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Topic: DTS reader light blinks irregularly

Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-07-2005 08:01 PM

quote: Monte L Fullmer
turn up that small trim pot up a bit with a VOM across the LED leads to get your 4volt adjustment range.

This is done without film threaded through the reader!! Its not adjustig the LED per say its a gain adjustment for the preamp that you're actually tweeking. This adjustment compensates for LED fading.

Mark

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Frank Dubrois
Jedi Master Film Handler

Posts: 896
From: Cleveland, OH
Registered: Mar 2005

posted 08-07-2005 08:29 PM

Since were on the voltage topic, I noticed the last time I adjusted the voltage, the highest it would go is about 3.5, thats maxed out and with out the film running. When the film was running through, they drop to about 1.1. Is that normal? This is all the readers, not just one particular unit.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-07-2005 08:36 PM

Then the LED is at the end of its life span. Both Gord Mc and myself are about to come up with an LED replacement for this reader. The number of 200.00 plus reader exchanges that I do for the sake of a 1.00 LED is approaching the rediclous point. Up north of the border the extra amount for duties and such are on top of that making it all the more rediclous.

Mark

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Frank Dubrois
Jedi Master Film Handler

Posts: 896
From: Cleveland, OH
Registered: Mar 2005

posted 08-07-2005 08:57 PM

I have to agree. Replacing the whole reader for the LED is just a poor design. Is replacing the LED something any semi-kowledgable projectionist can do, or will the unit still have to be calibrated/adjusted.

Also, about my last post, is it not normal for the voltage to drop to about 1 when the movie IS playing? Our units are about 5 years old.

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Bruce Hansen
Jedi Master Film Handler

Posts: 847
From: Stone Mountain, GA, USA
Registered: Dec 1999

posted 08-07-2005 09:15 PM

quote: Frank Dubrois
the highest it would go is about 3.5, thats maxed out and with out the film running

There must not be film in the reader when the adjustment is done. TP1 is on the reader side (after the first op amp), so if there is film in the reader, or dirt on the LED or lens, it will effect the reading. Also shining a light into the reader will effect the reading. When there is film moving through the reader, you will find square waves on TP1. A DC meter reading of the square waves on TP1 is meaningless.

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Frank Dubrois
Jedi Master Film Handler

Posts: 896
From: Cleveland, OH
Registered: Mar 2005

posted 08-07-2005 09:23 PM

Our lenses are clean. With no dirt, no film in the reader, I should be able to turn the voltage up PAST 4 without maxing the POT out. 2 of our units seem ok...the others get maxed out at 3.5....I was told that 3.5 is still fine. When the units were installed, the installer set them all to 2.0 Why? I know the LED gets dim over time. Does a new LED only need 2 volts where an older LED needs more juice to get the job done? Curious...

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 08-07-2005 09:31 PM

quote: Monte L Fullmer
I would then realign the scanner roller a bit and it would track back into play by taking off that seal sticker, and tweeking the pivot screw. Sometimes, you have to do that even though the sticker mentions factory alignment.

If you had good luck with this, OK, but it is definitely not something I would recommend for general practice. There is a reason for the sticker being where it is. To really align the reader properly, you need a special test film which, unfortunately, can not be bought. However, the reader dealigns itself rarely, I would say. But I wouldn't recommend people in general to peel the sticker off and play around with the reader.#
Since there is no error rate display (I think the DTS-XD10 has something like that, but I haven't worked much with that unit yet) on most DTS units in the field, there are too many imprecise variables to troubleshoot the readers and systems when you have bad timecode situations. Having people play around with them without proper testing equipment is about the last thing I would want...

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Richard May
Phenomenal Film Handler

Posts: 1057
From: Floral Park, NY USA
Registered: Aug 2004

posted 08-07-2005 09:37 PM

Thanks for all your comments about this problem. One thing I think everyone is missing here is the fact that 90% of the time, the light is perfectly steady and super bright. If it was a low LED, I would think that it would be blinking more often and not as bright.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-08-2005 02:02 AM

quote:
I was told that 3.5 is still fine. When the units were installed, the installer set them all to 2.0 Why? I know the LED gets dim over time. Does a new LED only need 2 volts where an older LED needs more juice to get the job done? Curious...

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Franbk,
Again, you are not adjusting the LED voltage at all. The LED voltage remains constant thourghout the LED's life span but it does fade. Its the preamp gain that you are adjusting with the pot. 2.0 is dead wrong and your tech ought to be shot. Re-adjustment of the gain pot required more often and the processor might not get good time code on a dense print. Get a different tech!

Mark

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Bruce Hansen
Jedi Master Film Handler

Posts: 847
From: Stone Mountain, GA, USA
Registered: Dec 1999

posted 08-08-2005 05:12 PM

OK, here is a question for Mark, and everyone else here. I have not heard anything about LEDs losing intensity over time, but than again, what manufacture would want to talk about something like this? They are solid state devices, they either work, or they don't. The multi element LEDs used for Dolby Digital and optical readers can lose over all intensity when indivual LED elements die (if for no other reason than they are run quite "hot"), but the LED elements either work or they don't. I did a google search on this subject, and the only thing I could find was about how LED flashlights lose intensity as the batteries die (dah). So, does anyone have any information or data on LED intensity over time?

