Film-Tech Forum ARCHIVE: Reverse Scan Reader Gain

This is topic Reverse Scan Reader Gain in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Richard May (Member # 2627) on 11-17-2009, 03:55 PM:

Do all reverse scan readers have an adjustable high/low gain setting?

Posted by Gordon McLeod (Member # 33) on 11-17-2009, 04:22 PM:

Most have a jumper you cut to increase the gain of each channel

Posted by Demetris Thoupis (Member # 833) on 11-17-2009, 04:40 PM:

and the reason for wanting this?

Posted by Richard May (Member # 2627) on 11-17-2009, 05:11 PM:

We are going through LED's too quickly at one place.

Posted by Demetris Thoupis (Member # 833) on 11-17-2009, 05:57 PM:

It would help to write the model of the reader and projector head or soundhead. Also a brief description of the set-up. Average life-span of LED is good well 5 years.

Posted by Richard Fowler (Member # 893) on 11-17-2009, 07:30 PM:

You may also have a bad reader board or solar cell....a lens problem...or power issues to the LED.

Posted by John Walsh (Member # 168) on 11-17-2009, 07:36 PM:

My experience is they last less; about 3 years. If the LED power supplies could be wired to the motor power, they might (depending on usage) last longer since they would not be on all the time. If the heat sink compound is not applied when replacing an LED, it might overheat, reducing it's life.

Posted by Mark Gulbrandsen (Member # 72) on 11-17-2009, 09:48 PM:

I typically get 5 to 7 years out of LEDs and here's why...

What Dolby reccomended a number of years ago to prolong LED life is to turn the preamp gain almost all the way up and set the LED current(or voltage depending on make of supply) down to get to Dolby Level then slightly balance out one or both channels as needed to get exact Dolby level. I've done this for years with systems that don't have excess noise in the preamp and it greatly prolongs LED life span. Also make sure that your power supply has been modified if needed. Some older Component Engineering and Kelmar units that were made for IR leds can not be turned down low enough and you'll have extremely short LED life span as a result. I reccomend checking the current through the LED to make sure it's at a reasonable level and the power supply is functioning properly. Also consider switching on and off all reverse scam power supplies in any complex. Since there is typically a delay before going to analog this is generally not a problem. You can sometimes use the generally unused exciter contacts in the automation to do this. The power supply can also be wired across the motor circuit. What ever is easier and meets local codes.

Hope some of this helps....

Mark

Posted by Monte L Fullmer (Member # 2797) on 11-17-2009, 10:14 PM:

Sadly, BACP units don't have this availability with adjustable LED current (there must be a reason why of this against other units that do have this..).

I just did a BACP LED replacement and reconnected the gain jumpers on the preamp card so I can have more headroom with the preamps levels since they were wide open and I couldn't get Dolby level prior to the install .. and I was getting some decent noise from the wide open preamp card on the CP55 unit.

But the original LED has to be a good 6 yrs old before starting to fade..which I think is a good enough life span.

Now, just need to figure out to be able to switch off the LED while machine is at rest and comes on when power to motor is enacted. This will buy tonnage on LED duration.

-Monte

Posted by Sam D. Chavez (Member # 1841) on 11-18-2009, 12:04 AM:

One reason BACP did not make the LED current adjustable is due the tendency of many techs to reach for the adjustment pot instead of looking at mechanical alignment to make sure you have max light before looking elsewhere.

So say you have an LED way out of alignment. You run the current all the way up to make Dolby level and exacerbate the LED wear issue by running the LED at 300 mA instead of the 150 mA we decided was the best compromise.

Mind you, we made sure there was 8 dB over Dolby level on a properly aligned reader with a new LED. This was intended to make the LED last 5 years more or less after cutting the gain jumpers at about 3 years. It has proven to be about right after 11 years out there in the field.

Posted by Monte L Fullmer (Member # 2797) on 11-18-2009, 12:48 AM:

quote: Sam D. Chavez
looking at mechanical alignment to make sure you have max light before looking elsewhere.

.. like loose holding screws, azimuth adjustments, oil-filled lens tubes, et.al.

quote: Sam D. Chavez
we made sure there was 8 dB over Dolby level on a properly aligned reader with a new LED

Fully agree here. After what I did with the reinstall and got everything back to square with the 'o-scope', I had to really turn down the trimpots on the preamp card just to get the proper dolby tone levels.

thx-Monte

Posted by Demetris Thoupis (Member # 833) on 11-18-2009, 01:26 AM:

I've never had any problems with the gain on the BACP. And been running for years now with no problems.
Demetris