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Cisco Router Switch- No Power Problem Repaired
The complaint was no power. The moment I connected the AC supply there was no sound and was very quiet. If there is sound or the fan rotates for a while and stopped then I can roughly know where to check for the problem. I opened up the casing and saw only two boards i.e. the power supply and the main board as seen from the photo below:
After that I tried to scan for any bulged electrolytic capacitor but all seems to be normal. Here is what I had observed:
1) The power supply was quite after turning it On
2) No bulged electrolytic capacitors
3) No burnt components
From above, my mind suggested that:
1) The main fuse could have an open circuit
2) One of the secondary output diodes could have shorted
3) No DC supply (VCC) or improper voltage to the power IC
4) Bad component/s in the primary section. It could be an open start up resistor, shorted run DC diode, or even partial shorts in the transformer primary winding.
The first component that I have check was the main fuse and it was Ok. Then I powered On the set and check if there is any 300VDC across the main filter capacitor. Yes the 300VDC was present and this suggested that I should perform the voltage test on the VCC line of power IC. When I was done placing the probes on the VCC supply line and to the hot ground and powered On, I got about 3 volts which were abnormal.
It should have at least 14 to 16 volts. Now I got the clue at where I should troubleshoot the fault. Here are the things that were in my mind again:
1) The startup resistor could have gone up in resistance value.
2) The filter capacitor for the VCC supply line to the power IC could have problem (bad ESR value).
3) The power IC (UC3845) itself could have problem and pulled down the supply voltage
4) The related component could have direct or minor short circuit that had pulled down the supply voltage.
Ok here is another round of checking. The startup resistor of 160K ohm checked to be within range. Next was the filter cap connected along the supply line. It was 10 uf 50 volt and checked to be good. Now left only power IC and the corresponding components. I desoldered the power IC VCC pin and powered On and the voltage was still 3 volts. This suggested that it has nothing to do with the power IC and the problem could be corresponding components.
I started to trace along the dc supply line and found that besides the supply going to the power IC (UC3845) it also goes to the VCC pin of LM393N. I took out my analog meter set to x100 and checked between the supply line and hot ground I found that it showed shorted reading. When I removed the VCC pin 8 of the LM393N IC the shorted reading was gone.
So I took out the IC and checked between pin 8 (VCC) and pin 4 (Ground) and the reading was shorted as seen from the photo below. A good one should not show two similar readings when checking on the VCC and the ground pin.
A replacement of LM393 IC cured the fault. Ok I’m not sure what are the procedures you use to troubleshoot power supply with no power symptom but the procedure that I have just described can be applied to any type of power supply in the market. The power IC has to get the right supply voltage in order to function well.
Now the above checking method is different if you are troubleshooting a power supply that has the standby power. The standby power supply has to work first to produce 5 volt to the mainboard before the MCU IC sends out an On signal to trigger the main power supply.
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