Crack & Stink Gigabyte GA7N400 Pro 2

[socketa]

Removed the south bridge and, unfortunately a couple of pads 😢
If i replace the pads, will UV-curing conformal coating cure if it's underneath the pad, so that it secures the pad?

[momaka]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

i had trouble seeing those resistor colors with a magnifying glass and a torch!
(wouldn't it be easier for all if they just wrote some numbers on it?)
It looks like black brown black gold
But since it tests as 50 ohms, i guess the first black must be "green"

Good point.
Probably a 51 Ohm resistor then as that's a standard value (i.e. Green Brown Black Gold).

i cut the resistor
now readings across diode are:
forward bias = 0.154v
reverse bias = 0.588v

Not quite the results I expected, but I will check my ISO PSUs and let you know. Did you make sure to leave the multimeter for a few seconds before nothing down the measurements. The capacitors on the output of the 5VSB can take some time to charge so that can mess up the results.
I'd expect to see about 0.5 to 0.7V in forward bias and higher (if any at all) reverse bias results. But again, I can check.

Also the resistor that is between the emitter of C945, and the emitter of C5353, had a hard short
(diagram attached)

Could be just a low-value resistor. Did you check its color bands? Resistors almost always fail open circuit. I haven't actually seen one fail as a short circuit. With diodes, it's the other way around.

As can be seen, for educational purposes, i spent the day tracing out some of the primary side circuit, after being inspired by everell's thread on the 311 mod:
https://www.badcaps.net/forum/showthread.php?t=16071

There could possibly be a mistake in it, even though i went over it twice.
Does it look right?

Hard to tell without the resistor values, but many parts of it look about right. I'll trace mine when I can and try to post it here.

Both of the windings are on the same transformer.
i think the drawing could be tidied up by removing the 2 long bottom lines, and replacing them with symbols for common, but i'm not sure if they are actually "common"

They could be "common", but it would be the primary side common (which could be 160V lower than ground). Secondary side common, on the other hand, is ground (or should be unless you are running ungrounded outlets).

Finding this quite intriguing, and am keen to give the mod a shot, sooner or later.

Yeah, everell's 311 mod is neat! I bought two 311 chips but haven't used them yet. A lot safer and a little bit more efficient than 2-transistor design. Should be easy for the most part, though.

[mariushm]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Yes, the multimeter has diode test mode
What does that do, that the continuity tester doesn't?
What does the voltage reading indicate?

Continuity tester measures continuity - at a resistance below a certain threshold (some have 20 ohm, some less) the meter will beep and later (if you keep probes connected) it may show the actual resistance.
If the resistance is above a threshold, it won't say there's continuity (try putting probes on the leads of a 100k resistor and see if it says continuity.

Diode test sends current from one lead to the other and measures the voltage drop on the diode. A good diode will show a voltage drop with the leads in one position and no voltage drop if you reverse the leads (the diode blocks current flow).

Exceptions are zener diodes and I think some more obscure diode types (but I can't say one off the top of my head). Zener diodes conduct when a particular voltage is reached, so the way to test them is to put a resistor in series to limit the current to a few mA and then use a power supply with current limit to slowly raise the voltage - when the voltage zener works at is reached, it will conduct and you'll have a short and you'll have those mA flowing through the diode and the power supply will enter current limit mode.

Resistance and continuity and diode test use different voltages sometimes. My ut61e uses -0.35v for resistance, -2.5v for continuity, 2.8v for diodes. The current can also vary - you may be able to light up leds using diode test, but the resistance modes may be limited to a few mA, a value below the amount a led would turn on.

