How to enable/disable checkbox in a grid using renderer

I need to load a grid based on the query results I get. I am not having any issues with loading the text fields in the grid but there is one particular column which is a check box type. I am using the xtype: 'checkcolumn' for this purpose. The object that returns the query results returns a "Y" or "N" for this particluar column. If it is "Y", I need to enable the checkbox and if it is "N", the checkbox should be disabled. I am using the following code for defining my checkbox field.

{

xtype: 'checkcolumn',

header: "Old User",

disabled: true,

disabledCls:'x-item-enabled',

width: 170,

dataIndex: 'oldUser',

itemId: 'oldUser',

renderer: this.checkOldUser

}

checkOldUser: function (oldUser) {

if(oldUser== 'Y') {

this.oldUser.setDisabled(false);

}

}

This renderer function is not working for me. How should the renderer look like?. Can you please let me know how to resolve this issue?. Thanks....

How to enable/disable checkbox in a grid using renderer