Checking for shorts

Discussion in 'General Electronics Chat' started by mbohuntr, Apr 17, 2011.

  1. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Since R!f@@ is shy, :D I'll put this into it's own thread. How many ways are there to check for shorts on a board? The only way I can think of without lifting connections is to check continuity from rail to rail. I'm completely inexpierenced at troubleshooting, and this should benefit many of us noobs...
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Start by using your eyes. A lot of short-circuits on newly built projects may be due to stray bits of solder or other debris bridging between tracks. Look carefully, as even a tiny strand of wire or fleck of solder can cause a problem: a magnifier may help you to do this more easily.

    When using strip-board, strips may need to be cut to avoid shorts. Has this been done effectively?
     
  3. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks adjuster, I was thinking more about troubleshooting existing boards like the treadmill. I tried looking over a digital guitar amp that wasn't well, but that was WAY outta my league... I wanted to learn preliminary tests one would do before lifting leads.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Hey !! What gives :confused:. I am not shy...put 'em up.!!

    What are u up to ?
     
  5. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    There you are!!! I figured since you didn't respond to my treadmill question,I figured you musta lit out quicker than Happy seeing a camera... ;)
     
  6. R!f@@

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    you had a treadmill ?
    I did not know that..must have been too much in my head. I tend to 4get things now.
    Seems old age is finally catching up to me..

    So, what's up?
     
  7. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I was following the thread on the treadmill board resistor burning and when you mentioned to check for shorts, I was asking what methods were to be used. As stated, I have no troubleshooting expierence. Cmon, lemee into your head....:)
     
  8. R!f@@

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    Ok. Basically checking for shorts is meant to check out the power components.

    What I do is just put the my fluke in diode check and check the Bridge, any & all power diodes, MOSFET and transistors that I see. Just check for shorts in the circuit.
    It's pretty handy for me, since I can tell from the reading tht there is nothing wrong that would blow the fuse. If in doubt, I remove them the abnormal ones and check 'em.

    Pretty simple. It's a tech thing. Which you will learn if you have been doing repair work as long as I have.
     
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  9. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks, You diode check BJT's by probing the Base /Emitter? Without lifting leads? What about stuff like triacs , SCR's and caps?:confused:
     
  10. R!f@@

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    SCR and triacs too but they seldom develop are low gate resistance and does not switch on completely.. the best option is to measure the voltage levels or to replace them
     
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  11. Jotto

    Member

    Apr 1, 2011
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    Caps very seldom short out, they just have increased ESR and change value. You can check a low voltage SCR by triggering the component with either digital or analog meter (analog meter works best use X1 scale), hook your meter up to the anode (neg) and cathode (positive), then add a clip lead to the neg side of the meter and touch the gate. After touching the gate it should show deflection an stay in that state until release. There is a downfall to this test in that the meter doesn't have enough power to hold high voltage SCR's in the working state. I would use this test for the SCR of the PFN of a radar. Triacs read basically like a transistor, nothing more then AC switch. The only time I remove the transistors is when I can't tell if its acting properly or I am reading thru something else and want to be sure its not the problem, then I will remove the suspected side but not the others.

    Some units use earth ground, like a SMPS, and then regular ground for the rest of the unit. If you are looking at the primary of the power supply you use earth ground, the secondary will be regular ground. You must be careful if the unit uses two types of ground.

    If I am looking for a short with no noticeable damage done to the board, I hook up to the ground and look around at all the transistors and diodes for a short. Once I find the short, I do remove one side to isolate the components to locate the defective part. I don't do this very often since I can usually find the problem with my meter. Also a good indication of where the defective component is the board changing color due to excessive heat build up.

    I believe you can look in the ECG or NTE manual for checking out a SCR, which is also filled with lots of good information on testing components. I haven't gotten one in years so I am not sure how good they are now. But the older ones were great.
     
    Last edited: Apr 21, 2011
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  12. Adjuster

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    How capacitors are likely to fail depends on technology, age, and application. The rising ESR / falling value scenario probably does account for a lot of the faults with modern electrolytics, as well as with "self-healing" non electrolytics which may have suffered degradation due to voltage surges. The dead-short capacitor is not unknown though, and neither is excessive leakage current.

    Older paper dielectric capacitors are notorious for leaking. If you are ever involved with valve (electron tube) amplifiers you would have to beware of coupling capacitor leakage, which can lead to spectacular meltdowns in some cases. Circuit impedances and voltages are large, so a little bit of leakage can do a lot of harm. The power tube(s) can get turned on so hard that they burn out, possibly taking transformers and rectifiers with them.

    Another problem may be seen if a circuit containing electrolytics is switched on after not being used for a long time. Imagine you get a "bargain" of a second-hand bench power supply. You get it home, switch it on, and all seems to be well. After a while though, you hear a sizzling sound, and then suddenly KABOOM! A loud bang, smoke pours out, and the fuses blow.

    What happened? During time in storage, the oxide insulation in the reservoir capacitors had deteriorated. Suddenly switched on again, the capacitors leaked, heated up and generated gas internally until they burst! Some people recommend a process of gradually applying voltage to try to regenerate capacitors in older equipment after long disuse, but maybe it's safer to fit new capacitors.
     
  13. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I'd like to find a faulty device or two to get some expierence on. I have a Hitachi-Denshi scope I like a lot. It works ok, except the smaller time steps cannot be displayed because they cannot be stabilized. It is old enough that likely the caps need replacing. I haven't looked into it as I am not good enough to attempt it yet, and it will probably cost more to fix than it's worth. I can't even find a schematic on it. Thanks for the replies! I'm getting a lot out of them.
     
  14. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    What I do is replace all the electrolytic in the PSU.
     
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