The PSR-500 looks to be the same layout as the Whistler WS-1040. Have you tried contacting Whistler to see if you can get a new membrane from them? Only other source I could think of would be on Ebay, either someone who has parted out a non-working unit or buying a non-working 500 with a good keypad. However, I didn't see anything like that when I searched just now.