Your Multimeters Millivolt scale can be used to locate shorts to ground.





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Published on Jul 10, 2015

Sometimes locating which component is causing a short to ground can be a time consuming process. You can waste a lot of time UN-soldering components to find the source of your short if you don't own a sensitive ohmmeter, but even if you don't there is another option you can use if your Multimeter has a Millivolt scale.
In this video I demonstrate how to use it in combination with your external power supply to isolate which of your components may be causing a short to ground.
Technically speaking the only voltage drop you really see in this example is from the shorted
diode. The other diodes direction does not allow any current flow, and capacitors do not pass DC. I should have used slightly different words to describe this example.
The TV used in this video is a Philips model number 39PFL2608/F7 The diode which I confirmed to be bad is a 30 volt 1/2 watt zener. Its location is D652.. DIODE part number is 1ZB30BB NDWZ0001ZB30.

Comments • 120

USS Liberty
USS Liberty3 years agoHighlighted comment
Thanks for sharing this technique, This video needs to be in Jestine Yong website !
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I'm a retired avionics tech. 30 years ago HP published a trick not unlike your own, using the mv on the DVM, but with no guessing. To find the short connect a current limited supply to the shorted supply rail at the spec voltage but with a safe current limit. Now connect the positive lead to the rail, and we'll use the neg probe to chase down the short. Clearly the millivoltage will grow towards the short, and if you probe past a node, the voltage will stop increasing.
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Great video, thanks! There's a potential (little) goldmine out there if you can get the "throwaways" before the "scrappers". And if they are repairable. I bet most are.
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Glen Whatley
Also, if the short is low enough resistance, you can likewise inject a voltage of much less than the voltage than the power supply and boost the current output until the shorted component heats up enough to actually feel with your finger (no using expensive freeze spray. I work repairing cable boxes for Comcast and have been able to repair a very high percentage of boxes this way. I Have used the milli ohm method as well as the ESR Meter as well as injecting voltage and tracing with the milli volt meter, too. All have their place and merit. With Shorted chip components on a highly concentrated board with as many as 10 decoupling chip caps on a power supply line all across the PCb, it could be anywhere on the entire board. Hands down the injecting low voltage at higher current and merely feeling for heat is the most effective. I haven't had one case in hundreds of repairs where I have ever damaged the upstream or downstream components. This is mostly with buck supplies.
Very good teacher.
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excellent, thank you for sharing your hard earned knowledge, much respect!
Actually, there is one thing here VERY WRONG. Measuring with current is ok, but 0.25A is TOO MUCH - that could've fried your diodes! Most diodes work on currents around 20mA (LED diodes for example) and that is a level you should have. In this example you where lucky that the diodes you had there where power diodes, but on laptop motherboards and elsewhere, the diodes are much more sensitive and they can get fried with as low current as 50mA. Of course, a short to the ground means that the power will go through the short and not through the diodes, but once the short is gone, all that power would go to those diodes. Another safe thing (except limiting current) would be to try to find the short by limiting the voltage to 0.25 V - at this voltage most of the diodes just don't work and no harm will happen to them even if the short will be eliminated. At 0.25V only thing that works is passive components (and shorts) - and those can handle that voltage no problem, so you don't have to limit the current so much when you are at 0.25V.
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This technique works very well, great on logic circuits were the 5v rail might go everywhere.
Usually to find shorts of that kind, i usually plug a lab supply across the shorted rail and inject some power and look for the hot parts.
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Clifton Jamison
Thanks much Grant you remind me of my uncle who had the 1st TV repair shop in the north Adirondacks.That was circa 1950? He was a communications guy in the Marines on Okinawa ww2
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