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Topic: Partial failure of reverse-scan analog LED

Christopher Seo
Jedi Master Film Handler

Posts: 530
From: Los Angeles, CA
Registered: Jun 99

posted 08-10-2002 01:42 PM

At the theatre yesterday, we got a complaint that "My Big Fat Greek Wedding" (optical SR only) was very soft. Cranking the CP650 fader from 6 to 7.5 seemed to bring the sound back up just fine with no other quality issues. I then discovered that the entire upper row of micro-LED elements in the analog LED were dead. Just have a few questions....

How common a problem is this? I have never noticed it before, but I was only recently made aware that the LED is actually composed of the 2 rows of (is it 11 or 12?) micro-LEDs.

I wasn't able to troubleshoot the problem, but I heard that a simple cycling of power to the power supply (Component Engineering) fixed it and the entire LED came back on. Could the power supply possibly affect only one row of micro-elements and not the other? In any case, would it still be prudent to replace the LED?

Just curious about these things. Also, is the LED for the Cat 701 reader identical to the analog one?

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 08-10-2002 02:04 PM

Oops! I reread your post and my answer did not fit.

But this one might fit.

Mark's post below has merit. The power supply may have "crowbared" itself and shut down. Upon re-boot of the power supply, they will come back up if shomething is not shorting the DC rail.

Keep an eye on it. When electronic switcher power supplies start to crowbar for no apparent reason, it is a warning sign of an impending failure.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-10-2002 02:04 PM

Yes, I ahev had several fail this way and I kow of a couple of other techs thathave had them fail the same way. Its probably safe to assume that with an array like this it will most likely not fail all at once. The only failure that way would probably be fomr a bad power supply. Its easy to tell if you use the illumination test film to check it out, and on a Cat 701 SRD reader you'll see big spikes in the video waveform dipping down into the signal incicating bad individual LED's in the array.
Mark

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Darryl Spicer
Film God

Posts: 3250
From: Lexington, KY, USA
Registered: Dec 2000

posted 08-11-2002 01:13 AM

wait a minute. Reading your post you are talking about the reverse scan LED analog readers in the soundhead correct. Not digital readers. All made by COmponent Engineering. Just clarifying this for others.

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Richard Fowler
Film God

Posts: 2392
From: Ft. Lauderdale, FL, USA
Registered: Jun 2001

posted 08-11-2002 07:33 AM

Most....if not all reverse scan analog and digital readers are using the LED-V chip as supplied by Dolby. It consists of two lines of multiple LEDs to create the slit like output. You can have failures of a few of the leds within the array due to age, power supply output problem. Dolby and the vendors have gone through a learning phase on how to power these ( I have 30 power supplies in our shop with "do not sell" since they are early versions ).
Richard Fowler
TVP-Theatre & Video Products www.tvpmiami.com

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