[socketa]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Good day to you.

i had trouble seeing those resistor colors with a magnifying glass and a torch!
(wouldn't it be easier for all if they just wrote some numbers on it?)
It looks like black brown black gold
But since it tests as 50 ohms, i guess the first black must be "green"

Yes, the multimeter has diode test mode
What does that do, that the continuity tester doesn't?
What does the voltage reading indicate?

i cut the resistor
now readings across diode are:
forward bias = 0.154v
reverse bias = 0.588v

Also the resistor that is between the emitter of C945, and the emitter of C5353, had a hard short
(diagram attached)

As can be seen, for educational purposes, i spent the day tracing out some of the primary side circuit, after being inspired by everell's thread on the 311 mod:
https://www.badcaps.net/forum/showthread.php?t=16071

There could possibly be a mistake in it, even though i went over it twice.
Does it look right?
Both of the windings are on the same transformer.
i think the drawing could be tidied up by removing the 2 long bottom lines, and replacing them with symbols for common, but i'm not sure if they are actually "common"

i have another iso psu, similar to this one, and also a dm311 chip out of a dead psu that i found.
Finding this quite intriguing, and am keen to give the mod a shot, sooner or later.

[momaka]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Just to make sure i've got this straight...

Looking at the example schematic (in attachment below),
the 'circuit' (orange) that goes to the ps-on terminal, is teed-off from the 5vsb output.
Also there is an independent 'circuit' (purple) that originates from the transformer, that feeds one of the pwm controller (TL494) terminals.

Am picking that the "secondary-side auxiliary rail" is the purple path - since the ps-on output is already live, along with the 5vsb output.

i can see that the orange circuit also powers a transistor, so am guessing that when the PC power button is pressed, that that transistor must liven up the TL494 controller.

Does all of that sound correct?

Yes, all of that is correct.

there is continuity from 5vsb output to one side of the diode, but no continuity across the diode.
forward or reverse bias, multimeter displays 50ohms

Should this diode have continuity in one direction, or is it faulty?
(photo of this diode in attachment below)

If your multimeter has a diode test function (usually, but not always, combined with continuity on most multimeters), then the diode should show a reading one way but not the other. HOWEVER, most 2-transistor 5VSB circuits tend to have a load resistor to keep them stable when there is not much load on it from the motherboard. 47, 51, and 100 Ohms are very common resistor values here. If your PSU has one, that can easily mess up your readings.

Looking at the last picture you uploaded, your load resistor seems to be 100 Ohms. Remove it and double check that diode again (that would be D23). If you still get 50 Ohms, then the diode could as well be bad.
... Or, remove the diode out of the circuit and measure it. It's almost the same work as removing the resistor (though, the diode has thicker leads, so naturally, it is easier to remove the resistor).

[socketa]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Thanks momaka,
Just to make sure i've got this straight...

Looking at the example schematic (in attachment below),
the 'circuit' (orange) that goes to the ps-on terminal, is teed-off from the 5vsb output.
Also there is an independent 'circuit' (purple) that originates from the transformer, that feeds one of the pwm controller (TL494) terminals.

Am picking that the "secondary-side auxiliary rail" is the purple path - since the ps-on output is already live, along with the 5vsb output.

i can see that the orange circuit also powers a transistor, so am guessing that when the PC power button is pressed, that that transistor must liven up the TL494 controller.

Does all of that sound correct?

i also checked this iso psu pcb with a continuity meter, and see that the 5vsb output comes from a inductor and then a diode, and then from the transformer.

there is continuity from 5vsb output to one side of the diode, but no continuity across the diode.
forward or reverse bias, multimeter displays 50ohms

Should this diode have continuity in one direction, or is it faulty?
(photo of this diode in attachment below)

[momaka]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

^ Excellent point.

[Agent24]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

For the record, ATX and similar supplies have standby power. Older PSUs like AT and XT don't.

[momaka]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

bad output caps >> incorrect operation of pwm controller chip >> excessive voltage across the 5vsb secondary transformer winding

Almost .
more like, bad output 5VSB caps >> excessive voltage across the 5vsb secondary-side auxiliary winding >> burnt resistor >> erratic operation of PWM controller

Better yet, let's have us some PC PSU "101 basics"...

Some common terms:
I/P - short for Input; likewise O/P is same as output.
When referring to a voltage that the PSU produces, the following terms are sometimes used interchangeably: rail, supply, line (or supply line, and output (IIRC, there was some confusion about that either in this thread or in the other, so that's why I added this).

To keep things simple, I will not get into the circuits of the power supply too much here, just the general concepts.

