GPU Dead! Fans Spinning, GPU heats up, Black screen on monitor. Bad mosfet?

Thread Starter

Johnston Cabrera

Joined Jul 28, 2019
3
Hi everyone.

I am new here so I hope that I am posting on the right section of the forums.

Here is my problem, the card I have is a GTX 750 Ti from MSI which is a Twin Frozr 2gb card. I was playing a game and suddenly the screen went black. CPU crashed. I thought it was nothing but when I powered the PC on, there was no display. After a few seconds or a minute, the GPU fans will start to spin 100% (based on the noise) and my monitor would detect a display although only black. I have already tried several troubleshooting steps including testing the card on a different PC.

The computer still detects the card because if I use my onboard graphics, the device manager still see's my card but there is an error code 43. I have also tried flashing the bios to no avail. I have had the chip reflowed to no avail. Only thing I can think of based on youtube tech videos is a short component. Most of the videos and forums I went into says that the mosfets are the problem.

So, I disassembled the card and found one mosfet, using my self made continuity tester (LED lights up when circuit is complete), I placed one probe on the ground and probed on the mosfet. Here is a picture



APL1117
CP94F

is whats written on it

So one of my probes is on the ground whilst I probe the feet of the mosfet both left and right (I don't know much about electronics so please forgive me) the led lights up. Could this mosfet be the culprit? sadly I don't have a multitester as of now but will be getting one soon.

Hope someone can help.

Thanks!
 

dl324

Joined Mar 30, 2015
10,505
Welcome to AAC!
So one of my probes is on the ground whilst I probe the feet of the mosfet both left and right (I don't know much about electronics so please forgive me) the led lights up. Could this mosfet be the culprit? sadly I don't have a multitester as of now but will be getting one soon.
All MOSFETs have a parasitic diode between the drain and source. What does your homemade continuity tester do when you forward bias a diode? Unless you have a dead short between two of the terminals, you can't test the device in-circuit.

I consider graphics cards to be a disposable item (not for end user repair). If the MOSFET is defective and you replace it, how can you be certain that there aren't other problems that you didn't find that will result in erratic behavior?

In my decades of using computers, many of them self built with cherry picked modules, I've had two graphics cards go bad. As soon as I isolated the problem, I swapped in a new card and didn't entertain a repair attempt.
 

Thread Starter

Johnston Cabrera

Joined Jul 28, 2019
3
Thanks for the reply.

I know that gpus tend to be disposable I also have an RX 470 which is what I am using now, I basically just want to learn something. If this card does not work, I might as well just re-purpose it. Maybe use its shroud as additional cooling system inside my case. If it does work then hey, thats something, maybe I'll use it as some trophy for venturing into the unknown.

All MOSFETs have a parasitic diode between the drain and source. What does your homemade continuity tester do when you forward bias a diode? Unless you have a dead short between two of the terminals, you can't test the device in-circuit.

Not sure what this means but, my DIY tester is very basic. has 2 probes, when touched, lights up a small led powered by 2 aaa batteries. So this mosfet when I put the black probe on the right leg and red on the middle which is cut light comes up. same as when red is in left leg. same if I do it in reverse.

I just want to know if bad mosfets like this can be checked with a continuity tester.

I appreciate your response. Thanks again
 

dl324

Joined Mar 30, 2015
10,505
All MOSFETs have a parasitic diode between the drain and source. What does your homemade continuity tester do when you forward bias a diode? Unless you have a dead short between two of the terminals, you can't test the device in-circuit.

Not sure what this means but, my DIY tester is very basic. has 2 probes, when touched, lights up a small led powered by 2 aaa batteries. So this mosfet when I put the black probe on the right leg and red on the middle which is cut light comes up. same as when red is in left leg. same if I do it in reverse.
What does your tester do if you test a known good diode?

I have a commercial continuity checker that consists of two AA batteries and an incandescent light bulb (3V flashlight bulb). When I use it to forward bias diodes, the bulb turns on; but not at full brightness. When I use it to forward bias LEDs, other than white or blue, the LED lights and the bulb lights dimly. When I use it on the coil of some 5V relays, I can hear the relay click (don't remember what the bulb did). For this to happen, the relay pick up voltage needs to be close to 3V; actually lower because I use NiMH batteries.
I just want to know if bad mosfets like this can be checked with a continuity tester.
You can't do any quantitative tests with a continuity checker. That would require more than a continuity checker or DVM and, in most cases, it can't be done in-circuit.
 

Thread Starter

Johnston Cabrera

Joined Jul 28, 2019
3
Not sure the tester has only one function lit up when + and - meets. No dimming of lights or other. Maybe I'll just use this card as a display. Thanks again.

Its sad to know though that the great engineering level that are put on to this cards simply goes to waste when they fail.
 

dl324

Joined Mar 30, 2015
10,505
Its sad to know though that the great engineering level that are put on to this cards simply goes to waste when they fail.
When the cost to repair and test something exceeds the cost of a new replacement, that thing becomes disposable. That encompasses a wide range of devices (laptops, televisions, computer modules, ...) and it isn't a recent phenomenon.
 
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