wilykat:
I got a non-working UNO. Loop-back test checked out so I know the cable and com and the built in 16u2 is fine. I swapped '328p chip for a known good one, it still doesn't work, Arduino can't communicate with '328 chip at all.
I've checked, there is no short, resistor and caps checked out, reset line isn't stuck low, traces (tx and rx + reset) between 16u2 and '328 is not broken. The chip does get +5v on Vcc and 0 on ground.
The only thing left is the 16MHz clock. On my UNO clone, it is a very tiny piece about the size of 0805 SMD resistor. I can't replace that but I was wondering if it's OK to solder the larger 16MHz can and 2x 22pF cap somewhere and wire in 2 leads directly to pin 9 and 10 of the socket? If so, is there a limit to how long the wires can be between the socket and the oscillator can?
Before you solder anything, take a 16 MHz crystal (HC49US package probably) and bend the two leads so that they line up with the 328P pins 9 and 10, then hold the crystal leads on the 328P pins, using the little "ridge" on the IC socket to wedge the crystal leads between the ridge and the outside edges of pins 9 and 10.
Don't force the crystal into the socket, lust use finger pressure to hold it in contact. Then see if your UNO boots up.
If it does, it's a crystal problem. If not, no need to waste time soldering in a new one.
If it IS the crystal, and you don't want to be soldering, and if you don't care about a precise 16.0 MHz clock, then simply enable the internal RC oscillator in the 328P and then you don't need a crystal at all.
(edit to add): If you DO need a crystal soldered in, you do not need the two loading caps (the 22pf parts you mentioned). The easiest place to solder on a crystal is underneath the board, directly to the IC socket's pins 9 and 10. I suggest removing the 328P before soldering, then re-insert it after (so you don't damage the chip with ESD or heat).
These little SMD crystals fit in place of the resonator quite nicely!
(click pic for full size)
