How to enable/disable combo inside a Form

I have this code i used to generate a Form and want to manipulate it, to enable or disable the objects inside a form:

i using: Combo_cuenta_activa.disable(disabled);
anyone have an idea how to access properly to the combo inside the form.

<script type="text/javascript">

    dhtmlx.image_path = "codebase/imgs/";
    dhtmlx.skin = "dhx_skyblue";
	var main_layout = new dhtmlXLayoutObject("winVP", '1C');
	var a = main_layout.cells('a');
    
	a.hideHeader();
	var str = [
        { type: "settings", position: "label-left", labelWidth: 260, inputWidth: 200 },
		{ type:"fieldset" , name:"_nuevacuenta", label:"Nueva Cuenta" , inputWidth: "auto", list:[
        {type: "block", width: 800, list:[
            {type:"newcolumn"},
    		{ type:"combo" , name:"cuenta_activa", label:"Cuenta Activa",
              connector: "opciones/cuentas/agregar/combo_options.xml"
            },
    		{ type:"input" , name:"_cuentacredito", label:"Numero de Cuenta" }
            ]},
        {type: "block", width: 800, list:[
    		{ type:"input" , name:"_cuentamadre", label:"Cuenta Madre" },
    		{ type:"input" , name:"_nombre_comercial", label:"Nombre Comercial" },
    		{ type:"input" , name:"_nombre_legal", label:"Nombre Legal" },
    		{ type:"input" , name:"_representante_legal", label:"Representante Legal" }
        ]},
        {type: "block", width: 800, list:[
    		{ type:"calendar" , name:"_fecha_fundacion", label:"Fecha Fundacion", dateFormat:"%d-%m-%Y", readonly:true, enableTime:true, serverDateFormat:"%d-%m-%Y ", options:{			
    		} },
    		{ type:"input" , name:"_rtn", label:"RTN" },
    		{type:"newcolumn"},
    		{ type:"combo" , name:"_sector", label:"Sector Comercial" },
    		{ type:"combo" , name:"_subsector", label:"Subsector" }
        ]},
		{ type:"input" , name:"_sitioweb", label:"Sitio Web" },
		{ type:"input" , name:"_estimado_compra_anual", label:"Estimado de Compra Anual" },
		{ type:"combo" , name:"_estado_posible_cliente", label:"Estado de Posible Cliente" },
		{ type:"input" , name:"_limite_credito_solicitado", label:"Limite de Credito Solicitado" },
		{ type:"combo" , name:"_sucursal", label:"Sucursal" },
		{ type:"button" , name:"_btn_agregar_cuenta", value:"Agregar Cuenta" }
		]  }
	];
	var form_1 = a.attachForm(str);
    //form_1.setImagePath("codebase/imgs/");
    //form_1.setSkin('dhx_web');
    //form_1.getItem("cuenta_activa").readonly(true,false);
    //form_1.getItem("cuenta_activa").disable(true);
    Combo_cuenta_activa = form_1.getCombo('cuenta_activa'); 
    Combo_cuenta_activa.disable(disabled);
    //form_1.load()
	var status_49 = main_layout.attachStatusBar();
	status_49.setText('');
</script>

Well,

to enable/disable combo item use:

form_1.enableItem("cuenta_activa"); form_1.disableItem("cuenta_activa");

to get combo object (then you can work with it using combo’s api):

var combo = form_1.getCombo("cuenta_activa");