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Using a multimeter on a circuit board

Discussion in 'Solid State' started by CavScout, Nov 6, 2018.

  1. CavScout

    CavScout Well-Known Member

    Messages:
    566
    I am fairly new to working on electronics but I am learning.

    One challenge that I have had is testing the parts (caps, resistors, transistors etc).
    The top of the board is almost impossible to take readings from and the back of the board is an incredibly hard to follow map (especially when you can't position the receiver in a position for easy board access).
    Online schematics are faded and hard to read.
    Is there a trick I am missing here ?
     

     

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  2. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    Other people will have their own methods, but when I can't make out what's what on the solder side of the board, I try to stand the unit on its side and shine a pen light from the component side. That usually helps me get close enough to figure out what I'm looking at.
     
    Lavane likes this.
  3. elnaldo

    elnaldo Addicted Member

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    +1. Also, try to locate some hole, heat-sink, screw, whatever you can see from both sides of the board, and use it as a reference, then mark with a marker on the solder side the components you need to measure. Checking continuity ("beep") with the meter from one side of the board to the solder side can help to verify you are at the right component.

    Anyway, I don't see any trick you are missing. If schematic is not readable, board not accessible, not too much you can do. I'd remove the board if needed. Find the less invasive method. Sometimes you spend a lot of time to do some work without disassembling , and it takes longer than disassembling, work faster, and assemble again.
     
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  4. CavScout

    CavScout Well-Known Member

    Messages:
    566

    That is what I am thinking.
    I had a guy on this forum tell me once " you should never ever ever remove a board from the unit because ALL work can be done without doing so"
    I am thinking of clipping the wires, remove the board.......test.......fix.......resolder the clipped wires.......test unit
     
  5. elnaldo

    elnaldo Addicted Member

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    I think somebody saying or writing that sentence would be thinking or referring to a particular unit. Or he was kidding. Definitely not all work can be done without disassembling. And sometimes the hardest part of the work is just disassembling-assembling, perhaps to replace a $0.05 capacitor.
     
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  6. Bob

    Bob AK Subscriber Subscriber

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    couple of points

    1. take pictures. or rely on your memory (we had a thread here regarding just two very different diodes
    and it took 20+ threads, wide advice variation, educated guesses, and it worked)
    2. if you cut wires, put little paper tags on both ends. tape the group of wires if they all go to a spot
    on the PCB AND label the pcb similarly. better to do all this and simply desolder.
    3. mark all electrolytic caps by polarity on the PCB (ignore stenciling if different) -
    confirm with schematic. use something like red marker.

    if you really want a good guide, take a picture of the top using a high quality camera not a dumbphone.
    then take a picture of the PCB solder side. reverse the top side, match scale to bottom side
    and print both out. this tells what the top side looks like when viewing the bottom. I leave the
    small details to the execution.

    find a good schematic and match to PCB components, and parts list, - all must agree.
     
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  7. teal'c

    teal'c It's all moo Subscriber

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    Yup, experience. Practice makes less pain in the ass.
     
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  8. CavScout

    CavScout Well-Known Member

    Messages:
    566
    Here is what I am struggling with .
    A monster receiver that I have to mount on its side to work on and getting on both sides of the board is difficult.
    But cutting all those wires looks intimidating to reattach ( a sh*tload of soldering)
     

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  9. elnaldo

    elnaldo Addicted Member

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    Seems you have a very good access to the solder side. I think you can remove almost every component from that board with a little patience.

    What do you need to do on that board?

    I wouldn't cut all those wires.
     
  10. Lavane

    Lavane AK Subscriber Subscriber

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    Using a light can help a lot. Unless your working on the newer boards with double layered traces. It can be like trying to figure out a complicated freeway interchange from the air. :D
     
  11. merlynski

    merlynski Curmudgeon Electronicist Subscriber

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  12. satellite65

    satellite65 AK Subscriber Subscriber

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    If I have a service manual with board layouts, I print one regular and one mirror image.
     
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  13. Scott75s1974

    Scott75s1974 Active Member

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    Yes, I'd see about getting one of those LED work lights that are stacked around hardware store checkouts--the kind with a strip of LEDs. Could you slide one behind the board? Easier than holding a flashlight in your third and fourth hands while desoldering.

    That doesn't look like too complex a board. I often reach around the board and pinch the part and its lead on the bottom to locate. Then I mark the pads of the one part I'm working on with a black marker. Desolder, replace, repeat. Occasionally I desolder the wrong lead. Not big deal.
     
  14. Binkman

    Binkman AK Subscriber Subscriber

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    I also use the flash lite and get a ball park on transistors. They are easy to identify. then you get reference points. We posting to a newbie will agree that you just need to keep studying and identifying. You can work both ways.. output transistors back to power supply transformer then trace back to outputs.. red, black markers wash off with iso pro. alcohol. Make notes on printed crappy sm and take pics suggested. You can make your own wire identification by writing on masking tape to numbered board point.
    we're here to help!
     
  15. avionic

    avionic " Black Knights " Subscriber

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    Buy new manuals.
     
  16. CavScout

    CavScout Well-Known Member

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    Yeah...off Ebay...for like $30-$50 ...when they are available
     

     

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  17. avionic

    avionic " Black Knights " Subscriber

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    What are you working on?
     
  18. avionic

    avionic " Black Knights " Subscriber

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    Don't cut the wires. Unsolder the wire wrap pins.
     
  19. avionic

    avionic " Black Knights " Subscriber

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    Wire wrapping is a better connection than solder.
     
  20. Binkman

    Binkman AK Subscriber Subscriber

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    Above noted Q of model?.. but whats wrong with it? was it working?
     

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