Possibly irrecoverable Uno R3?

Hello,

Quite a while ago I had an idea to measure the speed of one of my PC fans that were lying around. Uploaded the sketch (with analog read/serial write commands) and tested it out with a 9V DC power source powering the Arduino and the fan connected to Vin + the yellow wire which gives the readings connected to the correct analog pin - it worked. The next sketch I was tried uploading was a no go. It seems the random resp=xx response started happening. Tried many options that were written in the forums - none helped. The loopback test over 16u2 (in the beginning and after multiple proper reflashes) failed. I also did a not so well-thought experiment at a later time - shorted reset with Vin pin, and not noticing it afterwards pressed the reset button. I used a stripped CA-42 cable just now with rx, tx and gnd pins and checked what the serial monitor shows - it seems rx and tx are shorted together within the board. So now my Uno is capable of being just a sketchless 5V and 3.3V voltage source… Or is it? Maybe something is possible, such as replacing the 328 and programming it over the rx and tx pins? Also, an anomaly started happening - whenever I touch the pin 13 or a certain pin on the 328, the LED dims or even turns off completely, I think it’s related to this.

Swap the 328 with a new one PREPROGRAMMED(unless you gave another Arduino) with bootloader. You can have your UNO working,provided the rest of the circuit is good,which probably should.

-Malhar

But when I tried the loopback test without the 328, the serial monitor was spammed with gibberish. I'm not sure that just replacing the 328 will solve everything.

Remove the 328 and try the loopback again. If works, go for a new 328,else buy new arduino.

Just did. rx to tx, reset to gnd, 328 removed. Whenever I type something, I receive a fair amount of "ÿýý´ßOÿýÿÝÿÿ÷ýÿÿÿ". Should still I try getting a pre-programmed 328 and upload sketches over CA-42? Or is a new Arduino the only solution?

Sounds like your 16U2 has problems, or maybe your USB cable. That's what the loopback test is checking.

CrossRoads:
Sounds like your 16U2 has problems, or maybe your USB cable. That's what the loopback test is checking.

Oh yes. Try another cable. Try choosing a lower baud rate in serial monitor like 9600. Keep the TX-RX jumper very small.
-Malhar

You say the fan was connected to 9v - did you happen to look at the output from the fan speed pin with a scope or anything? My suspicion is that that would be 9v pulses (same level as the supply). 9v pulses into the Arduino can fry the inputs so you need to make sure you are not putting 9v into an input pin or anything you replace may get zapped as well.