I have just been told that the laser in my 15 years old Marantz CD80 is on
the edge of the red zone and that it is likely to fail in the not so
distant future. Can anyone tell me what this actually means? How are lasers
tested, does a laser in the red zone perform worse musically or will it
just become more difficult for it to recognise disks?
The laser diodes used in CD players do "age" over time. The laser's
optical output level begins to drop. Eventually, the amount of laser
light being emitted by the laser, and reflected from the disc, becomes
low enough that the photodiode(s) and tracking/decoding electronics
cannot read the signal cleanly. When this happens, the CD player may
begin to mistrack, or the bit-error rate of the data may increase to
the point where it overwhelms the error correction circuitry and you
begin to hear pops, clicks, stutterings, and so forth.
I'd guess that "red zone" is some manufacturer's term for an output
level which is low enough that mistracking or mis-reading may be
likely. This could be determined by actually measuring the laser
output, or by looking at the amplitude of the "eye pattern" (the
output of the photodiode and preamplifier, viewed on an oscilloscope),
or by looking at the bit error rate of the data stream.
It's sometimes possible to _temporarily_ correct the problem by
boosting the laser diode's operating current level (there's often a
small trimpot on the PC board to do this). However, the increased
current tends to cause the laser to age even more rapidly - it's a
slipperly slope which leads downwards to total failure of the laser.
Replacement of the laser diode subassembly is sometimes, but not
always possible (it depends on the specific transport mechanism), and
some amount of physical and electrical realignment is likely to be
necessary.
--
Dave Platt AE6EO
Hosting the Jade Warrior home page:
http://www.radagast.org/jade-warrior
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