Film-Tech Forum ARCHIVE: Help required Dolby Cat.150D

This is topic Help required Dolby Cat.150D in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Rob J. Buskop (Member # 3338) on 11-15-2012, 08:07 AM:

Have a Cat.150D on my bench that works well, with output on all channels, but the LED on channel 4 does'nt work.
Diode measuring through this led is normal.
Don't have a schematic but would like to sort this out.
Anybody with a schematic out there, or technical suggestion?
Thank you for your attention!
R.

Posted by Sam D. Chavez (Member # 1841) on 11-15-2012, 09:25 AM:

As I remember it has a current limiting resistor. I would leave well enough alone as there are some high impedance components you are likely to damage.

Dolby stopped publishing schematics years back and in any case that does little to tell you where the part actually sits.

Posted by Rob J. Buskop (Member # 3338) on 11-15-2012, 11:02 AM:

The basic set-up should be simple like rectifying the audio
signal at some stage, or possibly indirectly stimulating a VCA.
Should be rather easy to repair when you know the basic circuitry.
Anyway, thanks for your response, Sam!

Posted by Rob J. Buskop (Member # 3338) on 11-17-2012, 08:50 AM:

As it sometimes happens, the cure was relatively simple.
Took the boards apart and followed the signal path to the led.
Put my tweezers on the current reducer R to check and.... found one end of the resistor had'nt connected in the tin bath manufacturing process.
Problem solved.