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HI,

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I have a HP8595E with a problem of power supply. The power supply doesn't start when the line switch is on, all the power-supply LEDs remain off. I check the shematic diagram and i find a diode failed (short circuit) CR306 (HSMS-2800) on the PSU control board.?

I don't understand the function of the AOP U303D on the shematic diagram, so i unsoldered this diode CR306 and i don't replace it immediatly because i didn't have any. I then tried without this diode and the power supply was running well.?

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VSTBY is 15V, the voltage on the pin 10 U303D (KS_EN) is about 6V (VRAIL is about 320V and the resistor divider 632K and R311 12K1 provide this voltage). The voltage on pin 11 U303D is 7,5V by the divider resistor R309/R310 100K/100K. So the output pin 13 of U303D is Hi arround 13 V. When the switch line is on, the PWRON_L is connected to the FB_GND and so the there is no signal on TR301-7 and on the? anode of CR302, voltage on pin 6 of U303B is 0V the power supply is on.?

With these diode CR3036 on short circuit it was impossible to start the power suply because event if? there is no signal at the output of T301-7 and the anode of CR302, the output of U303D maintains a voltage on pin 6 of U303B and pin 8 U303C.

So what is the function of this U303D ? in my opinion the output of U303D never gone 0V and even if this output is at 0V (it would be possible if the +VRAIL failed) what? would be the desired function knowing that the shutdown of the power supply is conditioned by a hight level on the anode of CR306. If anyone understand the interest of this part of the diagram U303D I would apreciate the answer.