How to clear controlgroup control after submit button click

I want to clear my textbox and dropdown list and radio button.

code that i used
html code

<ion-header>
  <ion-navbar>
    <ion-title>Postpaid</ion-title>
  </ion-navbar>
</ion-header>
<ion-content padding>
<form (submit)="doSubmit($event)" [ngFormModel]="prepaidForm">
<ion-list>
 <ion-item>
	<ion-label>Operator</ion-label>
    <ion-select #item ngControl="operator">
        <ion-option *ngFor="let item of oprArray" value="{{item.oprCode}}">{{item.oprName}}</ion-option>
    </ion-select>
</ion-item>
</ion-list>
<ion-list >
  <!-- <ion-list-header>
    gdfgd
  </ion-list-header> -->
  <ion-item>
	  <ion-label floating>Mobile No</ion-label>
	  <ion-input type="number" ngControl="mobileNo"></ion-input>
  </ion-item>    
  <ion-item>
	  <ion-label floating>Amount</ion-label>
	  <ion-input type="number" ngControl="amount"></ion-input>
  </ion-item>        
</ion-list>
<button secondary round block>Submit</button>
</form>
</ion-content>

ts file code

this.postpaidForm = new ControlGroup({"operator": this.oprControl,"mobileNo":this.mobControl,"amount":this.amountControl });

this.postpaidForm.control["mobileNo"].reset();
this.postpaidForm.control["amount"].updateValue("");

but above code not work

Hey guys please help me…i am stuck here…