Electronics > Repair

Help Please, Switching Power supply shutting stright off.

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Hi, hopeing someone out there might be able to help me diagnose a problem i have with an old PSU.
When i first came to test it was completely dead and upon opening it up it was clear that the few of the capacitors had swollen and show signs of starting to leak, so i decieded to recap the board and replaced them all, before reassembling i test it and all apeared to be working. However once back together the supply was dead again, so i took it apart to further test but could not find the fault and when testing it all apeared to work again. All voltages present the only issue being that the 12v is more like 11.6 but thats not uncommen on psus form this era so i put it back together only for it to be dead again.
Upon futher investigation i have discovered that 99% of the time when out the case it starts up and works fine but as soon as it is screwed back in to the case it starts up very beifly then shuts straight off again. As you can see from the images attached the circuits at the 2 screw points are only connected by the case (the yellow wire has been added by me to create this connection for testing out of the case) and it is connecting these together that causes the supply to stop working.
To put it blunt connecting the capcitors at C2 onto the negative rail stops the supply working.
Has anyone any ideas as to what could be causing this?

Normally the first step in SMPS repair is to identify the controller IC and download its datasheet.
Very often the Typical Application circuit is copied quite closely by PSU manufacturers.

From your pic it seems the controller is on the daughter board.

Normal failure points
VCC flyback diode
VCC cap
HVDC dropper resistor or chain of.

Controller operation can be tested with no mains supply with just a DC supply to the IC and scope its output to the switching device.
Good luck.

Check the resistance between the yellow wire and the negative terminal of C5.

Resitance between yellow wire and c5 negative terminal starts at 5 mega ohms and then climes up as the caps in ciruit charge. Also if i disconnect the yellow wire and test the negative terminal of c5 to the ground side of c2 i get an open ciruit.

Look at two similar diagrams.
Maybe you will find something useful for yourself.


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