quote: Eric Robinson

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Other than a noticeable lower light output level, are there any other artifacts introduced in this scenerio. Distortion, loss ???

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You're correct but there is a little more to it. Add in drastically increased distortion and alot of phase shift. Most decoders cannot deal with this sort of incomming signal problem and they tend to end up doing some very strange things. you might be able to squeek by with a simlge segment failed till you can get back to replace the LED... perhasp a day or so but usually by then the intensity has also petered out for the most part. Any more than one element out and immediate replacement should be done. This is but one very good reason for installing BACP readers... Although I have yet to change even one of their LED's it can actually be changed while the film is running if need be! Levels can then be set when the show is over and the LED has at least had some time to settle in.

Mark
 

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Posted by Eric Robinson (Member # 2966) on 07-26-2006, 10:06 AM:
 
That's interesting to hear that there are artifacts such as distortion and phase shift. Correct me if I'm wrong, but doesn't Dolby logic decoding base its decoding scheme on the phase relationship of the left and right audio channel. If so, it would seem that phase distortion would cause the decoding scheme to produce unusual results. Is this true?

Also, it is somewhat easy to diagnose a failing digital LED by viewing the video signal on a scope. In the case of the analog LED; however, I'm unaware of any way to view its output performance other than the naked eye. Any suggestions?
 

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Posted by Mark Gulbrandsen (Member # 72) on 07-26-2006, 01:00 PM:
 
Best thing to do is check the current draw on the led. This is the best way to determine the end of the LED's life. Looking at it with the naked eye is not a good thing. I use a layer of darak film over the led.

Mark
 

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Posted by John Hawkinson (Member # 1135) on 07-26-2006, 01:14 PM:
 
Well, for an analog LED, I think the big issue is relative gain between the left and right channels. So as long as you run 69T and set your preamp gains appropriately, you are probably just fine.

If one channel is radically different from the other (e.g. several pot turns), then there's a good chance something is wrong (or needs cleaning).

--jhawk
 

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Posted by Steve Guttag (Member # 268) on 07-26-2006, 01:35 PM:
 
John, you are wrong...the LED in a typical reverse scan system is not as uniform as one would hope. If elements are out, then the uniformity of the light will be off. Merely raising the level based on Dolby Tone will not correct the problem since the light will remain inconsistant where the elements are bad.

Picture, if you will, a dark spot on your glasses...you eye may open its iris a bit more to let more light in...but you still have that dark spot as you look out. Now if you merely put a very weak sunglass over one lens such that your iris could balance things out, then that would be another story. In fact, it wouldn't surprise me at all to find that human eyes do not perceive light intensity identically within the same individual.
 

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Posted by Gordon McLeod (Member # 33) on 07-26-2006, 01:36 PM:
 
As the segments fail the eveness of track illumination gets worse causing level and pahse errors and distortion
 

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Posted by Kenneth Wuepper (Member # 1174) on 07-26-2006, 02:27 PM:
 
Does anyone use the uniform illumination test film "snake Track"?

That used to be a valuable tool for detecting sagging exciter lamp filaments and also crystallized filament wires.

I would think that this test would be equally useful in determining dead or dim elements in the LED array.

KEN
 

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Posted by Sam D. Chavez (Member # 1841) on 07-27-2006, 09:04 AM:
 
The snake track has been rendered obsolete by the Dolby 566 illumination uniformity track. It uses 6 tracks alongside each other on octave centers to produce a bar graph display on an RTA. Way better and you can see changes an adjustment in real time.

[ 07-28-2006, 01:15 AM: Message edited by: Sam D. Chavez ]
 

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Posted by Kenneth Wuepper (Member # 1174) on 07-27-2006, 03:17 PM:
 
Thanks, Sam.

I knew that there must be a current better "CAT" way of catching the mouse.

KEN
 

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Posted by Frank Dubrois (Member # 3042) on 07-28-2006, 01:00 AM:
 
that 566 test film seems to be some handy stuff. About time I get some.

Mark,

Do you know offhand if NT sells that film? I'm a fan of their stuff since you last brought them up. Thanks
 

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Posted by Sam D. Chavez (Member # 1841) on 07-28-2006, 01:13 AM:
 
Dolby is the only manufacturer of 566. It's very cool stuff in the last days of analog.
 

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Posted by Louis Bornwasser (Member # 3063) on 07-28-2006, 08:33 AM:
 
Because of the way 566 is made it is fairly unlikely anyone else will make it soon. It is the outgrowth of the amazing, 6 channel discrete Dolby format proposed in the early 80's. Dual inventory requirements killed it. Louis
 

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Posted by Mark Gulbrandsen (Member # 72) on 07-28-2006, 09:10 AM:
 
Frank,
CLACO can supply you with any test film you need in any length or quantity. Call Vince at our office after 9am Mountain time to order. We tke VISA/MC.

Mark

Office 801-355-1250
Fax 801-355-1259
 

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Posted by Eric Robinson (Member # 2966) on 08-01-2006, 03:55 AM:
 
Now to follow up on Marks comment about BACP readers!

I have only replaced Dolby LEDs so far, can you, (Mark), explain the method of LED replacement for the BACP readers? and why it is so easy compared to replacing an LED of any other brand?

Thanks...ERIC
 

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Posted by Charles Greenlee (Member # 3856) on 08-01-2006, 07:03 PM:
 
Also, where can one (person) get a BACP reader? And for how much (Simplex 5-star)? This is mostly for my own experimentation.
 

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