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221K 2KV Cap Cracked In Power Supply
Received one desktop computer for repair; stating the problem symptom of no power.
I have checked computer by applying the power to the SMPS but there is no sign of life. I have removed the SMPS from the PC and opened it for the inspection. On visual inspection I did not found any physical damage or burnout signs.
So I took out my Multimeter and started “Cold testing” of the equipment. (Here term “Cold testing” is the process to check the equipment without applying the power i.e. resistance checking and “HOT Testing” is for testing the equipment by applying POWER and doing Voltage and Current testing.)
On cold testing I found Fuse is open circuit so I have removed the fuse this is 3.15Amp 203V ceramic fuse . And attached 100w Bulb in place of fuse to find out that only the fuse is blown? Or some other culprits are cause to blow the fuse by short circuit.
So for confirmation I have applied the power to the SMPS, and bulb steadily glow Bright that means some other culprits are there (shorted components.)
Note: On this symptom of bulb – advisable to instantly “put-off” the power or you can lead to damage more components on the PCB.
On applying power I found one high value capacitor suddenly omitting black fume so instantly I have switched off the power and started the process to find culprit.
I have removed the high value capacitor which has value marked as 221K 1KV that is 220PF 1KV.
So I have again started cold testing on primary side to find what cause this part for failure. So i have start testing one by one surrounding components and I found one Zener diode shorted on PCB. I have taken out the Zener diode for the testing.
This Diode is 1N4734 5.6V 1W. I have removed this diode from the circuit because I have found one fusible .22 low Ω resister connected parallel with this diode and it may mislead you for finding shorted component.
Now I have removed the low ohm resistor and again checked surrounding components seems ok. So replaced high value capacitor and zener diode
and by re attaching the 100W bulb I have turned on the SMPS this time Bulb light up bright for a moment and goes dim. That means there is no short circuit. For conformation I have checked voltage on Power on Green wire on connector side and I got 3V DC.
That means SMPS is ready for running hence I have removed the multimeter probe from the connector and shorted green wire with black wire for starting the SMPS and instantly SMPS FAN started rotating that means now power supply is working. For my satisfaction I have tested voltages on the connector.
After this test I have connected my SMPS Tester unit and see the result.
So finally I have replaced these culprits to get back this SMPS in working condition.
This article was prepared for you by Yogesh Panchal who works as a Computer Hardware Engineer in Mumbai India.
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