Film-Tech Forum ARCHIVE: Analog sound problem...

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Posted by Tom Wienholt (Member # 1510) on 01-20-2007, 12:09 PM:

Equipment: Dolby CP-50 w/Cat280T's, Smart amplifiers, Kelmar reverse scan LED reader

Problem: Sound sometimes does not come on at the beginning of a show. Also, the sound will occasionally cut off for 10-20 seconds during a show. LED is visibly on, Amplifiers and Processor are on. Turing the LED off and then back on will sometimes solve the problem. I replaced the Cat108 preamp card in the processor but the problem continues. I also checked wiring to the processor and LED reader. Power rails on the backplane of the CP-50 were also checked and were at normal levels. I am confused as to what is causing the sound to cut off. Any help would be appreciated.

Posted by Sam D. Chavez (Member # 1841) on 01-20-2007, 12:39 PM:

Not much to go on here. Does it happen often enough for anybody to sit there and wait and watch?

he meters on the Cat. 109 would be a good place to watch or the signal presence lights on the Cat. 150 if it's new enough to have them. Either would at least tell you where and when the sound quits.

Any CP50 with 280's puts a strain on the power supply and can cause sporadic operation. There are newer versions of the supply around but they are getting very rare and it's considered impractical to spend much money on a very old (sob) processor.

You could try having the power supply rebuilt with new caps and diode bridges. Do you have a spare?

Posted by Louis Bornwasser (Member # 3063) on 01-20-2007, 01:08 PM:

The key here is that recycling the led helps some. We have found some Kelmar power supplies (older ones with a board with pin connections) develop a loose connection on the wires that feed the preamp powser supply. Although it could require replacing the board, so far, I have resoldered the output pins and the resistors near them and had no problem. These boards are off-the-shelf and Kelmar is not at fault. Louis

Posted by Tom Wienholt (Member # 1510) on 01-20-2007, 03:19 PM:

When the sounds cuts off, there is no signal to the 150 card (green LED's are not lit). Also, I have several spare power supplies and I tried replacing the power supply with no positive results. I suspect something is wrong with the LED reader.

Posted by Mark Gulbrandsen (Member # 72) on 01-20-2007, 05:47 PM:

I have had two intermittent solor cells in Kelmars in the past few years... and these were in the early Kelmars not in any recent units. I now carry an extra one in the van!!

Mark

Posted by Gordon McLeod (Member # 33) on 01-20-2007, 06:01 PM:

I have had several cell failures recently

Posted by Louis Bornwasser (Member # 3063) on 01-21-2007, 07:34 AM:

These were early Kelmar units also. I would not think the cell would be stressed or repaired by application of power; that is why I did not suggest that. The loose power connections would be so affected. Louis

Posted by Richard May (Member # 2627) on 01-21-2007, 07:43 AM:

Just a thought. I've had a few problems in the past with CP50's that have the DTS breakout board on back. Since that cuts off the analog signal, that may be something to check.......if it's on there.

Posted by Sam D. Chavez (Member # 1841) on 01-21-2007, 01:19 PM:

If the lights go out on the Cat. 150 when the audio is lost this means the problem is upstream of that. I assume the DTS board is downstream on the 150, no?

The next thing to try perhaps is running the CP50 in bypass for a while. If you lose the sound again, it is almost certain a reader problem.