Hi,
Today I noticed something really odd with one of my Uno's. The voltage at the 5V pin, when powered with a 9V battery connected via the DC jack, is only 3.30V. The 3.3V pin is at the same voltage. At first I thought that my battery was flat so I swapped it out only to see the same happen again. I measured the voltage at the DC jack when the Uno is powered up and I read about 8.5V or so. However, when the Uno is powered up via the USB connector I get 5V at the 5V. This is how I mostly use it so I had not seen this before. I then used the same 9V battery on another Uno and that board gave me the 5V at the 5V pin as expected. Any idas as to what might be causing this? It seems to be perfect 3.30V so perhaps a short on the regulator causing it to provide 3.3V instead of 5V? I don't have any jumpers or anything connected to the board. Any input is welcome! Thank you!
Can you test it with a power supply?
If it still reads 3.3V, and its an Arduino UNO, the there is a fault, if it's a clone, then who knows! It MAY have a 3.3V regulator as some clones ARE 3.3V based.
Maybe it has an incorrect regulator installed? Can you read the part number on the chip next to the barrel jack?
CrossRoads:
Maybe it has an incorrect regulator installed? Can you read the part number on the chip next to the barrel jack?
Wow! You are awesome!!! I never expected the regulator to be wrong IC... I quickly looked at it and you are absolutely correct! The one with the proper regulator has the text "AMS1117 5.0 DN911" written on it. The one with only 3.3V on the 5V pin has a regulator with the text "AMS1117 3.3 DN911" on it. I must never have used this board with a 9V battery yet...
And yes, this is a clone. I got what I paid for... 
:o That board is going to get a big "X" label on its DC jack... It seems to be worth every penny to get genuine boards!!