Quite often whenever electronic equipment don’t function or work, we would immediately suspect a faulty switch mode power supply. But do you know that defective or shorted components in the motherboard or major board could cause the power supply to stop working too?
Switch mode power (SMPS) are designed so efficiently that will whenever there is any short circuit happen in the main board the power supply would shut itself off and totally stop working. If you have no experience about troubleshooting switch mode power supply, you might think that the power supply have problem where in fact the main board is the real cause of no power problem.
Change mode power supply have a current sense circuit (if you look at UC3842 PWM IC pin 3, it stated I-sense which mean present sense) and if there is short circuit in the secondary side (either in secondary diodes or main board), the existing drawn would be increase and this can lead the PWM IC to stop generating output to the power fet and thus the power supply would shut down.
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All this happen in a split of seconds and you do not have the chance to know if there are output voltages on the secondary side.
Some older design of SMPS power supply do not use the PWM IC, but it do have the signal to detect over current drawn and shut itself down anytime it detects a shorted element in the secondary side. One good example was the power supply used in printer. Printers usually have two boards; one was the power supply while the other was primary board. If there is any short circuit in the main board, the power supply would not function. In order to isolate at where the is actually, one must remove the connector in the power supply board. Once the supply connector to the main board was taken out, you can now switch on the printer and check if there is any voltages existing at the power supply connector.
If you will find zero voltages measured across all of the supply (VCC) pins then we are able to conclude that the power supply have issue and you can put your whole concentration with this power supply board. What if there are voltages measured across the connector? This means that the primary board is causing the no strength problem most probably due to some shorted components in the main board.
For your details, dot-matrix printers usually required two voltages to function. One is the five Volts (for logic IC, eeprom and CPU) and the other will be 30+ volts for the motors. The question now is how do we know if the main board is the main cause that will shutting down the power supply? Very simple, simply use your analog multimeter set to X 1 Ohm and measure between the supply pin (say 5 volts pin) and the main board floor and then reverse the probes. A great board should not show two similar reading and if you get two similar ohms reading then this means that the 5 volts line had shorted to ground through some faulty components.
If you have confirmed that the 5 volts line have problem then how do we find out the culprit since there are so many components connected to this range? TTL IC’s, CPU, EEPROM, transistors, diodes and even small filter capacitors are all connected to the 5 volts collection. Either one of these components shorted could cause no power to the printer. You might remove each components lead (5 volt supply) in the main board plus hope that the short circuit will be gone. Assuming if you happen to remove one of the filtration system capacitor pin and the short circuit is fully gone then we can say that the real culprit is the filter capacitor.
The real is actually what if the board has many parts on it and this will consume lots of your time to isolate the problem simply by removing one pin at a time. It is not easy to identify the supply 5 volts pin to a spider IC that has 100 pins or more. Many spider IC’s have more than one five volts supply pin. Some even have 4 and some have 6 to 8 supply pins. Does this mean you have to check one pin at a time before you finally locate the fault? Not just that, to remove the supply pin in the spider IC’s and check for any kind of short circuit between the grounds required an excellent skill too. If you messed out there the circuit board track, the primary board can then be considered beyond repair. Even though you can repair the broken circuit track, this does not mean you have solved the actual fault!