Multiple DC Voltage Testing Points of Mainboard
If you are repairing many same model of mainboard, there are plenty of ways to troubleshoot it. One of the ways is to power On the device and measure the DC voltages on various points of the mainboard. You need to mark the voltage just like the photo below:
Typical example of marked DC voltages on various test points on a TV Tcon board.
Advantages
1) Once you have taken the important points of the mainboard, you can compare it with the board under test. If you found any point that is different from the value that you have marked, you can start to troubleshoot on that suspected area instead of the whole mainboard which is very time consuming.
2) It saves lots of troubleshooting time because now you know where you should test and what DC voltage to expect on those points that you have marked
3) You will be able to learn the skill of marking DC voltage at various test points and apply it on other type of mainboard with much more confident.
Disadvantages
1) If you are not working for a company that distribute the same device, then it is not worth to mark down the voltage because there are thousands of different kind of mainboard in the market and there is no way you can mark down all of the mainboard.
2) It takes time to measure and mark down all the important voltages
3) During the initial measurement of the various points, if you are not careful enough you may short your test probes to adjacent pins of IC and other components in the circuit.
4) You may need time to understand the circuit functions you are measuring because one cannot simply put a test probe on any point they like. Worst, if the distributor company that you are working for them does not have any schematic diagram at all for that particular device or equipment.
5) Some circuits that are faulty may still show good DC voltage on certain points thus you will not be able to locate to the exact point of the problem.
Conclusion- If you are knowledgeable about electronics theories and have the extra time, you can perform the measurement beyond the DC voltage test points. This means beside DC voltage, you can measure on the frequency, digital signals, captured waveform and etc. The more different signals you can measure on various points on a mainboard the faster you will be able to pinpoint the fault. You can also use special test equipment to compare the signature between a good and a bad mainboard. What is your favorite testing point method? Or do you have other special method to solve a mainboard problem? Please post your comment below that is related to this topic. Thanks!
This article is brought to you by Jestine Yong. He is from Kuala Lumpur Malaysia and he loved electronics repair and blogging about electronics repair information. He is the author of the famous Basic Electronics Repair and SMPS Repair ebook . He is also a trainer and conduct electronics repair courses at Noahtech Electronics Training Center.
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You may also interested in his previous repair article on The Impedance Of Multimeters
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24 Comments
Leave a Reply
Babu M S
November 28, 2021 at 12:27 am
Very good lesson learned. Thank you very much for sharing.
Jestine Yong
November 28, 2021 at 6:46 am
Hi Babu,
You are welcome.
Jestine
Albert van Bemmelen
November 28, 2021 at 1:03 am
When fixing computer mainboard faults it helps to study some equivalent service manual from another computer manufacturer if we can't find the exact same manual. And in many cases that may still be using the same controller chips from identical manufacturers. That often out about 48 pages or more of circuit schematics consisting service manual always shows what voltages are always active when the board starts to function. Those are named 5V_AL, 12V_AL and/or 3.3V_AL meaning always there before the cpu controller and other controller chips get there specific generated voltages.
Jestine Yong
November 28, 2021 at 8:06 am
Hi Albert,
True, even laptops motherboard also have AL voltages and all these are good testing points also.
Thanks.
Jestine
Robert Calk Jr.
November 28, 2021 at 4:13 am
Nice article, Mr. Yong. After you repair a mainboard, for instance in your own TV, you could record the voltages and waveforms of the mainboard so that if something goes wrong with it again, you could check your recorded test points to maybe help speed up the next repair.
Jestine Yong
November 28, 2021 at 6:53 am
Hi Robert,
It is very good to hear from you again! Thanks for the ideas and hope the readers can learn from your sharing.
Jestine
Imoudu Onwumah
November 28, 2021 at 5:04 am
Hi Jestine,
Thanks for this article, it never occur to me such documentation of voltage points could be handy for future reference. I think I will start this on universal mainboard TPV56PB816 which is coming more frequent on my work bench, even if it's China made though!!.
Jestine Yong
November 28, 2021 at 7:05 am
Hi Imoudou,
You are welcome. But one need to be hard working in doing the voltage test on various points. Recording down more than one signal test points (beside DC voltage) would give you lots more info.
Jestine
Sylvester
November 28, 2021 at 6:47 am
I start by looking for shorts to ground.Then checking the power supplies for each power rail.90% of the time that solves the problem for me.
Jestine Yong
November 28, 2021 at 8:04 am
Hi Sylvester,
Thanks for sharing. Sometimes beside dc supply power rail shorted to ground, it can also have partial short (leakage-not so significant) and open circuit (component or circuit track broken) thus having to check on the various DC test points are quite crucial also.
Jestine
Mark
November 28, 2021 at 10:30 am
Great article again Jestine!
Part of the problem is trying to locate the information and schematics for items being tested.
