Seller Note “Untested”
Summary
- This card has a nice looking design, very clean and still has its warranty sticker. I think I kind of fell for this and judging by the rather obvious issues (not detected, fans run full speed), the seller was not really honest in saying it was untested! Normally I would expect this card has a faulty core, but I will try to analyse it as far as I can as I have never seen a 780 like this and there appears to be little in the way of information.
- Resistances (low looking Vcore doesn’t look good)
- Vcore – 2.8 (seems a little low for this type of card, Core is GK110-300-B1)
- Vmem – 83.6 (good, chip is the commonly used Hynix H5GQ2H24AFR-R0C)
- PEX – 262
- 1.8v – Not present
- 5v – 1.977K
- 3.3v – 684
- 12v – $.5K+
- Voltages (not much in the way of minor voltages, not really what I expected, will dig deeper..)
- Vcore – 0.05V
- Vmem – 0v
- PEX – 0v
- 5v – 1.2v!
- 3.3v – Present
- 12v – Present
- No direct schematic, the board layout is quite different to other 780s I’ve got or have ever seen.
- It appears I might have been too hasty (or have a fall coming later), but with the almost complete lack of voltages (only 3.3v and 12v found) and a missing component, there is at least some more analysis to do before confirming the card is dead..
- Against my initial fears, this card is not only fixed, but seems like an excellent version of the 780! 🙂
Investigating U15 (5v source)
At this stage, I suspect U15 is the source of 5v (guess based on other 780s) and I can see clear issues around this voltage area. The chip is RT7257GLZSP http://www.datasheetq.com/RT7257GLZSP-doc-Richtek
| pin | Resistance | Voltage |
|---|---|---|
| 2 (VIN) | High, 12v, 4K+ | 0.86 v! Need to find why this isn’t 12v |
| 3 (SW) | 1.977K | 1.19v ! |
| 7 (EN) | 10MOhms? | 0.01v ! |
Mystery damaged / missing component!
Well, I don’t know how… but there seems to be a missing component responsible for the absence of the 12v VIN. Considering the card had its warranty stickers and seems in pristine condition, I don’t understand how this happened. I guess it could have just overheated and broken down.
From other 780 cards, I think the missing component could be a 3A 40v Shottky diode. Before I attempt to replace it, I need to try and understand if there is any obvious reason it seemingly vapourised?
- Overcurrent through the 5v rail – if so, why?
- Could the U15 be faulty somehow? Doesn’t seem shorted, VIN has a high resistance and there is about 2K on the 5v output
- The core resistance still seems a bit low to me, possibly some of the 5v supplied components have an issue? The VRMs seem uniform in terms of ESR, no obvious shorts.
- A faulty diode?
I will make some comparisons with other GTX 780s I have, in particular EVGA GTX 780 3Gb SC (Card B)
- My other 780s have the 5v resistance more like 2.5K than the 2K I have.
- Diode D4 seems to be the equivalent of D6 on this card, and is either a SS24 or A34 3A Shottky Diode.
- U15 seems to be a APW7142 Buck Converter on other boards https://datasheetspdf.com/pdf/659355/AnpecElectronicsCoropration/APW7142/1
I decided that the variances could be within tolerances, as the PCB layout does see different. So, I desoldered a A34 Shottky diode from the supespected dead EVGA GTX 780 and soldered it on the XLR8 after cleaning the pads.
After checking the diode was working properly (in both directions) and that resistances were normal, I powered on the card and to my delight there was a picture!
Key learnings
- Initial assumptions can be wrong, always check all the basic readings before jumping to conculsions! I feared this card would end up having a dead core due to not being detected and having it’s fans running at full speed, but it is now very much alive.
- I didn’t know anything about Shottky Diodes before working on this card. Now I will make sure I check them going forward.
Stress Testing
- MATS passed 3 tests – good.
- Komnuster HD, 3D TimeSpy – Passed.
- Under full load this card seems to be more efficient than the reference GTX 780. It is running a few degrees C cooler and using more power and the GPU core is seemingly clocking higher. I need to do more tests, as I am not sure the proper XLR8 BIOS is on this card. The memory and core clocks show too low in GPU-Z, plus it thinks its the reference when clicking lookup. I might try flashing a listed XLR8 BIOS. It is running at 1032 MHz core and 1502 memory.
- Subnautic is running nicely at 70 degrees C, better than the reference’s 79/80 degrees C.
- Overall, it’s the nicest 780 I have! 🙂