Found this (see attached picture) after reading all forum about non blinking LED on PIn13:
Well, I would like to fix this. Can anyone tell me what Led should I buy that fits on this board ?
Damt it !!!
is the R4 missing too ?
Hi.
Why ask here instead of at your (no doubt Chinese) seller ?
That is not an original Uno.
It doesn't have the USB / serial converter original Arduino's come with.
As long as the seller doesn't sell it as an original, it's no problem to do so.
The LED and it's accompanying resistor were probably left on stock at the production facility (if they ever were on stock at all).
You can find what resistors and LED's are in the original design, by downloading the appropriate files form this site.
But that doesn't tell you what LED and resistor your board was designed for, again, if any at all.
Perhaps, if you're really lucky, you can find the schematics for your particular board and find what parts were left out and / or changed, so you were able to buy your board @ $2, including shipping all over the world.
- because here is about the electronics related to arduino, or is it devoted to original ones only 3?
- the Chinese seller is only one from a million of Chinese people selling non original arduino. Raise a hand if someone did not purchase anything from a Chinese seller.
- I don't need the original USB/serial converter, the one which came is enough for me.
- Thank you, I will download the files from site.
I think you're missing MAS3's point; nobody can tell you what the original manufacturer planned to be in there.
The original Uno R3 uses a buffer to drive the led; your board does not seem to have one which is similar to the sparkfun redboard from that perspective. You can look at the schematic for that one.
I'll raise my hand 
I think that MAS3 mentioned that to point out that it's a clone; you might or might not have known.
I think you're missing MAS3's point; nobody can tell you what the original manufacturer planned to be in there.
That's the problem with the cheap clones. I don't think they engineered them at all, they just rely on copying someone else's work and then finding ways to make them cheaper through omission of components and inferior parts.
They offer no support and do not contribute at all to the overall hobby. Sparkfun, Adafruit, Teensy- some might have similar boards, but they also do the legwork on libraries that work with their other products and have forums to answer questions and address problems. Do you think they want to spend their time helping you when you didn't think it was worth spending the extra $2? Do you think they know what parts are/are not on that clone?
I can also raise my hand. Every microcontroller/ module I have are from one of the 4, Adafruit, Sparkfun, Teensy, or Arduino.
I'm quite certain the seller will refund you, or send you a replacement unit.
At least you should try to get one or the other.
AngeloGomes:
... the Chinese seller is only one from a million of Chinese people selling non original arduino. Raise a hand if someone did not purchase anything from a Chinese seller.
Of course I have bought Chinese electronics! I have an iPhone!
As far as Uno clones go, I've bought from Seeed and OSEPP, and they are both first rate. None of the components on the boards are missing.
Well, what does it mean "original Board"?
Seriously.
In my case, I would prefer an "original" board, although it is expensive (I still own a functional Uno - my first ever board - which was very resilient and easy to find info about it). This preference - to check and prototype.
After the first phase, for the final device I can go ahead with an (acceptable) clone. I use Arduino nano - which are of variable quality, although I buy from the same seller. Few look brilliant and does their job. One looked like passing trough a Cosa nostra revenge and a prolonged Vendetta (curve legs, pale components) and ended up in smoke. Still good as jewelry. Next victims I will paint in red or blue and hook to a chain or bracelet - nice, isnt it?
To me "original" mean therefore "good quality", so the "original" meaning of the word "original" is meaningless (that is a really weird sentence, I must confess :-)).
Measure size of existing resistor or led (or compare to part of known size) to see if it's 0603 or 0805 (looks like one of yhose)
Get some leds in that size, and some resistors in that size. I'd do 470 ~2.2k depending on how bright I like my leds. Digikey, mouser both have excellent search tools on site, and those parts are also abundant on eBay. Basically any led that looks normal in appropriate package will work, as will any resistor of appropriate size and value.
On digikey/mouser, sort by price, ascending. They sell some insanely expensive leds and resistors; the cheapest ones are fine for thus.
Then figure out the polarity of that led (with power off, use dvm to locate which of those 4 pads is tied to either vcc or gnd - from which you can deduce the polarity. Then get tweezers and temp controlled iron out and solder em in. Negative side of led is marked with an almost unnoticible green dot on most and leds.
DrAzzy, this kind of answer is what I was expecting since the beginning .
Thank you very much !