trying to modify dc-dc regulator output

[yonubear]

Hi all First let me say I am happy to be here and I hope that I am able to contribute.

I am hoping however that someone can help me out as I feel rather stupid I am attempting to change a 12v dc regulator from outputing 4.5v to 3v it is based off of a MC34063A IC the problem I have is this as it sets the sense resistor is 0.39ohm, R1 is 1kohm, and R2 is using 2 resistors in series that equal out to 3.7k Ohm. I am using this calculator http://www.nomad.ee/micros/mc34063a/ which is rather close at 4.5v to how it is currently configured. The problem starts when I attempt to change R2 to the 1.8kohm that is says for R2 the output drops to 0v. I am rather confused at this point. I am attaching a photo of what I am working with.

Thanks,
Yonubear

[pebe]

Is the circuit working correctly at 4.5V? If so you should arrange the values of R1 and R2 so that you get 1.25V at their junction for 3V output.
Values of 1.5K for R1 and 2.2K for R2 will give you 3.08V