This is topic Visable Red LED Life in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Francis Casey (Member # 514) on 12-10-2000, 08:21 PM:
Hello All
What kind of life are you all getting from you LED's used in basement readers for Dolby Digital .In one of our multi plexes which use component engineering readers in 5-star soundheads , i just had to replace 3 which had about 9000 hours on them because proper digital level could not be reached by readjustment of the power supply .Others in the same complex could be made to reach the proper level only by turning the power supply all the way up and i suspect i will have to be replaceing these within a year .
Regards
Francis
Posted by Randy Stankey (Member # 64) on 12-10-2000, 08:29 PM:
You're right about that. They WILL eventually die on you and will have to be replaced. At least the Component Eng. LEDs are easy to replace.(Ever do the ones in a Christie? They arent fun. I'm sure other guys here can tell me ones that are even worse.)
Anyhoo, it was originally thought that the LEDs would never have to be replaced, or at least not for many years. That turned out to be wrong. I have some... (From a brand of projector whose name rhymes with "misty")... that have only lasted for a little over a year!
Posted by Aaron Sisemore (Member # 145) on 12-10-2000, 11:08 PM:
Kelmar LEDs are hell-on-wheels to change out and then you have to partially disassemble the damned power supply to get to the current adjustment, which even on *new* Kelmar LEDs end up cranked to maximum just to get a decent voltage reading on the 'scope... 
Aaron
Posted by Brad Miller (Member # 2) on 12-10-2000, 11:37 PM:
Actually, I've found replacing the leds on a Christie to be incredibly easy.Just remember when installing a new led to turn the power supply all the way down and only turn it up to the point where you need it for proper operation. You will get the most life out of the led if you start low.
Posted by Randy Stankey (Member # 64) on 12-11-2000, 01:28 AM:
Well, they aren't really THAT bad, except you have to take the flywheel off and work from the back. Even that's not too bad. What I refer to mostly is that the trim pot to adjust the LED once it's been installed is jammed in the corner of the housing where you can't get to it except by reaching your tweeker in between the pulleys and belts. The Dolby LED is the worst. Of course you have to have the thing running when you do it. That's always made me cringe.Also, there have been a lot of times when I've gone to take off the wires and found that they were trapped between the bracket and projector frame when they were installed.
Posted by Brad Miller (Member # 2) on 12-11-2000, 03:45 AM:
If you are working with platters, remove the pulley for the take-up belt to get better access to the SRD led's power supply adjustment. 
Posted by Bill Purdy (Member # 202) on 12-11-2000, 09:31 AM:
Francis,
If you are getting 9000 hours for digital you are normal. Dolby's original expectations turned out to be a little optimistic, particularly with some batches of LEDs. Dolby is now testing them more thoroughly and the newer lots are holding up better. There are other things you can do to extend their life. Run them at the lowest current you can and still get adequate video level. Many folks start them out at a high current so that they don't have to fool with them. Switch them off when not needed. You have to be a bit careful about when you turn them On so that you have stable signal when you are ready for it. We are all still in a learning mode with these little jewels, so new ideas may well come along. There is work being done so that they can run at an even lower current, but that will affect mostly new installs.
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Bill Purdy
Component Engineering
Posted by Steve Guttag (Member # 268) on 12-11-2000, 09:20 PM:
...just getting back from changing out a pair of red LEDs.I've now changed out a few of the red LEDs on the CE system...piece of cake....with the right test gear.
Bill, when do you think Component Engineering will have these new faster lenses (ie need less light) for their analog and digital readers? I hear others already have them and Ioan was talking about them at the P-3 meeting.
Steve
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"Old projectionists never die, they just changeover!"
Posted by Bill Purdy (Member # 202) on 12-12-2000, 10:14 AM:
The faster lenses you hear most about are for digital where the need is the greatest. The trouble is that they are significantly more expensive, even in larger quantities. The other thing that is coming will be a more sensitive CCD which we all will have when Dolby gets them into production. This is why I said that these changes won't mean much to current readers. How many owners will spend a lot of dollars to upgrade a system which won't perform any better, just save them a few dollars on LED replacements?The lenses we are currently using are already a full stop faster than the ones we started out with, but we haven't seen the need to make a big noise about it. As with so many other things, it is all about economics.
We currently recommend an initial current of about 150 mA for analog and about 350 to 400 mA for digital. The LEDs we are getting these days will give you a lot of hours at those levels, with room to go up as they age.
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Bill Purdy
Component Engineering
Posted by Steve Guttag (Member # 268) on 12-12-2000, 01:21 PM:
Thanks for the info Bill, I'm looking forward to the new CCDs then....As always, I am very satisfied with the CE readers (analog and digital). Even ASR-40 went in fast and easy.Steve
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"Old projectionists never die, they just changeover!"