At the moment, I am struggling with a GScan automotive diagnostic tool. Without a schematic and my limited knowledge, I have been able to find a shorted IC, but am still looking for the reason behind the short. Time will tell 🙂
Keep up with the informative articles!
Jestine Yong
November 28, 2021 at 9:31 pm
Hi Mark,
You are welcome. IC short can be due to overvoltage or surge in the DC supply pin, quality of IC being used, circuit design such as where the IC is position/located (is it located at a component that run quite hot), dry joints in IC leads causing small spark between the leads and the circuit trace and etc.
Jestine
Similinga mnyonge
November 28, 2021 at 12:10 pm
I always use nodal impedance and diode reading to test.this is perfect it doesn't require device to be powered .it eliminate the possibility of shorting device pin.
Also VI curve tracer is most efficient on this.
Jestine Yong
November 28, 2021 at 9:24 pm
Hi Similinga,
Good sharing. Yes using nodal impedance and diode reading to test a circuit board is good without have to power it on. One can always check the IC input protection circuitry. Many problems in digital equipment in service are due to damage in the input/output region of the equipment. Having the VI Curve tracer is also an added advantage.
Thanks.
Jestine
Parasuraman S
November 28, 2021 at 3:44 pm
Very useful tips! Many thanks for sharing!
Jestine Yong
November 28, 2021 at 8:45 pm
Hi Parasuraman,
You are welcome.
Jestine
Paris Azis
November 29, 2021 at 1:25 am
Very interesting article, Jestine, and very controversial the topic among the technicians’ global community, especially as regards these pros and cons...
In any case, troubleshooting any kind of circuit without a schematic is like looking for fleas in the hay...
In such a case and if the device under repair has come for the first time, in my opinion it is not worth it to do all this endless job of keeping such a detailed voltage records. The only exception I see, is the possible very high cost for the owner of the equipment in order to get a new similar replacement, especially if we talk about industrial equipment. In this case, if there is a reasonable agreement with the owner about a proper payment for the intended repair, then is worth it to spend the needed amount of time in order to keep such a record (which could also be used for additional future repairs). Otherwise this effort is meaningless.
I (and many other technicians I guess) happened to make such repairs within this frame of evaluation criteria and with the owners’ agreement.
As regards the methodology itself, I not only agree that it is effective, but also I would say that the more detailed the record the more effective the method is. But, again, under the light of all said above.
Apart from that, I prefer the voltage testing method to be limited in key points of the circuit. In order to do that, a previous analysis of what I have in front of me is necessary. And above all, a thorough visual inspection is of first priority. Also if the circuit is dirty, a proper cleaning of it is the first step. Then the visual inspection follows and finally these specific voltage tests.
I also have got the habit to measure beforehand the power consumption of the device under repair and evaluate the reading carefully. After the repair, I check for any deviations from the first measurement. This is also indicative factor of the effect of the repair itself been done.
Now, in cases that there is expectancy for same devices to come for repairs, then it is worth it to keep detailed voltage records of the circuit.
When working myself with VTRs at the official services of Panasonic and JVC, long ago, I was using a similar alternative method instead of keeping detailed voltage records. I had been using extensively the signature analysis method, using the component tester of the oscilloscope, having a working PCB as reference and comparing any other similar one with it.
The same method is ideally applicable on stereo amplifiers, given that their structure is always symmetrical, that is, the defective channel (and in most cases only one is defective) is a replica of the other. Therefore a “mirror” point by point checking method is very easily implemented and extremely time effective and conclusive.
Personally speaking, I consider the signature analysis method much more conclusive than any other.
If a specific voltage testing method is not clearly successful in finding the culprit component, the signature analysis will surely succeed to spot it.
Jestine Yong
November 29, 2021 at 9:11 am
Hi Paris,
Amazing! Thanks for your superb explanation!
Jestine
Lynn Blakely
November 29, 2021 at 5:25 am
I am enjoying reading all of the article, and all the comments, I don't know the the term "nodal impedance" does it mean a resistance reading for a component to ground?
Jestine Yong
November 29, 2021 at 8:02 pm
Hi Lynn,
You can click on this link to read the explanation of nodal impedance.
Jestine
Yogesh Panchal
November 29, 2021 at 3:28 pm
Sir,
Thanks for sharing Tips.This practice is very useful when same type of circuits comes to you for the repair and you have references ready with you.
Jestine Yong
November 29, 2021 at 7:53 pm
Hi Yogesh,
Yes, very useful to troubleshoot same type of circuit board.
Jestine
Waleed Rishmawi
November 29, 2021 at 4:01 pm
it is like making your own (kind of) schematics with all these test points. yes it is lot of work but at the end it is a time well spent and will cut down on your repair time. that is an excellent idea. thanks for sharing
Jestine Yong
November 29, 2021 at 7:55 pm
Hi Waleed,
You are welcome.
Jestine