or down buttons on the remote control, and restore the television to normal tuning operation when the mute button was pressed again (mute off). Space phone Some models of Zenith's System 3 line of televisions made from the late 1970s to the early 1990s had a feature called the Space Phone by Zenith. It was basically a hands-free speakerphone built into the television set. It used the set's speaker and remote control, in addition to a built-in microphone. A Space Phone-enabled television would connect to a telephone jack (using a built-in phone cord), and making a call was performed by pressing a button on the remote to activate the Space Phone (which would mute and begin controlling the program audio going to the speaker). The telephone number is dialed using the numeric keys on the remote, which then displays the digits being dialed on-screen (using the on-screen display features of the System 3 line). The user could then converse with another caller hands-free, much like a regular speakerphone. The porthole television In the late 1940s, Zenith entered the television market. These sets were all-round tube sets. The main feature was that the entire round screen was exposed. They were available in 12-inch, 16-inch and 19-inch sizes. Later round-tube models had a switch that would show the picture in the 4:3 ratio, or have the entire round screen exposed. These sets are very desirable among television collectors. Many porthole sets used metal-cone CRTs, which are now scarce. It is not uncommon for collectors to replace a bad metal-cone tube with an all-glass tube. Zenith porthole sets came in tabletop models, stand-alone consoles and television/radio/phono combos. Hand-wired chassis In the late 1950s, many electronic manufacturers, such as RCA, General Electric and Admiral, were changing from hand-wired metal chassis in their radios and televisions to printed circuit boards. While circuit boards save time and errors in assembly, they are not well suited for use with vacuum tube equipment, in which high temperatures are generated that can break down boards, eventually causing the boards to crumble if one attempts to remove a tube. Zenith, and to a lesser extent Motorola, avoided this problem by continuing to use hand wired chassis in all their vacuum tube equipment. Zenith kept circuit boards out of their televisions until the Chromacolor line of the early 1970s, and even then used them only with solid state components, mounting the