HOW TO TROUBLESHOOTING A 5 VOLTS SHORTED ON A CIRCUIT BOARD with all the IC power connected to the same line?
Tuesday, November 24th, 2009 at
2:00 pm
All of IC were soldered directly on PCB, there is no way for me to remove each one just to find out which one is shorted.
Tagged with: pcb
Filed under: Circuit Boards
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Disconnect the power supply from the AC power line before replacing components to avoid the danger of electrical shock.
If any of the CMOS logic chips are replaced, use reasonable care to avoid damage due to static electricity. If the relative humidity is below 50%, use a grounded workbench and make sure that the PC board is grounded to it. To prevent static damage to a chip, do not touch any of the leads unless you are also touching the workbench, or you are connected to it through a standard high-resistance grounded wrist strap. (Such wrist straps are connected to ground through a 1 megaohm or more resistor, greatly reducing danger to personnel due to electric shock.)
Other components that are soldered in place may be replaced following the instructions covered in this section.
If filter capacitors are to be replaced, fasten them securely to the board using the original factory installation as a model. This will prevent them from breaking loose from vibration in the future.
It is important to use the correct technique for replacing components mounted on PC boards. Failure to do so will result in possible circuit damage and/or intermittent problems.
The circuit boards used in SMART units are double-sided plated-through type. This means that there are traces on both sides of the boards, and that the through-holes contain a metallic plating in order to conduct current through the board. Because of the plated-through holes, solder often creeps 1/16" up into the hole, requiring a sophisticated technique of component removal to prevent serious damage to the board.
If the technician has no practical experience with the demanding technique of removing components from double-sided PC boards without board damage, it is wiser to cut each of the leads of the defective component from its body while the leads are still soldered to the board. The component is then discarded, and each lead is heated independently and pulled out of the board with long nose pliers. Each hole may be cleared of solder by using solder wick or by carefully heating with a low-wattage soldering iron and sucking out the remaining solder with a spring-activated desoldering tool. THIS METHOD IS THE BEST METHOD OF CLEARING A PLATED-THROUGH HOLE OF SOLDER.
Another technique is:
Use a 30 watt soldering iron to melt the solder on the underside (solder side) of the PC board. Do not use a soldering gun or high wattage iron! As soon as the solder is molten, vacuum it away with a spring activated desoldering tool like the Edsyn "Soldapult." Do not overheat the board! Overheating will almost surely damage the board by causing the conductive foil to separate from the board. Use a pair of fine needle-nose pliers to wiggle the lead horizontally until it can be observed to move freely in the hole.
Repeat step 1 until each lead to be removed has been cleared of solder and is free to move.
Now lift the component out of the holes.
Bend the leads of the replacement component until it will fit easily into the appro priate PC board holes. Using a good brand of rosin-core solder, solder each lead to the bottom side of the board with a 30 watt soldering iron. Make sure that the joint is smooth and shiny. If no damage has been done to the plated through hole, soldering the topside pad is not necessary. However, if the removal procedure did not progress smoothly, it would be wise to solder each lead at the topside to avoid potential intermittent problems.
Cut each lead of the replacement component close to the solder (underside) side of the PC board with a pair of diagonal cutters.
Remove all residual flux with a cotton swab moistened with a solvent like 1,1,1 trichloroethane, naptha, or 99% isopropyl alcohol. The first two solvents are usually available in the supermarkets under the brand name "Energine" Fire proof spot remover and regular spot remover, respectively. The alcohol, which is less effective, is usually available in drug stores. Rubbing alcohol is highly diluted with water and is ineffective.
It is good policy to make sure that the defluxing operation has actually removed the flux and has not just smeared so that it is less visible. While most rosin fluxes are not corrosive, they can slowly absorb moisture and become sufficiently conductive to cause progressive deterioration of performance.
IC opamps are usually operated so that the characteristics of their associated circuits are essentially independent of IC characteristics and dependent only on external feedback components. The feedback forces the voltage at the (-) input terminal to be extremely close to the voltage at the (+) input terminal. Therefore if the technician measures more than a few millivolts between these terminals, the IC is probably bad.
Exceptions are IC’s used without feedback (as comparators) and IC’s whose outputs have been saturated due to excessive input voltage because of a defect in an earlier stage. Also, be sure that the voltmeter is not interacting with these sensitive points and affecting the measured voltage. However, if an IC’s (+) input is more positive than its (-) input, yet the output of the IC is sitting at -14 volts, this almost certainly indicates that it is bad. The same holds true if the above polarities are reversed.
Because the characteristics of SMART’s circuits are independent of opamp AC characteristics, an opamp can usually be replaced without need for calibration. However, some of the control circuitry is sensitive to DC opamp characteristics, like bias current and offset voltage. Because of this, high performance dual opamps are used. These devices must be replaced with exact replacements; garden variety IC’s are not satisfactory.
A defective opamp may appear to work, yet it may have extreme temperature sensitivity. If parameters appear to drift excessively, freeze-spray may aid in diagnosing the problem. Freeze-spray is also invaluable in tracking down intermittent problems. But use sparingly because it can cause resistive short circuits due to moisture condensation on cold surfaces.
I recommend that all plug-in or front panel PC cards requiring repair be sent to our factory, if at all possible. We can normally turn a repair around in a short time and get it back into the customer’s hands far faster than would be the case should someone attempt a repair with no experience with SMART products. This also allows us to add reliability data to our files so that future revisions can be undertaken if necessary to improve the long term reliability of the system.
It is an excellent idea to have at least one set of the critical PC cards on hand in each multiplex for any backup use that may become necessary. These spares may be purchased separately.
Without a circuit diagram or schematic it makes it difficult but you need to figure out where your 5 volts originates from in the circuit and trace it from IC to IC until you lose it. If 5 volts comes in but does not come out that IC is bad. You need a dvm or a scope. Some circuit boards have marked test points but most do not. Really to approach this with any logic you need to have some idea what is happening in the circuit and a schematic is the best tool to do this with.
Simple. Visually.
5 volts is usally a biasing voltage that opens up a transistor. This almost exclusively comes from the powersupply or linked voltage divider. Like the other guy said, trace it back until it disappears. My other thought is that it could be a stuck logic state. +5Vdc is usually a logic high. It really depends on what side of the IC the 5 volts is on. I could help more if I knew the exact location of the fault. It could also not be the problem at all. It could be the IC’s clock signal that your meter cannot detect due the high freq that often serves as the clock. Or it could just be fvcked up. It’s hard to tell with that info. Hope that helped.
Snoopy, Gemma and Fifty don’t know what they’re talking about. On a circuit board with many components connected to a common supply point, it is impossible to go from one component to the next to see which one kills the voltage. The measured supply voltage will be the same at every point on the board that used that voltage. In other words, if the 5 volt supply is loaded down to 0.2 volts, you will measure 0.2 volts at the power supply point of every component that uses it. I would first check for which IC chip is hot. Verify that the 5 volt supply is good AND strong enough to supply the circuit. If it’s a 200mA supply and you need 2 amps to run the circuit, there’s going to be trouble. First the voltage will drop a lot and the supply will get hot. If the supply has the balls for the job and still gets sucked down quite a bit then there almost has to be a component eating the power. That results in heat, usually a lot of heat. If the 5 volt line measures almost exactly zero volts then there most likely is a short between the positive and the return somewhere on the board. Check the solder connections with a magnifying glass and strong light. Look at the component side of the board for shorts between connections, perhaps a small stray piece of wire shorting a component. Good Luck!