I know what the question is, and your way of using EditView.Text = "" is correct. If the Edit Box is defined in the Designer though it isn't needed to be Initialized in code. I think you may have View Names and Event Prefix Names Mixed up.
In your question you have a Sub Button1500a_Click, but below that you say you have a button called "1500a_click". The names are not very descriptive, but if they are exactly how you put it they look incorrect. With 5000 lines of code and being here longer than me I'd assume you know the basics, but:
Lets say your button is called 1500a in the designer and your Edit Box is called 1502a. B4a usually defaults to Event Prefixes with the same names as the views, so unless you changed them the click event for your button would be 1500a_click. For example's sake we will assume those are the only two controls. To access the edit box you need to dim it in Sub Globals as- Dim 1502a As EditText. Then in the 1500a_click Sub you 1502a.Text = "". I don't know the limits on variables starting with a number or if they need a char first like some languages, but this should get you going.