Repair of PSU Litepower 650w

[cekata345]

Hi,
PSU did not start.
I can`t find it wiring.
But...
On purple wire (Vsb) i have 5.1V
On green wire (PSON) i have 2.62V. On PCB it is
connected via 470 ohm resistor to pin #4, but in datasheet is
given to be 300 ohm resitor. Also it has capacitor to GND. Also on
PCB there is no 10K resistor to 5VSB.
This PSU use IC WT7502V
On pin #7 (VCC) i have 4.56V. This pin
have this voltage from purple cable via diode. It also
has second diode from +12V rail.
On other pins i don`t have any voltages.
What next to check?

 

[cekata345]

Now i notice that post look very strange.
I will upload text as picture

 

[sagor1]

Is this still in the device, or on the test bench. This sounds like a computer power supply. To run on the bench, you must short the green wire to ground (black wires) and most supplies need a small load on the +5v rail I order to start properly.

 

[cekata345]

Is this still in the device, or on the test bench. This sounds like a computer power supply. To run on the bench, you must short the green wire to ground (black wires) and most supplies need a small load on the +5v rail I order to start properly.

It is from computer, not bench.
I know for shorting green to ground. This doesn`t work. Also in computer did not work.
I try to fix it.

 

[Nigel Goodwin]

Historically, I've repaired a number of PC power supplies over the years, and the usual fault was failure of the start-up resistors, as they commonly used two resistors in series, which makes for an unreliable design, and very prone to failure.

 

[cekata345]

Historically, I've repaired a number of PC power supplies over the years, and the usual fault was failure of the start-up resistors, as they commonly used two resistors in series, which makes for an unreliable design, and very prone to failure.

Did you mean 2 megaohm resistors before diode bridge? Here there is no such resistors. I have fuse - LF!, capacitor, LF2, two small (blue) capacitors from every line to ground and after them is diode bridge.

 

[Nigel Goodwin]

Did you mean 2 megaohm resistors before diode bridge? Here there is no such resistors. I have fuse - LF!, capacitor, LF2, two small (blue) capacitors from every line to ground and after them is diode bridge.

They are usually after the bridge rectifier, and provide start-up power for the chip - assuming your specific PSU uses them?.

If you can identify what chip it uses, try and download it's datasheet, that should give you a good idea of what's going on.

Bear in mind the entire primary side is live mains, so you can't use an earthed scope on it, unless you use an isolation transformer.