On Mon, 24 Mar 2008 11:21:16 GMT, "Arfa Daily"
wrote:
For a lasting professional repair, tweaking the laser power pot is never a
recommended strategy for the short term failure reasons that you state.
Yep. However, there are three adjustments on most cdrom drives. The
laser current is not available on all models. However, there are
adjustments for the focus coil bias (sets the height) and servo loop
gain (sets ability to track warped disks). Both of these can be
adjusted to compensate for "normal" mechanical wear. See:
http://www.fortunecity.com/meltingpot/india/245/index5.htm
http://www.repairfaq.org/REPAIR/F_cdfaq7.html#CDFAQ_005
There are a few exceptions to this rule - notably Pioneer lasers - where
setting the laser current is an official documented part of the overall
setup procedure.
Umm... I don't think Pioneer has an accessible laser current (power)
adjustment. (I don't have a schematic or manual to be sure). I
couldn't find anything in Pioneer section of the RepairFAQ proceedu
http://www.repairfaq.org/REPAIR/F_cdfaq8.html#CDFAQ_014
I was talking more in terms of CD player lasers when I referred to Pioneer.
On almost all of their models, adjustment of the tiny laser power pot on the
flexiprint that goes down to the main board, is a proper part of the
adjustment procedure.
Well, I'll admit that I've never read or seen the official adjustment
procedure.
I was just trying to convey to to the less experienced
readers, that it is an exception rather than a rule, to adjust laser power.
Agreed.
I was interested to know exactly what wear you feel warrants adjustment of
the focus bias (offset) ?
I have a home made laser output measuring device. It's just a tiny IR
photodetector on the end of a plastic stick that I can shove into the
drive when it's running. That gives me an indication of the laser
output as it's looking for a disk. I've noticed that on some models
(i.e. Panasonic) the output was rather low. So, I cranked it up and
magically fixed a few cdrom drives. Unfortunately, my guess(tm) that
what's happening is that the laser diode has partially unglued itself
from the substrate, thus causing excessive heating and eventual
failure. I don't have the equipment (or talent) to do the necessary
autopsy to be sure. However, the subsequent total failure of the
drive after my adjustment seems to suggest that the diode was on its
way out, and I just accelerated the process. Incidentally, there's a
very noticeable apparent drop in laser output when comparing old and
new drives. It's impossible to tell how much using my home made
tester.
Adjustment of the gain and offset pots is to compensate for dirt in
the focus mechanism and mechanical damage caused by various cleaning
aids. There can also be some wear on the traveler rails and head
assembly if the drive does considerable seeking. I used to maintain
an array of 12 SCSI cdrom drives acting as a database server. (This
was before cheap DVD's). The CD's were changed (updated) roughly
every week. The drives were constantly seeking, making wear a
possibility. Drive lifetime was about 2 months after which I started
getting initialization errors. I initially compensated by tweaking
the gain and bias, which bought me another week or two. Eventually, I
compared the run-out and position of the traveler with a dial
indicator and found that it had substantial irregular wear. I
switched to a molybdenum disulfide based grease lubricant, and largely
eliminated the wear and tear. Unfortunately, the system was upgraded
to an array of DVD drives, and I only had about 3 months of operating
time for testing. There were no seek failures during the 3 months. I
saved some of the worn out drives (because SCSI CDROM drives are
difficult to find).
There's some stuff on adjustments at:
http://www.cyber-mag.com/station/laserPS2.htm
Back then, SCSI CDROM drives were about $250/ea so it was well worth
doing the repairs. (I think I paid $600 for my first 1X cdrom drive
with a tray and proprietary controller card). Today, cdrom drives are
about $30 and not worth fixing. The exception are drives that have
been "propietarized" for use in dedicated hardware, such as various
game machines. Also those that have unique plastic cosmetic parts
that are difficult to find. As I previously indicated, the only
reason I tweak the various adjustments is in vain hope of preventing
the drive from becoming e-waste.
I would agree that it doesn't hurt to slightly up
either tracking or focus gain, to improve 'playability' with some discs. The
setting of these controls is something of a 'manufacturing compromise' in
the first place to ensure that the servos have sufficient gain to maintain
lock under all conditions, without causing the lens suspension and drive
coils to 'whistle' too much on marked discs.
Adjusting the gain and bias seems to increase the range of travel,
which is what compensates for the wear.
However, short of serious wear
in the spindle motor causing the turntable to drop, I can't think of any
wear that would indicate a change in focus bias being needed ? The focus
servo has a pretty wide 'window of lock' anyway to accommodate warped discs,
so tiny amounts of wear in the motor will not have a lot of effect on the
nominal focus point.
Well, according to what I've read and seen, that's exactly what
happens(1). The traveler wears down as it slides along the rails. The
wear isn't much, but sufficient to impair focus with a warped or
marginal cdrom. However, I don't think that's the only source of
deterioration. A decrease in laser output, filth in the optics, wide
variations in reflectivity from various media, and bad cd burns, all
contribute to read errors.
Disclaimer: I are not an expert on cdrom operation and repair.
(1) Next chance I get, I'll tear apart a traveler from an old and well
used cdrom drive and stuff it under the big inspection microscope at
the local machine shop. It should easily tell me how far out of round
the holes have worn.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558