Break IC, Recover MCU, Microcontroller Reverse Engineering

Archive for the ‘PCB Repair’ Category

Under the circumstance of the absence of circuit schematic diagram, if you intend to repair a stange and complex faulty PCB circuit board, so called ”maintenance experience of PCB repair” can’t take its effect anymore, Even the veteran electronic technology hardware repair personnel get half the result with double the effort, so what can be done to better improve the repair efficiency? The following principles will make the repair task being finished in an orderly and smooth way:

1 Visual inspection before any movement:

Inspect the PCB circuit board ready for repair visually first, take the advantage of magnifying glass when necessary; at this point you have to emphasis on:

1. Whether there is an existence of OPEN or SHORT circuit, especially checking if the connecting wires of printed circuit board have occurred disconnecting and adhesive;
2. Whether there is an existence of disconnection of component lead like resistor, capacitor, inductor, diode and transistor;
3. If the PCB circuit board has been repaired, and if the components on it being modified already?

And whether show the existence of cold solder，no solder and wrong/reverse component?

After all the conditions described above being excluded, now can use the multimeter to test the resistance value among power layer and ground layer, normally speaking the value shouldn’t less than 70Ω, if the resistance value less than that, for instance, only a few or a dozen ohms, that can illastrate that component(s) being breakdown, and then measure should be taken to find out what component(s) being breakdown, the most direct measure is supplying electric power to the circuit board (Note!!! Before that you have to figure out the working voltage and postive/negative polarity of the circuit board, the voltage must not allow higher than specified working voltage, otherwise circuit board will be damaged by it), point-temperature tester being applied to test the temperature of each component, the one whose temperature rise significant would be viewed as highly suspect item.

Once we can confirm the resistance value is normal, multimeter should be used again to test the diode, transistor, FET and bandswitch, etc. The purpose of all these actions is to ensure all the components tested are all normally function. Don’t let the problem go further and become complicate if it only need multimeter to address it.

2 Check exterior and then interior:

When you inspect the PCB circuit board by using on-site circuit repair tester, it is more preferable that you can have a same normal function PCB circuit board in hand for reference, through which you can make the comparison test among these 2 PCB circuit boards by VI curve scanning function, PCB circuit board connector can be viewed as the starting point and then go further, especially for capacitor testing, which can solve the problem that multimeter can’t if capacitor is leakage or not.

3 From easy to difficulty:

Before use on-site circuit maintenance tester to test the PCB circuit board, in order to ensure the accuracy, technical treatment can be done in advance to avoid the interference occurred during the testing, measures see below:

1. Preparation before testing

Make the crystal become SHORT circuit ( please be noted you have to clarify which 2 pins are the signal output pins and then make them SHORT circuit, under this condition the 2 other power pins must not be shorted of 4 leads crystal ), for those electrolytic capacitor, you have to make it become SHORT circuit first, because charging and discharging large-capacitance electrolytic capacitor will have interference too.

1. Elimination method for component test:

When use the on-site circuit maintenance tester to inspect all the components on the PCB circuit board, if the components pass the test and/or test result show it is normal, you can confirm the test result directly and make the record, for those fail to pass the test and/or test result show it is abnormal, test them one more time and if they still fail to pass the test then you can confirm the test result first, follow these ways and keep on testing all the components untill finish all the component test.

For those fail to pass the test, some testers have provided some very practical approach: this kind of tester can provide power supply to PCB circuit board through its clips, so in the same way it can supply power to component’s POWER and GROUND pins, if lift the POWER pin of component from PCB circuit board manually then this component will be out of the power supply system of PCB circuit board.

At this time use the on-site tester to inspect component, because the other components can’t being supplied by power so it will eliminate the interference effect and the testing circumstance equals to “quasi-off test”, the percent of accuracy can be improved significantly.

1. Use ASA-VI intelligent curve technology can be applied to the comparison test of any component, as long as the component can be griped by test folder and moreover an reference PCB circuit board. Through comparison test it can also have strong fault detection and estimation ability, this feature offset the insufficiency of component library when operating on-site inspection, at the same time fault detection range has been extended effectively. Unfortunately in the reality the reference PCB circuit board usually can’t be found, and the electronic schematic structure of PCB circuit board ready to be repaired don’t have any symmetry, under this circumstance even the ASA-VI intelligent curve technology can’t have positive effect, and as a result of insufficiency of component library, the on-site testing can’t inspect each component on the PCB circuit board.

4 From static to dynamic

Because on-site circuit maintenance tester can only inspect the components assembled on PCB circuit board and have the static characteristic analysis, therefore the way to conclude that whether the faulty PCB circuit board has been completely repaired is install it back to the original equipment and then test-run the equipment. Under this circumstance all the power supply of this equipment should be checked and ensure it can correctly provide power to the PCB circuit board, moreover all the connectors on it have been installed in good condition, the negative impact of incorrect connection of PCB circuit board must be eliminated completely.

