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Thread: Zygo 7702 Stabilized laser unable to lock mod.

  1. #1
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    Default Zygo 7702 Stabilized laser unable to lock mod.

    So I ended up with a Zygo 7702 dual freq head. I rigged up a power supply and it just would not lock eventually the service light coming on. Reading though the laserfaq they mentioned that one of the test points needed to swing though at least +/-7v as the tube does it's mode sweep and sure enough mine was sweeping about -4 to 8v. The tube has about 30k hours on it so it was just getting weak, still putting out about 400uw but just not enough.

    There was a mention about possibly adding an extra opamp to amplify the signal a bit to get over the threshold. There is a LF347 quad opamp on the board that amplifies the signal from the photodiodes so I figured there ought to be a way to modify that to get it back into range. Tracing back the test point I found the quarter that handled the diode and traced out the basic circuit, just a pretty generic inverting amplifier. So I traced out the feedback resistor which is R17 for those playing along at home which is a 1 Meg resistor. I popped that off with SMT resistors and added a 1Meg axial to the pad and then started adding resistance till I got it to sweep in the range it wanted, which turned out to be about 1.6Mohm. Eventually the thing finally locked!

    So, it would probably be easier to put in a small multi turn trimmer than do what I did, but it worked. So for reference to anyone else that gets one of these heads and the power is too weak to lock in, try this.

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  2. #2
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Get Sam's little USB photodetector measurement board and some surplus HP plane parallel interferometers on EBAY. Most fun I've had with HENEs in years.
    Congrats on the repair. I now love stabilized lasers!


    Steve
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  3. #3
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    Are you talking about this board?

    http://www.repairfaq.org/sam/uMD1/uMD1.htm

    Unfortunately that wont work with the Zygo since it is a 20mhz beat system. Unless you were thinking of something different?

    I do have a few of the Zygo 7006 plane interferometers though.

  4. #4
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    Which wonder, could you use a divide by 5 on the output of the ref and signal feedback from the sensors to get it down to the frequency that the uMD1 works at?

  5. #5
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Check your PMs.

    Steve
    Last edited by mixedgas; 05-05-2018 at 17:10.
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  6. #6
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    So I talked to Sam about this and he says the output from the Laser ref is ECL logic. I am buying a 7081 receiver from him too which I am assuming is ECL out as well. He thinks as long as I mix it down instead of dividing it I should be able to preserve accuracy while using his board. So I figure it is worth a try. Ill get a couple ECL to TTL IC's and find a schematic for a digital mixer. I have a couple little GPSDOs that could be a nice, stable reference clock.

    I have never messed around with ECL before so this ought to be interesting.

    I also have access to a ZMI-1000A board that will work with the receiver but it is VME bus and Zygo wont give me any information on it. I might be able to get ahold of a couple newer ZMI-2001A boards that will work too but again, VME.

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