Evga 850 bq psu problem

[HelpMeRepair]

Hi guys and thanks for your help !

I woke up today and 1 of my electric panel breaker was off , and I found out that it was caused by my computer, so I decided to troubleshoot the PSU

evga 850 bq , those 2 parts : d111 a diode & q119 green circle ceramic resistance ( not sure but the emblem is like a rez ) both have no value writed on them.

They both gone bad , the res is blow on 1 side and the diode is just not doing Its job.

If someone can help me out to find what are the value of those 2 parts it would be really appreciated,I dont have the money to afford a new psu atm =\ ( hope that right to repair is coming soon xD )

Have a good one guys

[momaka]

Re: Evga psu problem

Sounds like another case of bad/blown APFC circuit (not particular to this unit, but to PSUs with APFC in general)... but I'll reserve my judgment for later.

If you could, please post some pictures of the power supply and the areas of the affected components. And please do it with the forum's built-in attachment feature.

Also, this forum has a dedicated section for troubleshooting power supplies here:
https://www.badcaps.net/forum/forumdisplay.php?f=32
... but no need to create a new thread there. Just ask a mod to move your post there.

[HelpMeRepair]

Re: Evga psu problem

Thanks for your answer , ill try to find out how to adk a moderator yo move the post , here some picture like you asked me

Hope they are good enough !

[HelpMeRepair]

Re: Evga psu problem

I forgot that picture sorry

[momaka]

Re: Evga psu problem

Hmmm... yup, most or all of those parts are part of the APFC booster circuit.

Start by making sure the primary caps are discharged. I'm sure they would be at this point, but it's always worth checking before doing any measurements. Use multimeter to check voltage. Should be 0 or very low (under 2-3V, in which case short their leads to drain the rest.)

Next, start checking all components on the input and the primary side.
- Check the fuse. It should have low resistance and not be open-circuit.
- Check the yellow "disc component" next to it - that's an MOV, which is used for clamping surges on the input line. I can't tell if the dark spots on it are because it's blown or because there's black silicone glue on it. If the former and/or if it measure low-resistance across its leads, remove from the circuit.
- Check the bridge rectifier (BD1) diodes
- The green disc component is an NTC thermistor (a resistor whose resistance decreases with temperature.) It's used to stop large inrush currents from going into the big caps when the PSU is first plugged in... but then as it warms up due to current flowing through it, its resistance decreases to decrease heat losses through it. Some PSUs will even have a relay to bypass it to further reduce losses, but I don't see one in this PSU. In any case, the NTC should have a few Ohms of "cold" resistance across its leads when measured with a multimeter, and slightly lower resistance when hot/warm. It's best to remove it from the circuit to check its resistance. Post what you find here.
- Check the two big electrolytic capacitors for short-circuit with a multimeter. With the multimeter hooked to their leads, you should NOT see any low resistance across them whatsoever after a few moments of them charging up.
- Additionally, if you have one of those component testers, remove the primary caps and check their capacitance and ESR. While Nichicon is a really good brand, APFC circuits are known to roast even good capacitor brands over time. The result is usually dry or semi-dry primary caps with partial or no capacitance and high ESR... which can cause other components on the primary side to blow up.
- Remove the diode you pointed to and test it out of circuit again. This diode is actually not part of the APFC circuit, but rather the main PS. The main PS appears to have a double-forward transistor topology. If that diode is bad out of circuit, then the MOSFET you pointed to in this picture is probably bad too... and likely would be a good idea to replace the other diode and MOSFET pair that's part of this circuit.

While waiting for / to order the replacement parts, you can do the following to further test the primary side and confirm that at least the 5VSB circuit is running:
1) Place jumper wire across green disc component (NTC thermistor)
2) If fuse is blown, remove and connect a 40-60 Watt incandescent or halogen bulb in its place. If not blown, connect a 40-60 Watt incandescent or halogen bulb in series with the Live AC wire. This is called the dim bulb trick (or DBT for short). The idea is that if there is something else shorted on the primary side of the PSU, the bulb will light up and stay lit when you plug in the PSU. If nothing else is shorted, the bulb will light up briefly (while the big primary caps are charging up) and turn off afterwards.
3) With the DBT setup and all other shorted components removed, plug in the PSU and check if you get 5V on the 5VSB wire of the ATX connector. The DBT should stay off / not be lit up. At this point, DO NOT attempt to turn on the main PS by jumpering PS-ON to ground.
4) Carefully check the DC voltage on one of the big input caps as you unplug the PSU (NOTE: you're dealing with fairly unrestricted high-voltage DC here, which can be life-threatening if not careful around.) The voltage should decrease somewhat gradually. If it instantly goes to 0 or some very low value (a few volts or low tens of volts), suspect the primary caps to be bad too. This test can be used if you don't have a component tester to check on these caps... but don't consider it very accurate. It's best to either get a component tester and check the primary caps or just simply replace them.

And that about covers most of the tests/checks you can do at this point.
If you find the diode and MOSFET you pointed to are also bad, there will be some additional components to check for the main PS (particularly the main PS PWM controller and parts associated with driving the MOSFETs' Gates.)
.
.
Yes, this can get to be quite an involved repair, but not always. Sometimes, only a few parts blow, and a simple replacement will get the PSU running again.
In any case, post the results for all of the tests and checks you find that I mentioned above.

[HelpMeRepair]

Re: Evga psu problem

Wow , I did not expect such a complete walk around from my little question in this forum ,your help is much appreciated friend !

Ill do what need to be done and test it this today or on the week end ,

I'll give you news soon

[HelpMeRepair]

Re: Evga 850 bq psu problem

hi friend , this is what i found so far , i havent made yet the dbt and tested the 5vrail , but i did un solder and test these component .

sck1r58 NTC thermistor had : 2.5 ohm

big caps was starting a 7m ohm and going down slowly

d111 was good ! ( the one circled in red behind the mosfet and the thermistor )

fast diode d102 was shorted ! ( byc10-600) 1 of the 2

q119 n 120 was good ( not common wit mosfet testing but i refered to spec sheet n tested source , gate n drain ) 0.3 ohm at source drain diode mode one way.

edit:adding picture