Kfa2 GeForce GTX 750 Ti 2GB GDDR5

Seller Note “When installed in pc the fan spins but with no output to the screen unfortunately otherwise in great condition”

Pre Checks / Multimeter Only

Test / ChecksResultNotes
1. Visual Inspection Eye / Magnifying GlassOKNo missing components, but a bit dirty.
2. Visual Inspection Microscope
3. Check for shorts on 12v PCI, 12v PCI-E, 3.3vOKCouldn’t find any obvious shorts.
4a. Resistance VCore (~0.5)Ω
4b. Resistance VMem (~60Ω) Ω
4c. Resistance 3.3v Rail (~960Ω+) Ω
4d. Resistance 5v Rail (~5KΩ)K Ω
4e. Resistance 1v PEX Rail (~123Ω+)Ω
4f. Resistance 12v BUS (14.7kΩ )
4g. Resistance 8.8v Rail (18.7KΩ )

Power On / Test Rig Checks

Tests / ChecksResultNotes
1. Observations (e.g. PCB heat noticed,
blank screen, power light on monitor,
bios screen, o/s loaded, artefacts, )
No display (no light on monitor)
Fan spins, GPU never gets warm (no power). Some warmth in the VRM area.
No power to CPU.
2. Voltage VCore (~0.88v)0.914vOK
3. Voltage VMem (~1.4v)~0v (4.3mV)!!! Need to investigate further
4. Voltage PEX Rail (~1v)?!!!
5. Voltage 3.3 BUS3.3vpresent
6. Voltage 12 Ext12vpresent
6. Voltage 5v Rail5vpresent
7. Voltage 8.8v Rail

That memory area needs further investigation, access is a bit hard with the cooler in place. The board is quite small and the heatsink is quite large and gets in the way of diagnostics. I will see if it can be run with a make shift heat sink. Alternatively, I will try and rig up a lab power supply to a riser.

Next Steps

Actually there is no need for a heatsink, as the GPU remains cold. There is some warmth towards the IO shield / VRM area, but hard to pin point. Still cannot find any trace of the 1v PEX rail. I am guessing VMem is missing perhaps because PEX is also missing. Hopefully there is a chance that the core and memory are ok and there is some failed power switching/delivery component. Interesting!

Deep Dive

Used my cheaper microscope to take a closer look at the PCB, in particular components relating to possible minor voltage production (VMem 1.5v, PEX 1v). Found two Low Dropout Regulars to investigate (refer to Fig 1 & 2 in photo above for positions) :

Fig1 U503
Fig2 U501

I struggled to find a datasheet for these exact components, but I did find this seemingly very helpful image from a Russion site:

Taken from https://cxem.net/sprav/sprav139.php (worth a look for the other information there).

Also here is a link for a similar chip http://images.100y.com.tw/pdf_file/38-RISHTEK-RT9018.pdf

Time to check voltages on these chips:

PINExpected VActual V
1. PGOOD (Dot)From vCore?0v Missing
2. EN2v+ ? 3.07v OK?
3. VIN3.3v0v Missing
4. VDD5v5v OK
6. VOUT1v0v Missing
Fig 1 U503
PINExpected VActual V
1. PGOOD (Dot)From vCore?0v
2. EN2v+ ? 5v OK?
3. VIN3.3v0v Missing
4. VDD5v5v OK
6. VOUT1v0v Missing
Fig 2 U501

Seems we could have a problem with the 3.3v rail.

TODO Test resistor R649

Want to understand more about the Vmem rail.

RT8120A http://www.chipset-ic.com/datasheet/RT8120A.pdf (single-phase synchronous buck PWM DC/DC controller designed to drive two N-MOSFET) I am guessing this drives the two MOSFETs in the memory VRM.

Also there is a chip marked 0T=1C 622 which I think is a PWM used to drive the Vcore VRM http://www.smdcode.net/uploads/Files/Datasheet/0T/RT8811A.pdf

Update: Unfortunately, I accidentally shorted to of the Vcore mosfet pins whilst measuring with my new meeter, smoke seen and now no VCore – dam it! The driver appears to have a burn mark on it, would imagine at least the driver and the mosfet need replacing. This is now long fix territory, as I need to find replacements and hope for the best in terms of no further collateral damage.