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 main board could cause the power supply to stop working too?
Switch mode power supply (SMPS) are designed so efficiently that whenever there is any short circuit occur 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 may think that the power supply have problem where in fact the main board is the real cause of no power problem.
Switch mode power supply have a current sense circuit (if you look at UC3842 PWM IC pin 3, it stated I-sense which mean current sense) and if there is short circuit in the secondary side (either in secondary diodes or main board), the current drawn would be increase and this will lead the PWM IC to stop generating output to the power fet and thus the power supply would shut down. All this happen in a split of seconds and you do not have the chance to know if there are output voltages at the secondary side.
Some older design of SMPS power supply do not use the PWM IC, but it do have the circuit to detect over current drawn and shut itself down whenever it detects a shorted component 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 main board. If there is any short circuit in the main board, the power supply would not work. In order to isolate at where the problem is, one must remove the connector from the power supply board. Once the supply connector to the main board was removed, you can now switch on the printer and check if there is any voltages present at the power supply connector.
If there are zero voltages measured across all the supply (VCC) pins then we can conclude that the power supply have problem and you can put your whole concentration in this power supply board. What if there are voltages measured across the connector? This means that the main board is causing the no power problem most probably due to some shorted components in the main board.
For your information, dot-matrix printers usually required two voltages to function.
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One is the 5 Volts (for logic IC, eeprom and CPU) and the other is 30+ volts for the motors. The question now is how do we know if the main board is the main cause that shutting down the power supply? Very simple, just use your analog multimeter set to X 1 Ohm and measure between the supply pin (say 5 volts pin) and the main board ground and then reverse the probes. A good 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 line? TTL IC’s, CPU, EEPROM, transistors, diodes and even small filter capacitors are all connected to the 5 volts line. Either one of these components shorted could cause no power to the printer. You may remove each components lead (5 volt supply) in the main board and hope that the short circuit will be gone. Assuming if you happen to remove one of the filter capacitor pin and the short circuit is gone then we can say that the real culprit is the filter capacitor.
The real problem is what if the board has many components on it and this will consume lots of your time to isolate the problem 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 5 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 until you finally locate the fault? Not only that, to remove the supply pin from the spider IC’s and check for any short circuit between the grounds required a very good skill too. If you messed out the circuit board track, the main 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!