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  1. #1
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    Jun 2008
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    1

    Problem with Maho Mh700 Y axis

    I am the owner of a Maho Mh700C with cnc control Philips 432-9 .

    I recently blew up a driver card (lm/lm drive )while i was repairing a bad ridden optical reader wire. I had the card repaired locally and some op-amps were replaced with some supposed equivalents (old ops were UA714 , new ones are OP07) . The apparent fault on the screen was repaired but however i still have a fault on the y axis . when the machine is powered the table is stable . However once i put the hydraulic on the table starts to go down slowly on its own until it reaches the end limit . We have checked the inputs on the y axis drive motor and have found that the reference voltage on the driver (1trm3 ports E1A/E1B) is approximately equal to 0.3Vdc .
    What should the value of the reference voltage be ? Can this be adjusted ? Are the replacement op-amps suitable for the card or are they causing this fault ?

  2. #2
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    Apr 2011
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    In my experience the reference voltage is 0V DC. Shifting to positive up to + 5V DC moves the axis to negative, and to - 5 V DC moves the axis to positive values.
    Since your fault started with replacing old OP AMPS and since those were working between differential "+" "0" "-" voltage by using a resistive circuit to balance the [0 V] on the "+" input of the OP AMP, it is very possible that the resistors of the voltage divider on your driver PCB card don't have the appropriate values and precision to keep the "+" input to 0 V, hence when the "-" input of the OP AMP gets signal and the comparison starts (negative reaction loop), the table moves as you described.
    Now you need to experiment by using a good multi turn shaft potentiometer or trimmer and a very precise multimeter to determine again the [0V] reference on the "+" input of the OP AMP that you changed.
    Note that this might not be the only problem, you might want to determine why the UA 741 OP AMPS got burned in the first place. Also there will be a speed problem that will come after you solve this one.

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