Defective Discs - Sony CDP-CX235 Training Manual

Defective Discs

Various problems can be found in CDs that cause skipping or clicking
sounds (momentary muting) when played. Close examination of the RFO
waveform can identify a defective disc.
Normal RFO
At start up, laser light from the optical assembly is reflected from the infor-
mation layer of the disc to the optical assembly detectors. This returned
laser light causes the RFO voltage to go initially HIGH.
RFO at start of Play
Initially HIGH
T
1
LOW voltage when a pit is encountered
The information layer consists of a reflective mirror area and data pits of
variable length. These pits are of precise width and depth so light cancel-
lation can occur. As the disc begins to spin, the laser falls on a data pit
and its light is reduced (canceled) so less light is returned to the detec-
tors. How long the light reduction takes place is dependent upon how
long the pit is. Light reduction yields a LOW RFO voltage.
In summary, it is the transitions from the mirror area to the pit area that
create the HIGH to LOW transitions of the RFO voltage. Therefore the
Vp-p transitions of the RFO are dependent upon the reflection quality of
the mirror area and the depth of light cancellation in the pit area.
Disc Defects
Disc defects affect the RF signal in several different ways:
iii
Center Hole Misalignment
This causes a wobble in the disc and a corresponding bounce in the RF
waveform. If the off-center hole is real bad, the disc will not play.
C D hole off
center
(exaggerated)
Bubble in the Plastic CD
The bubble diffracts the laser light so less light is returned. This causes a
LOW in the RFO waveform.
Information layer is Peeling off, or too Thin,
This causes a reduction of RFO level similar to a low output laser or dirty
lens. Try a known good disc or the YEDS-18 test disc (P/N = 3-702-101-
01). If the Vp-p RFO is still low, clean the lens.
Missing Information Layer
When a chunk of information is missing, there is a loss of light returned to
the detectors. The RF level drops low at this time and there is a click in
the audio when this disc area appears because the audio cannot be com-
pensated for, so muting is the last alternative.
P M 3 3 9 4 , F L U K E & P H IL IP S
T
Missing info layer (skipping)
Black spot
If the stamped disc remains too long on the die, the plastic may be burnt
black. During PB, this black spot causes a momentary loss of RF, which
momentarily mutes the music (causing clicking sounds).