In general, there are actually two power supplies in a PC PSU: the stand-by supply and the main power supply.
The stand-by supply is always turned ON whenever the PSU is plugged in the wall. It is meant to provide only a small ammount of power to the motherboard and the PSU's chips (i.e. PWM controller and other ICs). The stand-by supply generates the 5VSB line and the so-called (by me, at least) "secondary-side auxiliary supply/rail". 5VSB is usually only used by the motherboard. On the other hand, the secondary-side auxiliary rail is not available to the motherboard at all and is entirely dedicated to providing power to the PSU's ICs. So, in other words, when the computer is OFF but plugged in the wall, the motherboard is provided with 5V via the 5VSB line and the PSU's IC's are provided a voltage from the secondary-side auxiliary rail; that way, they are ready and awaiting a "turn-ON" signal (usually known as PS-ON). When the signal is given by the motherboard (that is, when you press the power switch on your PC), the PSU's ICs "wake up" and turn ON and control the main power suppy.
The main power supply is meant for providing power to the PC whenever it is ON (and sometimes in S1 stand-by). It generates 3.3V, 5V, 12V, -12V, and sometimes -5V (usually older PSU designs - though I should note that a lack of -5V does not mean the design of the PSU is newer).

[socketa]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Yes,
just found out that another psu has short circuit protection, when i accidentally shorted
it - thought that it would have blown a fuse, but it started back up again , no problem.

so it seems that there is some component/circuit that automatically resets itself after a short circuit condition

Failure of the PSU likely led to the incorrect voltage

referring to post #34:
what is meant by "secondary-side auxiliary supply"?

does that just mean that a greater voltage is now on the secondary-side of the 5vsb transformer because the pwm is not controlling the voltages correctly as a result of a bad output cap?
i.e.,
bad output caps >> incorrect operation of pwm controller chip >> excessive voltage across the 5vsb secondary transformer winding

[Agent24]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

So was thinking that this "short" could be on the motherboard, which, in-turn , could upset the PSU.

Unlikely it happened that way around as most PC PSUs just shut down when their output is shorted, so the only way to kill a PSU with a shorted motherboard is if the PSU itself is a totally trash design that has no short circuit protection - which would be a PSU not worth using in the first place, and its death would be of little importance...

(Note, I am ONLY talking about PC\ATX PSUs here, MANY other PSUs (including SMPS and linear) do NOT have short-circuit protection and shutdown)

The fact you already have bad capacitors shows it's most likely the PSU that went bad first. momaka already explained the most logical scenario in post #34.

The bad capacitors in your PSU caused instability and\or other improper operation and likely resulted in the resistor burning out in the PSU.

Failure of the PSU likely led to the incorrect voltage going to the motherboard on one or more rails, which killed it.

[socketa]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

The pic was only to make sure that the component circled was a zenner diode - that's all

Anyway, i didn't know that those chokes got hot.
So, perhaps that also contributed to the resistor burning up, since it was directly underneath it, and also surrounded by 3 caps.

Again, as long as you have those load resistors on the PCB

Which PCB are you referring to?
i was clearly referring to the measurement that i got from the motherboard.
But if by "load resistors" you mean "minimum load resistors" it seems that you are referring to the PSU.
Furthermore, i understood what agent24 said.

the motherboard doesn't need one because it's not a PSU, it is a load.

before submitting my last post.

If you re-read my posts on this page, i tried to make it very clear that i was referring to the motherboard when i took this 6.2ohm measurement.

This line of thought was initially prompted when agent24 said:

Resistors usually burn when something they supply power to is shorted, or they see an overvoltage condition

So was thinking that this "short" could be on the motherboard, which, in-turn , could upset the PSU.

OK, so was using the term "rail" incorrectly,
and should be using "output" instead.
Lesson learned

[mariushm]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Again.

There's no 5v SB rail... 5v SB (stand by) is just ONE WIRE. The red wires are 5v which is produced by a different section of the power supply.
So you have 5v standby , normal 5v , 3.3v , 12v , -12v and possibly -5v. SEPARATE 5v SB and SEPARATE 5v.

You have two capacitors on each 3.3v/5v/12v rail because you have on each rail a PI filter ... capacitor -> inductor -> capacitor. This helps smooth out the ripple, makes the voltage more stable. Those coils of wire on a black tube are the inductors.

