We've looked at what action to take if we have a short-circuited ATX PSU fuse. This means that the problem is somewhere in the high-voltage part, and we need to ring the diode bridge, output transistors, power transistor or mosfet, depending on the model of the power supply. If the fuse is intact, we can try to connect the power cord to the power supply, and turn it on with the power switch located on the back of the power supply.
And here a surprise may await us, as soon as we flip the switch, we can hear a high-frequency whistle, sometimes loud, sometimes quiet. So, if you heard this whistle, do not even try to connect the power supply for tests to the motherboard, assembly, or install such a power supply in the system unit!
The fact is that in the duty voltage circuits (duty room) there are all the same electrolytic capacitors familiar to us from the last article, which lose capacity when heated, and from old age, their ESR increases, (in Russian for abbreviated ESR) equivalent series resistance ... At the same time, visually, these capacitors may not differ in any way from the working ones, especially for small denominations.
The fact is that at small denominations, manufacturers very rarely arrange notches in the upper part of an electrolytic capacitor, and they do not swell or open. Without measuring such a capacitor with a special device, it is impossible to determine the suitability of work in the circuit. Although sometimes, after soldering, we see that the gray strip on the capacitor, which marks a minus on the capacitor case, becomes dark, almost black from heating. As the repair statistics show, next to such a capacitor there is always a power semiconductor, or an output transistor, or a duty room diode, or a mosfet. All these parts generate heat during operation, which adversely affects the life of electrolytic capacitors. I think it will be superfluous to explain further about the performance of such a darkened capacitor.
If the cooler of the power supply unit has stopped due to the drying out of the grease and clogging with dust, such a power supply unit will most likely require replacing almost ALL electrolytic capacitors with new ones, due to the increased temperature inside the power supply unit. The renovation will be quite dreary and not always advisable. Below is one of the common schemes on which Powerman 300-350 watt power supplies are based, it is clickable:
Powerman ATX powerman circuit
Let's take a look at which capacitors need to be changed in this circuit, in case of problems with the duty room:
So why can't we whistle a PSU to an assembly for testing? The fact is that there is one electrolytic capacitor in the duty room circuits, (highlighted in blue) with an increase in the ESR of which, we increase the duty voltage issued by the power supply to the motherboard, even before we press the power button of the system unit. In other words, as soon as we clicked the rocker switch on the back of the power supply, this voltage, which should be equal to +5 volts, goes to our power supply connector, the purple wire of the 20 Pin connector, and from there to the computer motherboard.
In my practice, there were cases when the standby voltage was equal (after removing the protective zener diode, which was in the short circuit) +8 volts, and the PWM controller was still alive. Fortunately, the power supply was of a high quality, Powerman brand, and there was a 6.2 volt protective zener diode on the + 5VSB line (this is how the duty room output is indicated on the diagrams).
Why is the Zener diode protective, how does it work in our case? When our voltage is less than 6.2 volts, the zener diode does not affect the operation of the circuit, but if the voltage becomes higher than 6.2 volts, our zener diode goes into a short circuit (short circuit) and connects the watchdog circuit to ground. What does it give us? The fact is that by closing the duty room to the ground, we thereby save our motherboard from supplying it with those same 8 volts, or another nominal overvoltage, along the duty room line to the motherboard, and protect the motherboard from burnout.
But this is not a 100% probability that in case of problems with capacitors, the zener diode will burn out, there is a probability, although not very high, that it will go into an open circuit, and thereby not protect our motherboard. In cheap power supplies, this zener diode is usually simply not installed. By the way, if you see traces of burnt PCB on the board, you should know that most likely some semiconductor went into a short circuit there, and a very large current flowed through it, such a detail is very often the cause, (although sometimes it happens that it is also a consequence) breakage.
After the voltage at the duty room returns to normal, be sure to change both capacitors at the duty room output. They may become unusable due to the supply of overvoltage to them, exceeding their nominal value. Usually there are capacitors with a nominal value of 470-1000 microfarads. If, after replacing the capacitors, we have a voltage of +5 volts on the purple wire, relative to the ground, you can short-circuit the green wire with the black one, PS-ON and GND, by starting the power supply, without the motherboard.
If at the same time the cooler starts to rotate, it means with a high degree of probability that all voltages are within the normal range, because our power supply unit started up. The next step is to verify this by measuring the voltage on the gray wire, Power Good (PG), with respect to ground. If +5 volts is present there, you are in luck, and all that remains is to measure the voltage with a multimeter at the 20 Pin power supply connector to make sure that none of them are dragged out too much.
As you can see from the table, the tolerance for +3.3, +5, +12 volts is 5%, for -5, -12 volts - 10%. If the duty room is normal, but the power supply does not start, we do not have Power Good (PG) + 5 volts, and there is zero volt on the gray wire relative to the ground, then the problem was deeper than just with the duty room. We will consider various options for breakdowns and diagnostics in such cases in the following articles. Successful repairs to everyone! AKV was with you.