Help! Kegerator down! Can't find C02 Leak!

Having a tough time with my new kegerator and I'm hoping you all can help.

My wife got me a kegerator for Christmas. Right now I'm pouring DFH 90 out of a sixtel until I can get my double tap system going and get some homebrew in my corny.

This is a Nostalgia Electrics base model kegerator, so I'm assuming it's not the best model on the market but it does keep my beer cold! The kegerator came with what I believe to be a defective regulator - I hooked up my C02 bottle and the pressure release valve on the Co2 regulator started going nuts. Checked the connection and tried it again, same deal. I wasn't sunk because my parents had bought me a kegging setup last Christmas as I was talking about doing a keezer conversion. So I go grab the regulator that came with that set, moved the gas valve on the regulated side from the original regulator to the new regulator, checked my connections, and I had draft beer.

Just this week I opened the fridge to see if I could fit both my sixtel and the corny inside - I looked at the gauge and the tank was empty. No serving pressure, and no reserve pressure. I know as recently as last weekend I had pressure on the original 5lb tank I had, and pours from the kegerator were doing fine.

I connected up my backup cylinder, checked all the connections, did the soapy water test over all the connections and came up good. No leaks detected (This was maybe Tuesday night). Tonight is Friday. I come home, and I check the regulator - now my backup tank is empty! GRRRRR!!!!

I know my CO2 is going somewhere, but what else can I do to track down this leak?

Thanks for any advice!!!

Start with the regulator/CO2 tank connection: remove the regulator and make sure there's a gasket fitted to the coupler. If it's missing it could easily explain how you drained two tanks of gas so quickly...

Cheers!

There's an O-Ring that's attached to the fitting that couples to the tank. The tank is coupled to the regulator with pretty decent force. I'm not sure if it is leaking from there, as after I did the soap test, I removed the tank to put everything back in the kegerator and it did start to bubble when I loosened it. I guess the O ring could be bad, but I'm sure with those kinds of pressures it would probably be a little more vocal about it?