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Thread: DAC signals ????

  1. #1
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    Default DAC signals ????

    I finally got all the wiring done and was in process of checking it all over once again to find all ready for power on. Everything is working as it should , but the green blanking signal seems stuck on outputting 2.16 volts all the time. So I checked a scanned image and sure enough....the trace lines were there.Dim but there. I have looked for the ground loop and other problems to no avail. Almost ready to buy another DAC....and can't afford to smoke credit cards any more......but I am so close to the finish. Any suggestions as where to look from here?
    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

  2. #2
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    Which dac do you have?

    Are you looking with a CRO, as a multimeter is probably going to distort the value
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  3. #3
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    From our conversations earlier: Blown TL074. Should be an easy solder job.

  4. #4
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    Quote Originally Posted by dave View Post
    Which dac do you have?

    Are you looking with a CRO, as a multimeter is probably going to distort the value
    It is a constant voltage on the easylase DAC with or without a signal input.
    Thanks to Spec I am searching for the op-amp to replace it. Thanks again for the help and I hope this will complete the repair.
    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

  5. #5
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    Quote Originally Posted by MechEng3 View Post
    It is a constant voltage on the easylase DAC with or without a signal input.
    You can't check with a volt meter you need to check it with a scope.

    Jim

  6. #6
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    Unhappy

    It has been checked several times, all 3 signals, with a DVOM. It is not a cheap unit either. It is ESD protected as well with a third lead. I have started to out think myself on this one. Chime in at any time.......since one of the three dac chips is working with a single supply signal, the negative offsets -if any from input- would be clamped to ground yielding no out put or 0 volts, Right? So then only pos. signals are outputted if they are above threshold of the chipset. Could there be an issue with the address decoder within the chip? I have no way to safely varify this ....as for now I am going to assume , on Spec's resolve, that a replacement of the 074c op-amp needs to be performed. It controls the final output from the dac chip itself. Still wondering and double checking.............
    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

  7. #7
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    You can measure with a voltmeter, even XY outputs. That "no voltmeter" thing is just a rule of thumb that you can safely ignore as long as the signal isn't changing and it isn't driving a capacitive load. I just make a test frame with a single white point in the upper right corner. The problem should be relatively straightforward to track down. If your opamp is indeed broken, you can deadbug a surfacemount TL071 or TL072 on top of the opamp with the broken channel and wire it in.

    http://www.digikey.com/scripts/us/dk...=84584&Site=US

  8. #8
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    When I was testing my homebuilt DACs, I made a test frame with a dot that slowly went around all 4 corners of the frame (this is in popelscan). I used that with a DMM and a logic probe to make sure both DAC channels were getting digital signals and that their outputs were correctly going from positive to negative. After that, it was simple to adjust the range & offset pots to make sure they both went +/- 5v

  9. #9
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    That's a good idea, we should make a "Photonlexicon" standardized ilda file for testing with multimeters and the like. I would have loved to had access to it when I started out...

    Maybe a sequence of frames which can be played continuous and slowly goes through the colors then moves through the corners and centers itself then repeats?

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