Step 1: Ask customer about the breakerdown appearances of damaged electronic device as well as on-site situation;

Step 2: Analyse the root cause of breakdown according to customer description;

Step 3: Dissemble device and clean it thoroughly; confirm the components being damaged and then assess the feasibility of can-be REPAIR;

Step 4: Analyse the circuit schematic diagram according to the location of damaged components, find out the root cause of components being damaged and prevent similar situations happen again;

Step 5: Communicate with customers about the components list that need to be replaced, collect comment from customer, start Repairing after receipt of customer confirmation on quote;

Step 6: Repair service include eliminate the existing problems and faults, replace obselete or damaged components, clean the entire device thoroughly and required maintenance;

Step 7: Instruct SIMULATIVE LOAD TEST after repairing process finish, once pass the test will send to customer for further on-site testing.

1 Power Supply Circuit:

During the circuit board repair process, power supply part should be inspected at the first hand and then come to the other part. +/-5V,

Power supply failure can occur under below several situation:

(1) NO power supply voltage or power supply voltage is deficiency, NC system is often used +/-5V, +/-12V, +/-15V and +/-24V, a few of them use +3.3V, and the varied or unstable voltage of power supply will cause the system working impropery;

(2) Use voltmeter to test the voltage of power supply, and the result show is normal. Voltage waveform detected by oscillograph has shown the existence of big ripple. This situation maybe caused by open circuit of Filtering capacitance, bad rectifier diode or cold soldering, but sometimes it could be caused by an overloaded component which has been brokendown and damaged by power.

(3) System can run properly when it is just being opened, after a while of operating, the voltage begin to drop off. This is usually caused by voltage stabilizing circuit and large power triode.

(4) The voltage decrease accompany with temperature increase maybe cause by component cold soldering, it is electrical connection can be affected negatively when the temperature high up.

(5) NO power supply voltage or supply voltage decreases significantly will cause the system to interrupt or stop working, this fault can be detected and spotted easier.

(6) When the capacity of power supply load drop off or filter circuit become invalid, it will cause the system halt suddenly, this situation which is very difficult to justify can bring damage to the equipment and facility even get personnel wound.

2 Clock Circuit:

Clock circuit mainly exist on the systematic motherboard, it is the foundation of large-scale integrated circuit system through which it can work, it can generate the constant square signal in the circuit base upon the crystal oscillator (commonly known as crystal), Once the crystal oscillator stop working, it is same as the heart of human being stop beating, the whole system will fall into the status of paralyzed, only after crystal can work under normal condition, the systematic circuit can operate under the command of CPU according to the frequency of crystal. The number and frequency of the crystal could be different due to the variety of numerical control system, but generally will at least one crystal, different clock frequencies required by the other circuit can be solved by frequency dividing circuit or other crystal. Crystal possess a higher rate of failure or damage, below are some malfuntions of crystal:

(1) Leakage: Use multimeter and switch to P*10K level to test it, if the resistance is infinite, then it can be viewed as normal;

(2) Internal OPEN CIRCUIT: Value of resistance is infinite tested by Use multimeter, can fail to generate oscillating pulse in the circuit;

(3) Alternative crystal: Due to the transformation of crytal will cause its internal parameter change which can only be detected by osillagraph or cymometer. Although the crystal can still has oscillation, but the clock frequency is deviated from its nominal value, and the still existing oscillation pulse can’t make system circuit work properly due to the value of oscillation is wrong. Only cymometer can test its tolerance now.

(4) In clock circuit, the both ends of crystal would be connected to ground by one ceramic capacitor value range from several picofarads to tens picofarads, failure of the clock circuit result from this capacitor leakage, deterioration is also more common.

The best tool to test whether the crystal is good or bad could be oscillagraph or cymometer, multimeter is very difficult to detect the root cause.

3 Reset Circuit

Reset circuit also exist in the circuit system of motherboard, it is unique set of large-scale digital integrated circuit. Microprocessor and interface circuit are all possess reset terminal. Reset pulse generate by reset circuit will clear the progam counter, force CPU recall the orginal files from the memory, execute initiation process on all the controller chips, system will occurs the phenomenon of disorder or crash as a result of faulty reset circuit, the method of using cymometer to illustrate the reset pulse is turn on/off power supply repeatly, observe and record the pulse value at the right moment of power supply on/off since it should be the normal square wave-form. If there is no existence of reset pulse, all the resistor, capacitor and transistor in the reset circuit should be detected. The reset terminal of integrated circuit should be regular low or high electrical level, or else, it is probably the malfunctional reset circuit or damaged integrated circuit.