Frequently whenever electronic equipment don’t functionality 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 supply (SMPS) are designed so efficiently that will whenever there is any short circuit occur in the main board the power supply might shut itself off and completely stop working. If you have no experience regarding troubleshooting switch mode power supply, you may think that the power supply have problem exactly where in fact the main board is the source of no power problem.
Switch mode power supply have a current sense circuit (if you look at UC3842 PWM IC pin 3, this stated I-sense which mean present sense) and if there is short circuit within the secondary side (either in secondary diodes or main board), the existing drawn would be increase and this may 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 associated with seconds and you do not have the chance to know if there are output voltages in 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 attracted and shut itself down whenever it detects a shorted element in the secondary side. One good instance was the power supply used in printer. Ink jet printers usually have two boards; one was your 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 is actually, one must remove the connector through the power supply board. Once the supply connection to the main board was eliminated, you can now switch on the printer plus check if there is any voltages present at the power supply connector.
If you can find zero voltages measured across all of the supply (VCC) pins then we can conclude that the power supply have issue and you can put your whole concentration within 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 2 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 major 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 supply pin (say 5 volts pin) and the main board ground and then reverse the probes. A great board should not show two similar reading and if you get two comparable 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 classes 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 range. Either one of these components shorted might lead to 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 eliminated. Assuming if you happen to remove one of the filter capacitor pin and the short circuit is fully gone then we can say that the real reason is the filter capacitor.
The real is actually what if the board has many components on it and this will consume lots of your time to isolate the problem simply by removing one pin at a time. It can be difficult to identify the supply 5 volts pin to a spider IC which has 100 pins or more. Many index IC’s have more than one 5 volts supply pin. Some even have got 4 and some have 6 to 8 provide pins.
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Does this mean you need to check one pin at a time before you finally locate the fault? Not just that, to remove the supply pin through the spider IC’s and check for any short circuit between the grounds required a very good skill too. If you messed away the circuit board track, the main board can then be considered beyond restoration. Even though you can repair the broken circuit track, this does not suggest you have solved the actual fault!