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Trying to repair some clamp meters

Discussion in 'Repairing Electronics' started by BGAmodz, Sep 25, 2013.

  1. BGAmodz

    BGAmodz Member

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    Hi everyone .

    So some days ago i broke one clamp meter due to a high voltage measurement that my device could not handle .

    Now after getting a new one , the responsible of the products warranty sold me 4 clamp meters with a nice price , those ones are similar to the other but they are bigger in size and handle very high current ( 2500 A ) , but they are not working .

    So i just decided to try my luck and see if i can get at least 2 of them fixed up and sell them to buy an ESR meter , i have 4 clamps ; 3 of them boot but some functions don't work .
    1 measures everything but AC voltage ( iso tech )
    2 cant measure anything at all except the buzzer
    - the last one don't start at all .

    After analyzing the one with AC voltage dysfunction , i checked all the caps , transistors , resistors , diodes / zener diodes , and they all seem working fine , no shorts or anything abnormal .
    Now things get serious in checking 6 ICs that i suspect at least one of them is responsible for the AC voltage measurement problem .

    Here is a list :
    NJM022B : dual low power operational amplifier
    TS27L2C : Precision very low power cmos dual amplifiers
    OP07 : Ultralow offset voltage operational amplifier
    AD736 : Low power , true RMS to DC converter
    AIC1578 : High efficiency step down DC/DC converter
    HEF4066B : quadruple bilateral switches
    This one is the main core processor it is labeled NJU 9210 : 3.3/4 digit sigle chip digital multimeter LSI , and am suspecting is the one responsible for the Clamp problem .
    A picture of the actual clamps model i got and i hope i get some opinions from you guys .

    Cheers
    [​IMG][​IMG]
     
    Last edited: Sep 25, 2013
  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  3. BGAmodz

    BGAmodz Member

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    What is the recommended AC signal i should apply to that AD736 ? and i guess i should apply it to the COM and IN pins ???
    [​IMG]
    I connected the power supply to it ' 9v ' battery , and still nothing , i will focus on this one later .
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Don't apply to the AD736. Use the normal meter inputs. You should see an AC signal at the input of the IC and a DC signal at the output. The voltage should be grossly attenuated when it hits the IC.

    I would suggest 24 VAC or less to the NORMAL METER AC input, just for safety sake.
     
    Last edited: Sep 25, 2013
  6. BGAmodz

    BGAmodz Member

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    Thanks for that , so this IC is responsible for the V-AC measurements ??
    I will try to apply this tings and see if i get a signal
     
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The AD736 is just a place to start without a schematic Probably because of the TRMS capability, the attenuator has to be designed differently. The AD736 takes an AC signal and converts it into an RMS DC value, so it's a good place to start when only the AC measurement doesn't work.

    Unless the OP amps are acting as comparators, looking at the difference between the inverting and non-inverting inputs would be something else I would look at. Check to see if the outputs are at a rail. The difference should be less than a few mV.

    We don't know what the analog switch does, but you could look create a table of the logic inputs and measuring mode, to see if their is correlation. It MIGHT route the converted AC signal to the DVM and the other signals. The 4066 is a relatively fragile device, if the supply rails are exceeded. Then you can compare the inputs and outputs using the truth table definitions. If the state changes are dynamic, then it becomes more difficult.

    Nothing is certain. You can only hope to find something wrong. Once you have one working, it can be come the basis to fix the others.

    One heard part is finding the "reference". The power pin for the AD736 looks like a good choice. you might try to figure out if this is connected to meter COMMON with an ohmmeter.
     
  8. BGAmodz

    BGAmodz Member

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    Thats my vision too , this IC converts AC signals to DC so they can be converted to digital data , so maybe this one is faulty .

    And indeed the meter's common is directly connected to the pin number 8 of the AD736 :

    [​IMG]
     
  9. BGAmodz

    BGAmodz Member

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    I think i will post pictures of the mother boards
     
  10. BGAmodz

    BGAmodz Member

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    Ok i set the meter to V AC and put under test a 24 V AC , now what are the pins i shood check in the AD736 IC ???
     
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  12. BGAmodz

    BGAmodz Member

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    Hi again and thanks ,
    I am trying to power up the meter but it seems the battery is getting low , but before i loose power i have had an AC signal of 5 VAC between COMM and IN pins , i will try to find out the output voltage after i get the meter back on the road .
     
  13. BGAmodz

    BGAmodz Member

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    Do you have an idea on how to simulate this IC on a simulation software , i have ISIS
     
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    What would be the point? - you've already been told what to do, just do it - 'playing' with a simulator isn't going to help you in any way.
     
  15. BGAmodz

    BGAmodz Member

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    Well to see how it works out of circuit , i cant get a clear idea on the IC when i simulate it ..
     
    Last edited: Sep 27, 2013
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    There really isn't any point of simulation. We are trying to answer two questions:
    1) is an AC voltage getting to the IC?
    If it is getting to the input, then is it converting it to DC
    If it's not getting to the input, then who knows where to look.

    The block diagram of the IC is really all you need. It only has one needed capacitor and one optional one.
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    As we've already said, no need - and no point.
     
  18. BGAmodz

    BGAmodz Member

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    OK , i checked the IC input its reading is 5 Volts AC , now for the output should i check the voltage between the pin 6 and COMM ???
     
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Yes, that should be a DC voltage, scaled appropriately. It might read 0.5 VDC which is just a guess.
     
  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You also need to vary the AC input, and conform the DC output varies accordingly.
     
  21. BGAmodz

    BGAmodz Member

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    Hi again am back with results of the tests , i have checked 3 voltages 24 VAC ; 220 VAC and 6 VAC , this is what i get on the AD736 input and output :
    24 V-AC measurement : Input : 4.3 V-AC ; V+ :4.3 V-DC
    6 V-AC measurement : Input : 1.7 V-ac ; V+: 5.3 V.dc
    220 V-AC measurement : Input : 6 V-ac ; V+: 5.3 V.dc
     
    Last edited: Sep 29, 2013

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