Film-Tech Forum ARCHIVE: Cat 701 Dolby reader LED voltage test points?

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Posted by Frank Dubrois (Member # 3042) on 05-06-2005, 11:10 PM:

I have looked through the manuals and other diagrams but cant find any test points to test the voltage that the LED is operating at. I know you can hook up an oscilloscope, however I dont have one, nor does the theater. Yhe LED replacement guide references test points for connecting the oscilloscope and I was wondering if I could hook up a simple multimeter the same way? Thanks.

Posted by Rick Long Jr (Member # 541) on 05-07-2005, 06:39 AM:

The test points are not in the reader. They're in the processor. A scope is required not only to show the amplitude, but to ensure the light source is positioned properly for a nice flat trace. Its all covered in the DA-20 manual. Don't use a meter.

If you have a CP 650 the latest software revision will report video strength on boot-up, as well as in the setup software.

Posted by Frank Dubrois (Member # 3042) on 05-07-2005, 10:57 AM:

We have a CP500

Posted by Bruce Hansen (Member # 281) on 05-07-2005, 11:35 AM:

LEDs are current opperated devices. Measuring the voltage across them will not tell you much. What is important is the amount of light, and that the light is hitting the correct place on the film. Therefore, you look at the reader, not the LED itself. In a DTS reader the test point is after the first op amp (the MAX474 is a dual op amp), but a DTS reader is only reading in one dimension, so a meter can be used to check the amount of light.

Posted by Mark Gulbrandsen (Member # 72) on 05-07-2005, 11:43 AM:

Follow the procedure in section 7.1 of the CP-500 manual.

Mark

Posted by Frank Dubrois (Member # 3042) on 05-07-2005, 01:40 PM:

I did follow that proceedure, and it did not affect the problem at all. The problem we seem to have is we are getting a consistantly high error rate with our films on this unit. It was suggested that possibly the LED wasn't as bright or strong as it once was and was unable to read the track as well. The entire projector and film path was cleaned and a new film (as well as an older film) were run through the house. We were getting error readings of 7, 7. and F. I know its possible to adjust the CCD board, however, it sounds like an oscilloscope is necessary to get it right.

Posted by Mark Gulbrandsen (Member # 72) on 05-07-2005, 02:48 PM:

Ya can't really tell what is going on without a scope(60mhz at least)! If you don't get the 4 volta peak to peak video level or there is a strange looking waveform instead of the mormal waveform then replace the LED. Age and running time is anoter good indicator. Has the 791 always been left on, or switched with the motor circuit? How old is it... 6 or 7 years of coutinous on operation and you definately need a new LED. A strange waveform would indicate missing segments and these are easu to spot just by eye.

Mark

Posted by Frank Dubrois (Member # 3042) on 05-07-2005, 02:55 PM:

Its about 6 years old. It is always on. I dont know how to work an oscilliscope, but even if I did, we dont have one.

Posted by Mark Gulbrandsen (Member # 72) on 05-07-2005, 05:49 PM:

With that much running time on th LED I would just relpace it.
You WILL need to get e tech in to do it though. Wire the 701's ac cord in across the motor to double the new led's life span.
Mark

Posted by Gordon McLeod (Member # 33) on 05-07-2005, 05:57 PM:

It wounds like it needs a complete alignment call in your service engineer

Posted by Frank Dubrois (Member # 3042) on 05-07-2005, 06:00 PM:

I wish "just replacing it" was an option. Unfortunately, unless the LED is completely out, it wont be allowed to be replaced. I was hoping to squeeze a little more life out of it.

Posted by Sam D. Chavez (Member # 1841) on 05-09-2005, 12:12 PM:

Frank,

You need to tell the owner or manager/dickhead that LEDs do not burn out, the light output just declines.

If they are waiting for the buldb to burn out, you won't be playing SR.D films for many years.

Posted by Mark Gulbrandsen (Member # 72) on 05-09-2005, 01:57 PM:

Actually its very possible and even probable that some of the segments are burned out. Explain to him that there are two rows of 12 LED's(correct me if I'm wrong Sam!) in there and thet you need all 24 leds working in order for the thing to work properly at all.

I've lately had droves of LED's with bad segments..... all were at least 6 years old. My god.... time does fly!

Mark

Posted by Frank Dubrois (Member # 3042) on 05-09-2005, 09:16 PM:

I know about the LEDS. I'm not sure if there were 4 rows of 6 or 4 rows of 8, but I know of them. On the unit in question, the LEDS were blinking all over the place...which I know is bad. I loosened the two hex screws on the LED head and adjusted it until the LEDS stopped blinking like crazy. We are getting all solid LEDS with an error count of 4 now....not perfect, but for a 6 year old LED, very acceptable.

Posted by Sam D. Chavez (Member # 1841) on 05-09-2005, 09:33 PM:

Frank,

I believe you are confusing the display LEDs in the cinema processor with the tiny segments in the 2 x 12 illumination LED on the reader itself that Mark alluded to.

The Reader LEDs do not blink. If they do, you've got a serious problem.

I know you are trying, but you have a lot to learn.
Did the manager authorize you to tinker with the reader?

No good deed goes unpunished you know. You should consider Dolby school.

Posted by Richard Hamilton (Member # 321) on 05-09-2005, 09:41 PM:

Frank,
Sam and Mark are talking about the L.E.D. in the soundhead, not the processor.

Posted by Frank Dubrois (Member # 3042) on 05-09-2005, 10:30 PM:

I know what LED they were referring to...and no it doesnt blink, i didnt think it could. As far as the tinkering goes, yeah, she knows. It was dropping into SR, now the error rate is around 4 and doesnt drop into SR...what was wrong with what I did?

Posted by Mike Pennell (Member # 1692) on 05-09-2005, 10:39 PM:

Frank they're just trying to caution you, the equipment is very fragile. Also many technicians get upset when people tinker with the machines without their knowledge, and in the past, has led to many a person being fired. Problems sometimes beget more problems if you are not sure what you are doing,even with the best of intentions. [beer]

Posted by Frank Dubrois (Member # 3042) on 05-09-2005, 11:17 PM:

I hear ya...