The big round things with wire are chokes/inductors but they're used in a different way.

Those inductors will be HOT during operation, especially at high currents. The load resistors on each rail will also be HOT. The fan moves the air around so there won't be during normal operation spots of heat but still that area will get warm.

Capacitors heat up due to electricity going through them, they heat up due to leads and traces bringing heat from the inductors and the load resistors and so on. In time, this heat will damage very slowly the capacitors, the capacitors will change their electric characteristics which may just mess up the circuit and cause some part of the power supply to work erratically or "stress" itself more than normal, which in turn can damage other parts.. you basically have a cascade, a chain effect...


gives a continuous beep, and 15.3 ohms, across the position where that that big red cap was on the 3.3v rail.

Again, as long as you have those load resistors on the PCB, they're going to screw up your measurements.

Your multimeter shows continuity as long as the resistance is below a threshold, let's say 20 ohm (it varies from meter to meter).

It then says 15 ohm, which could be the load resistor, it could be something else. The load resistor is between + 3.3v and ground and the capacitor leads are on +3.3v and ground, so you're measuring across that load resistor.

15.3 ohms is about right for a 15 ohm +/-5% resistor and about right for a load resistor . V = I x R => 3.3v = I x 15 => I = 0.22A so the resistor makes sure there's always at least 0.22A drawn from the 3.3v rail.

ps. And I don't know what the hell you mean with the first picture because - as I already complained in a previous post - you cropped the picture (again) in such a stupid way that I can't be bothered to try to determine where it's located in the power supply.

[socketa]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Yes, i get the pin-out
Had a tiring day and wasn't thinking straight and said 5v, while meaning 12v
(had being so focused on a 5vsb diagram, that the number 5 was stuck in my mind)

So, could that 6.2ohm low resistance on the 3.3V rail (on the actual motherboard), a possible trigger that could have initiated the frying of that resistor on the PSU?

i get the feeling that that was probably not the case.
And that this low resistance fault? is possibly due to a south-bridge that was damaged by the PSU
(seem to recall reading that the south-bridge can sometimes be powered by the 3.3v rail)

Now regarding the PSU,
i now see, after following the traces, that the small green cap is on the 12v rail,
the 2 middle sized green caps are on 5vsb rail,
and the big orange one is for 3.3v rail,
(thumbnail picture provided below)

Additionally,
Testing from either side of that burned-out resistor:
No hard shorts to any rails or ground
One side goes to the Vcc pin of the 16-pin PWM control pin, and the other side goes to a diode (which shows no continuity, but gives good diode test - so that looks like a zenner diode)

Is the most likely initial cause of this incident (the dead mobo+PSU) due to the worn-out caps degrading the performance the performance of 3.3v, and the 5vsb rails?,
(where excessive voltage on either of these rails, would likely result in motherboard damage)

Further testing, in continuity mode, gives a continuous beep, and 15.3 ohms, across the position where that that big red cap was on the 3.3v rail.

cheers

[mariushm]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Hence, was thinking that this low resistance to ground (on the motherboard 3.3v rail), perhaps via a burned-out southbridge - may have cooked the resistor,
as agent24 suggested

But now can see that if a psu has a minimum load resistor on a rail (to ensure a minimum current), then so might a motherboard,
so the 6.2ohm reading might be due to a minimum load resistor on the mobo - rather than a short.

No, motherboards don't need load resistors. In order to adjust the number of pulses going through the transformers, the power supply needs to monitor the output voltage on a rail and adjust accordingly. However, the power supply is tuned to keep the voltage through a wide range of current output, let's say from 1 to 30A.
It's just not able to monitor the output voltage properly at extremes (0A for example) so it adds that resistor to use 0.3-0.5A all the time.

