Film-Tech Forum ARCHIVE: Mysterious Sound Dropout

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Posted by Martin McCaffery (Member # 37) on 10-05-2010, 08:32 PM:

Dolby 650, Kelmar basement readers.

Film has a damaged digital track, so reverted to SR

About 10 mins into the film the sound looses most of the volume and most of the depth. Sounds like it is running through the Center horn only.

All amps are lit and indicating on. The sound was bad in the monitor, so not a speaker problem. 650 meter shows all channels are working, and we are getting faint sound out of the LR on the monitor, though had to boost the volume to 9+. After about 10 mins, sound pops back on and all is well (so far).

We've been running the print since Saturday and haven't had this problem. Any suggestions as to where to look for a cause, and possible tests to replicate it?

Posted by Geena Phillips (Member # 3726) on 10-05-2010, 09:15 PM:

What kind of damage does the film have? If it's sprocket damage, it's possible that the film was thrown slightly off the path. That could also explain why it's not happening every time.

Posted by Sam D. Chavez (Member # 1841) on 10-05-2010, 09:27 PM:

I would guess the LED current is dropping from a bad power supply perhaps so your Dolby Level is no longer anywhere near correct. This would cause the Noise reduction to mistrack severely and explain the weird sound.

I would run a loop before the show and set the level and again at the next opportunity after the show without switching off the reader LED. Just a guess though.

Posted by Martin McCaffery (Member # 37) on 10-05-2010, 10:06 PM:

Not sprocket damage, just scratching and worn Dolby track.

Would the bad power supply just go out suddenly then come back on?

Posted by Marty Carr (Member # 5267) on 10-05-2010, 11:11 PM:

Earlier Kelmar power supplies have a problem of over heating at the plug connection. This will cause the problem your are having. i have changed out a lot of these boards over the years. Open up the power supply and you can see the darkness around the plug.

Posted by Gordon McLeod (Member # 33) on 10-06-2010, 01:09 AM:

I have had that issue with all the kelmar supply's

Posted by Manny Knowles (Member # 1171) on 10-06-2010, 09:34 AM:

So have I. The soldering came undone where the connector is attached to the board.

Posted by Martin McCaffery (Member # 37) on 10-06-2010, 01:14 PM:

No visible signs of burning anywhere I can see without removing the unit.

I'll keep an eye on it and see if it happens again.

Posted by Manny Knowles (Member # 1171) on 10-06-2010, 01:27 PM:

They didn't "look" burnt, but the soldered connection became intermittent and the levels were way off.

Here's a blast from the past:

Blast from the Past

Posted by Louis Bornwasser (Member # 3063) on 10-06-2010, 06:20 PM:

They are right. The power supply that goes intermittent supplies the preamp board on the basement reader. You won't know you have this problem if the digital is running. Likewisr both LEDs look OK. Louis