New project, a Dell Model L350N-00 350 watts. Opened the case and found four bulging capacitors. So removed the pc board from the case and replaced the bulging capacitors. Then applied power to check out the power supply. All voltages came up fine. So I sat back looking at my newly repaired psu, and about two minutes after power up, there was a bang, crackle, sparks flying. I shut off the power and sat back looking at my newly fixed wreck, wondering what happened when I began smelling paper burning. Didn't see anything......so lifted up the power supply pc board and the paper it was sitting on was on fire! A small fire which I quickly extinguished, but a fire none the less. So I sat back looking at the mess......WOW!!!
Guess I better take a look see and find out what happened. Smoke covered about 75% of the bottom of the board. So I got out the alcohol and brush. It cleaned up well, so well I couldn't see where the damage was. Thinking the sparks probably came from the switching transistors shorting, I measured them with ohm meter. Everything looked good. Checked 5vsb power transistor...also good. Checked the output diodes.....all good. Checked the fuse....also good. What in the world is going on???
On one corner of the bottom of the pc board, the manufacturer had put a bunch of that white silicon stuff, then attached a piece of plastic about one inch by one inch. I suppose this was to prevent arcing to the case. So I decided to remove it. As I was pulling the plastic away, the white stuff started crumbling, and throughout the white stuff were small black pockets. Using screwdriver, I scraped away all the white stuff, then cleaned the board with alcohol. All that was covered was the snubber for the main switching transistor. The protection diode was shorted, and the filter capacitor 2.2 uF 450 volts was very high esr, and the filter resistor checked out OK. Some of the circuit trace had been blasted away.
With pc board cleaned up and damaged parts removed, I decided to try powering up without the snubber - as it is only a protection circuit. I turned on power, and the power supply came up just fine. All output voltages normal. Then shut down power as I do not like running without the snubber.
Root cause analysis of this failure: The white stuff goes conductive with age and causes some power supplies to go ........ snap, crackle, and fart (err....pop).
Guess I better take a look see and find out what happened. Smoke covered about 75% of the bottom of the board. So I got out the alcohol and brush. It cleaned up well, so well I couldn't see where the damage was. Thinking the sparks probably came from the switching transistors shorting, I measured them with ohm meter. Everything looked good. Checked 5vsb power transistor...also good. Checked the output diodes.....all good. Checked the fuse....also good. What in the world is going on???
On one corner of the bottom of the pc board, the manufacturer had put a bunch of that white silicon stuff, then attached a piece of plastic about one inch by one inch. I suppose this was to prevent arcing to the case. So I decided to remove it. As I was pulling the plastic away, the white stuff started crumbling, and throughout the white stuff were small black pockets. Using screwdriver, I scraped away all the white stuff, then cleaned the board with alcohol. All that was covered was the snubber for the main switching transistor. The protection diode was shorted, and the filter capacitor 2.2 uF 450 volts was very high esr, and the filter resistor checked out OK. Some of the circuit trace had been blasted away.
With pc board cleaned up and damaged parts removed, I decided to try powering up without the snubber - as it is only a protection circuit. I turned on power, and the power supply came up just fine. All output voltages normal. Then shut down power as I do not like running without the snubber.
Root cause analysis of this failure: The white stuff goes conductive with age and causes some power supplies to go ........ snap, crackle, and fart (err....pop).
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