CD player. I compare the output signal such as TE/FE/RF with
another one.
The voltage level/signal amplitude of tracking error/focusing error are quit
similar to the good one.
However, the amplitude of the RF signal is lesser and the DC level is
shifted up. What should I do next to fix the circuit?

Thanks a lot
 
M

meowbus

Jan 1, 1970
0
Thank you for your kindly help.

I have cleaned the disc and the lens carefully.
I use a analog oscilloscope to monitor the RFI. But I can't see the eye
pattern. However, the variation of FE and TE are quite small.
Would it be possible that even the FE and TE are small, it is still not be
able to read the disc due to the total reflected power is not enough?

Joseph Pang
 
C

Chuck Simmons

Jan 1, 1970
0
meowbus said:
Thank you for your kindly help.

I have cleaned the disc and the lens carefully.
I use a analog oscilloscope to monitor the RFI. But I can't see the eye
pattern. However, the variation of FE and TE are quite small.
Would it be possible that even the FE and TE are small, it is still not be
able to read the disc due to the total reflected power is not enough?

Joseph Pang

It is possible for the power to be low but unlikely. The normal power
for CDs is about 700 to 1200 microwatts. Most of the optical units I see
these days (Aatek, Gbond, Pioneer, Ricoh etc.) have between 0.25 and 1
volt peak to peak on the RF. The data AGC can usually handle about a
20db range. Normally the RF is balanced (differential).

There are two ways of getting the eye pattern. One is to trigger on the
RF itself. This should give a fair eye pattern if the RF has data in it
at all. The other way is to trigger on the recovered data clock which
goes from the read channel and the controller. If you can get an eye
pattern with the first method but you cannot with the second, the read
channel is not locking to the data.

One thing to establish is that focus is really working. The focus and
tracking will often look normal even when focus is not working. If you
watch FE and TE while you tap on the disk, you should see quite a bit of
activity on FE and TE. You can also look to see if the lens moves up and
down during start up. The up and down motion is focus search. The lens
can move about 1mm but the focus linear range is perhaps 10 microns,
hence the search.

If there are focus or tracking problems, you might check the power
driver(s). These will probably be Rolm (or whatever) and usually have BA
numbers. These are combo drivers and each part drives more than one
motor (typical CDs have 5 motors).

If your optical unit is bad, it is usually not possible to repair. In my
lab, we throw away the entire loader if the OPU is bad (except we used
to replace Aatek OPUs). If the read channel, servo DSP, or power drivers
are bad, you may be able to find replacements.

One last thing. I can see a CD laser at read power reflected off of a
piece of paper held over the lens. It is very dim but I can see it in a
darkened room. If you can't see it this way, the laser may be bad.

Chuck
 
C

Chuck Simmons

Jan 1, 1970
0
meowbus said:
Moreover, I see a large variation at TE ( about 600hz and >1Vp-p) before it
becomes small.
However, the RF signal are larger when TE variation is large.

Good. Focus is obviously working. The 1v p-p TE is track crossings due
to disk eccentricity. Tracking can't capture until the rate of track
crossings is small. Moreover, tracking may make several attempts to
capture.

RF should be near its maximum value when tracking captures and becomes
quiet. It is possible that the larger RF when not tracking is induced
noise from the tracking servo. I can't think of any other reason for RF
to appear lower during tracking.

Chuck
 
Top