The motherboard doesn't produce output, it just takes whatever it gets so there's no load resistors. You probably have a short somewhere. In theory, you could determine the short location with a good multimeter - put it in resistance mode, put black probe on ground somewhere and with the red probe, you can follow the pcb trace for 3.3v. As you get close to the short, the resistance should decrease (but by milliohms, so again you need a good multimeter)

So that yellow area contains the output caps for the 5vsb
Noticing how it wiggles to the left a bit, does that imply that it would also include the cap that i removed?
If so, then it is that little one in post #21 - (the one that has bulged and opened-up the most)

No. Colors are (sort of) standardized. Orange is 3.3v, red is 5v, yellow is 12v.
There's only one wire for 5v standby which is usually purple, white is -12v and blue is -5v
But since -5v is no longer required and they may have used blue for 5v standby. Just check the atx connector, see the wire color... i posted the atx connector pinout in another thread of yours but if you can't find it just google "atx psu pinout" or something like that.

[Agent24]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

But now can see that if a psu has a minimum load resistor on a rail (to ensure a minimum current), then so might a motherboard,
so the 6.2ohm reading might be due to a minimum load resistor on the mobo - rather than a short.

The PSU has minimum load resistor(s) to keep it working correctly and stable, the motherboard doesn't need one because it's not a PSU, it is a load.

[socketa]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

Just in case there is any misunderstanding,
When i said that "there is a short to ground (6.2ohm) on the mobo 3v rail",
i was not referring to the 3.3v rail on the psu - but rather, the motherboard

Hence, was thinking that this low resistance to ground (on the motherboard 3.3v rail), perhaps via a burned-out southbridge - may have cooked the resistor,
as agent24 suggested

But now can see that if a psu has a minimum load resistor on a rail (to ensure a minimum current), then so might a motherboard,
so the 6.2ohm reading might be due to a minimum load resistor on the mobo - rather than a short.

Anyway thank you, guys, for highlighting the caps with rails colors and for the explanation/interest.

Am guessing that since the psu diagrams, that i'm looking at, don't have that 8-pin DIP IC, that transistors would be doing that job in those diagrams

So that yellow area contains the output caps for the 5vsb
Noticing how it wiggles to the left a bit, does that imply that it would also include the cap that i removed?
If so, then it is that little one in post #21 - (the one that has bulged and opened-up the most)

[momaka]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

just found the culprit that made the stink (and possibly the crack)
am guessing it was caused by one or two bad caps (photo in previous post) stressing the resistor out?

I'm guessing this is the same PSU in this thread:
https://www.badcaps.net/forum/showth...392#post463392

I traced the circuit. That resistor supplies the PWM controller with power (that 8 pin DIP IC). Looks like indeed scenario #2 (from post #13 I made in that thread above) was what happened here - i.e. caps on the output of the 5VSB went bad, so the 5VSB regulation went bad too. This made the secondary-side auxiliary supply for the PWM controller go over-voltage and probably killed the PWM IC ar/or made it do weird things.

Third time I see this now on a 2-transistor 5VSB design. I just recalled that I had another PSU - a Morex MXA-235PTF1 with the same problem (though, it's resistor didn't get to cook quite yet and so PWM controller seems to have survived). I recapped the 5VSB output on it with good caps, and it's rock-stable at 5.02V now.

there is a short to ground (6.2ohm) on the mobo 3v rail

Most likely you are reading a minimum load resistor, as mariushm suggested. Not uncommon to see 2W or 3W resistors between 4.7 Ohms and 47 Ohms on the 3.3V rail. Your 5V rail seems to have one too. Probably 27 Ohms or 47 Ohms (I find these most common).

[mariushm]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

thanks

there is a short to ground (6.2ohm) on the mobo 3v rail

No, there's minimum load resistors on 3.3v and 5v to keep the output within regulation. Think there's even minimum load resistors on 12v and -12v but not sure.

3.3v 6.2 ohm .. v = i x r => i = 3.3/6.2 = 0.53A P = IxIxR = 0.53 x 0.53 x 6.2 = 1.7w

That looks like a 3w resistor to me, so the math works out:

[Agent24]

Re: Crack & Stink Gigabyte GA7N400 Pro 2

If the output capacitors died in the PSU it's possible the PSU became unstable to the point that components would be damaged, and also resulted in the death of the motherboard.

I don't know where the resistor goes from your photo, unfortunately, but if you're not going to attempt repair I guess it doesn't matter.