Circuit Troubleshooting

Short notes about Circuit troubleshooting.

– Check the circuit for mechanical failures, like disconnected wire, broken vias on the board, and connections between two adjacent vias which are not to be connected.

– You should compare values of components and their connections with values and connections between components on the schematic.

– Measuring DC voltages at certain points of the board, and comparing these values to the ones on the schematic. So, by knowing the operation of the circuit you start the process of elimination to find the “suspect” component.

– If there are several “suspects”, and this is not a rare occurrence in complex devices, the testing is divided into groups of components, you start checking in reverse soldering order, this means that you start with components last soldered, because those are the most sensitive components on the circuit like integrated circuits, transistors, diodes, etc.

– Check electrolytic capacitors, since they have a somewhat limited lifespan and leave resistors and block capacitors as last in line since they are sturdy little thingies and can take a lot of beating.

– Grid transformers are tested by measuring the resistance of the copper wire on the primary and secondary coil. Since the primary coil has more curls than the secondary one, and is wound using a thinner wire, it’s resistance is higher, and it’s value lays in range between several tens of ohms (in high power transformers) to several hundreds of ohms, even to kilo ohms (in low power transformers). Coils can be tested in the same way as transformers – through their resistance. Infinite resistance still means disconnected coil.

– DC capacitors should produce an infinite value on the instrument. Exceptions are electrolytic and very high value block capacitors. When the positive end of an electrolytic capacitor is connected to a positive probe of an analog instrument, and a negative end to a negative probe, needle jumps to the lowest value and then gradually comes back towards infinity. This is a proof that the capacitor is ok, and the needle’s movement is the charge stored in the component being discharged. (Even small capacitance components get charged while testing, but their discharge time is very short, so the needle doesn’t have the time to move.)

Amr Ayoub

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