BASIC METHODS OF TROUBLESHOOTING OR FAULTFINDING COMPONENTS ON A PRINTED CIRCUIT BOARD
Basic methods of faultfinding... in no particular order of preference. 1. Rigorous inspection, just look for wrong c omponents/shorts etc, 2. Study the circuit diagram and try and workout what fault would produce the symptoms. 3. Look at circuit nodes with an oscilloscope, comparing these with expected waveforms, using a working reference circuit board, or the designer's/service manual's notes /waveforms 4. Break the circuit into 2 sections to isolate which area the fault is in, keep repeating this procedure until you get down to the actual component. Note this is difficult if there are lar large ge feedback loops as in a switch mode power supplies. 5. Link in sections of a good reference board to isolate which section is bad. You can use a razor blade to cut a trace to test an individual component and re-solder the trace. You can de-solder the component and test out of the circuit. Use the LeakSeeker http://eds-inc.com/leak.html very good tool $229 Heating or cooling a component can help you locate a bad component (see video in the link above for a demonstration using heat and the LeakSeeker). (sometimes heating a joint or skimming the iron over the traces would reveal a problem's location. This in combination with close-up viewing under the glass, of course) Dry joint is a legitimate fault in electronics equipment. It's short for a "dry solder joint". Occurs when a joint is solderd and disturbed before it solidifies creating a loose joint resembling a short this can cause other components or circuits to fail. When completed, a solder joint should be clean, shiny, and show a perfect adhesion to both the component lead and the PCB. If there is any sign of the solder being "frosty", sitting on the PCB as a "blob", or not flowing up the component lead in a nice smooth arc, then the solder joint is incorrectly made. Too little heat makes a "cold" joint, where the solder just sits in blobs but does not make a metallic bond.
Look for capacitors that are bulging sometimes from the top sometimes on the side. Wir Wire Name ame (Oz (Oz)Wire )Wire Col Colour ¹
Also know known n as ...
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Brow Brown, n, Red, Red, Blac Black k
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Neut Neutra rall
Blue Blue,, Bla Black ck,, Whi White te
cold cold,, com commo mon, n, grou ground nded ed cond conduc ucto torr (US (US), ), minu minus, s, negat negativ ive e (as (as abov above e for for the the las lastt two two))
Eart Earth h
Gree Green/ n/Ye Yell llow ow,, Gre Greengrou enground nd,, pro prote tect ctiv ive e ear earth th,, ear earth th grou ground, nd, safe safety ty eart earth, h, grou ground ndin ing g con condu duct ctor or (US) (US)
Note 1 - Be careful with wire colours. The standards are gradually changing worldwide to the Brown, Blue, Green/Yellow scheme, but a great deal of older equipment will use one of the old standards - and it might not be o ne ever used in your country! Make sure that you treat all incoming mains wires that are not connected directly to the chassis as hostile.
TIPS
Like Del pointed out , it is indeed patience testing workout , it is a huge task and to the best s atisfying if you make it to work . you have to know circ uit diagrams for transistors , mosfets , diodes and other components with there specs , have a circuit diagram at hand , make primary observation and let your eyes , noses sense for burned component , w ith the help of multi-meters check resistances , diodes ,fuses , transistors ,checking mosfets is different from checking transistors , divide the circuit in part of there operation from the circuit diagram with thin marker , check the relative components also. replace all the mosfets even if one is defective , change tr ansistors with there relative components like diode , capacitor if found .this quiet tedious but observe economics as Del mentioned replacing a set of components should not over ride the cost of getting new one forget time you put for fault location Back when I was repairing stereos, I had an advantage if only one channel was defective. The other channel would usually be working! So, I'd take my multimeter and started comparing voltages at each transistor. Eventually, I find a big enough difference to identify a defective part. For televisions, I usually relied on the schematic. Most schematics had voltage test points with the expected voltage printed on it. If there was a difference, I'd change the most likely part. When was troubleshooting pcb boards, I would take a good pcb and start measuring the voltages of each component moving from the output of the pcb to the input of the pcb in the area of circuitry that I'm experiencing the problem. I would write them down as I'm doing this. Then I would do the same with the bad pcb and compare the voltages. As you compare the voltages from output to input, look for the area of the circuitry where the voltages start to match up. That is usually where the problem lies.
