Troubleshooting And Repairing Mainboard 

If you are working in a distributor company that sells electronic equipment for a particular brand, you will definitely have gone through the training provided by the company or at least you have all the information about the equipment such as the schematic diagrams, service manual, tips of repair and etc. The electronic board that I’m referring to can be a Mainboard/Motherboard or Card as long as it has lots of ICs in it. As mentioned above, if you got all the information about those electronic boards, in fact you have already won 50% of the war. What if there is an electronic board that you have less or no information about it? Where and how are you going to start/test?

The electronic board/card can be broken down into three types:

1)    Electronic board that contains only the TTL IC’s (use in older type of electronic equipment)

2)    Electronic board that contains of TTL ICs and Customized ICs

3)    Electronic board that contains only the Customized ICs (like modern LCD Monitor/TV board/Display card/ Automobile board and etc)

1) Electronic board that contains only the TTL IC’s

Since you do not have the schematic diagram the only thing that you can do is to test out all the TTL (Transistor transistor logic) ICs with a digital IC tester (TTL IC tester). With the help of a special desoldering gun that have the vacuum pump in it (see the photo) you can easily remove the TTL IC and place it onto the tester socket. If the IC is bad then replace the IC and retest the card again. If it is good then solder the IC back and remove another IC and follow the same procedure again until you found a bad IC.

If you do not have the IC tester (this tester is expensive and not many technicians could afford it) you can always use a logic probe to find out the bad IC. For your information you can’t simply just test the pins of the TTL ICs, you have to find out the part number (like 74LS 04, 74LS86 etc) and see what type of IC you are checking. You need a truth table to guide you about logic probe result. If you get something that is different from the Truth table then suspect the IC might have problem. You can only do this kind of test when the power is “On”. The electronic components like resistors, network resistors, capacitors, and transistors rarely have problem and if you have the time you can test them first before you test on the TTL IC’s.

Assuming if you have a board that contains of more than 100 pieces of TTL ICs and you found that the supply line has shorted (when you use your ohm meter and test the supply line between the VCC and ground) it will be time consuming trying to locate at where the short circuit is. You have to use a special tester that can easily locate a shorted component like the Leakseeker  82B by EDS. Some technician calls it as Current Tracer. The function of this Leakseeker is to produce a series of “beeping” tones and LED light, and the “beeps” grow louder (or the LED light grow closer) when you move the probe closer to the point of the short. With this method, it can detect a shorted component to the exact location thus saving up your time.

Electronic board that contains of TTL IC’s and Customized ICs

If you come across such board the first thing you can do is to follow the procedure above by checking either the electronic components (resistors, capacitors, transistors, crystal and etc) first or the TTL IC’s. But before that you have to make sure there is supply voltage going into all the IC’s. Check that the voltage regulator is producing good output. Once you have confirm that all the components are okay then only you proceed to check on the other types of IC’s like the MCU, Rom , Ram , EEprom and etc. You can check the SMD components with the help from your normal meters or use a Smart Tweezers.

The question is how are we going to test those ICs above if we do not have information or data about it? This is the most challenging questions I often got asked by my ERG members. The answer is we can’t test it because the data/programs stored in the MCU/EEPROM/ROMs are unique and is not easy available from the market. Unless you have the same type of board where you can do the comparison but again where to get such parts (IC’s) for replacement? Some IC’s part numbers are very unique and you really do not know it belong to which type if you do not have the schematic diagram. Most of the customized IC’s can only be available from the manufacturers/distributors and normally they would not sell to anyone.

The closest we can do about this kind of board that uses many customized ICs is by comparison with a known good board with the help of Oscilloscope and Huntron tracker. And if you have lots of similar board then you can salvage some of the customized IC’s.

Electronic boards that contains only the Customized IC’s

I still could recall the first time I repair the Samsung LCD Monitor Mainboard few years back. It contains only the customized IC’s and do not have any TTL IC’s in it. What I did was to check on all the components first with my meters and once I found that all of the components are working good then I suspect it is either the EEprom, ADC/Scalar IC or the MCU IC fault. I waited for quite sometime before another customer sent in the same model that have power problem. Once the power problem solved and the Monitor working good I took out the Mainboard and extract the EEPROM and MCU for comparison purposes. Guess what? I found that the MCU was the cause of the problem.

Since I could not find the MCU ICs (good IC must have program in it) and I do not have programmer to reprogram such IC, I bought over those spoilt unit from customers (LCD panel cracked, power side hit by lightning and etc) and salvage the good MCU IC. Getting a new board is too expensive and most of the time the customer would not repair it due to high cost.

The other day I met a repair friend who is specializing in Audio/Tv repair and he told me that “Jestine, whenever I come across DVD board that have sound problem I will give back to customer. I asked him why and he said that usually would be the Audio SMD chip problem and rarely is the passive components” and since he could not get the chip from the market, he had to return the set back to the customer. The real problem is the maker of DVD does not use a common chip part number and for those part numbers, you could not find any data from the internet.

Conclusion-If you come across a board that have lots of component in it, do not give up first until you have test all the components that you could test. If all of them check to be okay then only you suspect a bad customized IC. If you have the part then replace it and retest again and if you do not have any just send the board back to customer. This is the only current steps that we can do unless you really know where to source parts for the board that you are repairing. Have a great and a wonderful day! 

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