true / false to Y/N Ex

I am trying to change the default value sent from a checkbox. I have this code:

xtype: 'checkcolumn',
                header: 'Contacto de Emergencia',
                dataIndex: 'contactoEmergencia',                    
                listeners: {
                        beforecheckchange: function() {
                            return false;
                    },                          
                },
                width: 100,
                flex: 1,
                editor: {
                    xtype: 'checkbox',
                    cls: 'x-grid-checkheader-editor',
                    inputValue: 'Y',
                    uncheckedValue: 'N'
                }

I am also set my model like:

{name: 'contactoEmergencia', mapping: 'CONTACTO_EMERGENCIA', convert: function(v) {
        return v === 'Y';
    },
    serialize: function(v) {
        return v ? 'Y' : 'N';
    }},

But it´s not working. The default value true/false does not change to Y/N. anyone knows why is not changing ? what I am doing wrong ?