I did a bit more reading, my curiosity piqued. Voyager's main visible light cameras were Vidicons, i.e. analogue devices of the type that used to be fitted to video cameras, with their output subsequently digitised at a resolution of 800 x 800 pixels. At the time Voyager was launched, CCDs were tiny and I doubt they would have been reliable enough.

The Instrument Information page at SETI has more data for the terminally curious.