DMM not reading AC voltages !!!

Discussion in 'General Electronics Chat' started by LETITROLL, Oct 9, 2013.

  1. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    Hi everybody , and i want to thank you for the great electronics lectures available on this website .
    I am a 2nd year student in automation systems based on programmable automata and microcontrollers .
    So i have a Digital clamp meter that measures everything but the AC voltages .
    I have already posted my problem in another forum but didn't achieve much progress in the troubleshooting that was mainly based on guessing .
    The reason i am planning to fix my meter is that it is a high quality meter that measures high currents in both AC & DC , and it is about 300 dollars .
    What could be causing this problem ??
    [​IMG]

    list of ICs on the motherboard :

    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
    NJU 9210 : 3.3/4 digit sigle chip digital multimeter LSI
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,566
    2,379
    So this is a current and voltage meter?
    And it is reading everything but AC voltage?
    And like your other experiences, the answers here can only be based on guesswork, unless someone has experience exactly the same effect.
    I assume obviously for voltage you use probe lead inputs? Then it could be anything from switch contacts to the AC input circuitry, which in all probability, the first component is a rectifier set up of some kind?
    Max.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,566
    2,379
    A search turns that up as a clamp on current only?
    Did you mean, does not read AC current?
    My fluke meter reads strange results when the battery is low, and before the low battery indicator come on.
    Max.
     
  4. w2aew

    Member

    Jan 3, 2012
    219
    64
    According to this site:
    http://www.tes.com.tw/2000.htm
    It does AC and DC volts as well. Of course, you MUST use the probes to measure voltage. Were you using probes?
     
  5. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    Yes it is a current & voltage meter.
    and yes i use probes , they are on the lower part .

    In other forum , me and a member suspected that the problem could be caused by the microcontroller or the AD736 RMS to DC converter but this SMD card is hard to analyse due to circuit complexity , and the circuit schematic is not available .

    i checked all the resistors and found a bad one , replaced it but still same broblem .
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    and did you check the fuses?
     
  7. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    There is no fuses , only some protectors ( i don't know if they are termistors or varistors ) in series with the inputs of probes .
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,566
    2,379
    For AC and DC voltages, buy a simple VOM?
    I always use these as two separate devices anyway.
    Max.
     
  9. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    Yes but i don't see any reason to buy a VOM , the ones i got seems fine , am sure the problem is from IC or the microcontroller .

    You can take a look at my list of components exisiting on the meter's PCB .
     
  10. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    What type of circuit are you trying to measure? If the power source isn't supplying a clean sine wave it might have problems reading the rms voltage.
     
  11. #12

    Expert

    Nov 30, 2010
    16,345
    6,831
    There are some very sneaky fuses nowadays. They are often disguised as resistors. Check again.
     
  12. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    HI

    are those with 000 on them considered fuses ??
     
  13. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    well i just checked them and they seem acting normal with 0 ohms .

    any ideas on the ICs i posted ??
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,070
    3,841
    If a fuse or other protection device failed, and the device works for all other functions, then diagnosing it will be very difficult.


    Not that it will help with this problem, but hopefully a path forward...
    When I pay $300 for a volt meter, I expect that replacement parts, technical support, and/or a service center is available to fix it. Fluke, Agilent, Tektronix, are all good options. Otherwise, I try to spend $50 and treat it like a disposable item.

    This one seems to have similar functions as yours for $50 at Sears.

    http://www.sears.com/craftsman-digi...1x000001&kpid=03482369000&kispla=03482369000P
     
  15. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    This one was belonging to an retreated electricity engineer and i bought it off him for less than that price actually , the engineer have worked with it for almost 5 years , and was doing serious AC/DC measurement with it as you can see it can handle till 2500 A in both currents .
    So fixing it would be really beneficial .

    So the idea is if some people here are well experienced on multimeters or fault diagnosis , i can post pictures of the unit i got and we could single step through each suspicious part of it .
     
  16. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    I think this is a good question, and it should be answered. Let's make sure your version of ac is the same as the meters. You might be asking it to do something it cannot.
     
  17. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,061
    Seems fine, except it won't measure AC voltages. How is that "fine"?

    You have a complex and (maybe) broken piece of equipment that lacks a function you could replace for almost free with a VOM. Is it worth the time and effort to repair the meter? Maybe, if you enjoy the challenge.
     
  18. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    Well with the help of guys in this board , am sure we can achieve something
     
  19. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    I dindn't mean VOM ( voltmeter ), i meant MOV , like termistor or varistor , because some one told me to check fuses .
    . my bad
     
    Last edited: Oct 10, 2013
  20. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    Here is a simple schematic i made , i focused on the AD736 rms ic connections , witch i guess is responsible for AC signals .

    [​IMG]
     
    Last edited: Oct 10, 2013
Loading...