Contax RTS : Invisible Aperture Scale

Having just acquired a tidy Contax RTS I'm disappointed that the aperture scale and attendant pointer in the top of the viewfinder are well-nigh invisible. With very bright light coming through the lens I can just make out the numerals and pointer, which appears to be moving correctly. This scale is apparently not electrically illuminated, but backlit from the mirror. Does anyone have any ideas on what could be causing the problem?

Is it a RTS3 ?

No, it's the original RTS. Much as I'd like it to be a III...