Doing some quick math, there is only about 8ma flowing through the red LED in a DTS reader. The red LED and it's current limiting resistor (R15 360 ohm) are directly across the 5 volt supply to the reader. Say there is a 2 volt drop across the LED (some what normal for a red LED), there would be 3 volts across the resistor. By ohms law: 3v/360ohms = 8.333ma. This would be will below the maxim rated current for the LED, and it would not get hot (the major cause for falure of solid state devices).

So, if the LED is not changing intensity over time, what is the cause of the problem with the reader not being able to produce that 4 volt reading at TP1? Is the LED getting skuffed, and the light is being defused? Is the alignment changing? Is the solar cell losing sensitivy over time, or just getting dirty (too many farts in the booth)? The MAX474 dual op amp is also a solid state device, and should either work, or not work (me thinks). One of the major problems with electronics today is electrolitic caps losing the moisture in the electrolite over time, and losing "value". There are two of these caps before TP1 (C3 and C8). I could see C8 losing value, causing a loss of level. It would be interesting to measure the light output of the LEDs in a "good" and "bad" reader. I have a feeling that DTS knows the answer, but is not going to tell us. So let's put our heads together, and figure this out.

Another thing to think about, since the LED is run well below it's max rating, could you change the value of R15 to allow more current to flow through the LED, and produce more light (if you cannot produce that 4 volt reading at TP1? If you put a 470 ohm reistor across the 360 ohm resistor that would give you about 15ma through the LED, and more light out of the LED. Could this keep your reader working for a little longer? LEDs of this type are usually rated for 20ma or more.

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Frank Dubrois
Jedi Master Film Handler

Posts: 896
From: Cleveland, OH
Registered: Mar 2005

posted 08-08-2005 06:10 PM

Its a good question, I'd like to hear a good answer. I, personally, dont think that boosting the power would be a great idea. I would think that it could have some effect on the sound quality. I'll be watching this. Although the voltage reading in our readers is decreasing, the LED looks as intense as it did on day 1. Also, when turning the voltage up/down, the apparent intensity of the LED doesnt seem to change. The movie will no long play in DTS, however, the LED is still on and bright. Hmmmm....this is an interesting topic. Can't wait for an answer!

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Bruce Hansen
Jedi Master Film Handler

Posts: 847
From: Stone Mountain, GA, USA
Registered: Dec 1999

posted 08-08-2005 06:28 PM

quote:
I, personally, don't think that boosting the power would be a great idea. I would think that it could have some effect on the sound quality.

The reader is just reading time code. Allowing more current to flow through the LED would not effect sound quality. The audio is stored on the CD ROMS.

quote:
Also, when turning the voltage up/down, the apparent intensity of the LED doesn't seem to change

The adjustment (R16) does not change the voltage (or current flow) through the LED, it adjusts the gain of the reader side of the time code reader. LEDs are current operated deivces, not voltage operated. They are diodes (the D in LED), and are therefore avalanche mode devices. Once the P/N junction brake over voltage is exceeded, an LED will go into very heavy conduction, if the current is not limited, the LED will burn out right away. That is why there is a current limiting resistor in series with LEDs, in this case R15.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 08-08-2005 07:03 PM

LEDs do indeed loose intensity over time. Who ever said they are so stable as to just work, or not?

If you ever saw a Dolby multi segement LED loose a segment, you'd know it...first off it would look odd and second off...the mistracking on the 2:4 decoder would be quite dramatic.

I had much less problems with my DTS readers fading than my Dolby Digital or analog readers.

You are correct though...the harder your run them (higher current), the more they deteriorate. Note too, this is not as predictable as you might think. I've had some that got Brighter over time...some last a real long time...others that just seem to start going downhill from the word go.

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 08-08-2005 07:38 PM

Can that be cause by impurities in the doting?

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-08-2005 08:01 PM

quote: Michael Schaffer
Can that be cause by impurities in the doting?

Michael,

I think you mean impurities in the doping stage. Doping is just one stage of making a semicondictor......

One problem I've noticed is that the LED on the DTS reader is nopt "thermally connected" to its mount(or heat sink) and I believe this is one reason why I've seen some of these units LED's decay so quickly. On my last service trip through Montana the reader at West Yellowstone wouldn't even make 3.6 volts and it had a 2003 date on it. Now that theatre only runs one show a day there too! Ditto for one of two readers that had to be replaced in Idaho Falls. The other one was still of the non-adjustable type and had also quit reading time code reliably. I probably see on average anywhere from 3 to 6 readers a month that our customers have had to pay in excess of 200.00 each for when all that was wrong was a faded out $2.00 LED. The labor to replace it is about 1 to 1.5 hour shop time or in our case $65.00 an hour plus the $2.00 LED. Its pretty clear that DTS is taking ALL of us for a big ride on this simple and what should be a really inexpensive to repair problem.

quote: Bruce Hansen
Another thing to think about, since the LED is run well below it's max rating, could you change the value of R15 to allow more current to flow through the LED, and produce more light (if you cannot produce that 4 volt reading at TP1? If you put a 470 ohm reistor across the 360 ohm resistor that would give you about 15ma through the LED, and more light out of the LED. Could this keep your reader working for a little longer? LEDs of this type are usually rated for 20ma or more.

Bruce,

You're exactly correct and this is a big defficiency in that reader. Its obvious that it was kept as inexpensively as possible to produce and you could certainly change the resistor or insert a small pot in there to keep the led useful until the end of its life span. This is exactly what we do with the 24 segment Dolby LED's!! However I am not out to change DTS's circuitry but just make LED replacement inexpensive and very feesable to do at least in the shop.

Mark

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