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Originally Posted by Tom Bavis
The 12DT8 is the only tube used for FM, and not AM. So try it first.
The dot before 100 is for Zenith's own FM station in Chicago. A handy dial calibration point at the factory?
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That little white dot can also be used as an alignment aid if you ever have to realign the set. The symbol marks 99.5 MHz and was, as mentioned, the frequency of Zenith's owned and operated (O&O) FM in Chicago in the '50s through about the mid-'70s. The station was known as WEFM in those days; it is now WUSN, "US99" C&W (country/western). Most of Zenith's radios from the '50s until the end of the '70s had the dot at 99.5, but by 1976 or so this was just a carryover from an earlier time (the WEFM era) and was kept on the radios only for nostalgic value. Both my Zenith K731 and C-845 have the 99.5 marker on their slide-rule tuning dials, but the '845 has two parallel horizontal lines, rather than a dot, at this point. My C845 also has the two parallel horizontal lines on the AM tuning dial at 640 and 1240 kHz (the old Conelrad emergency warning frequencies) in addition to the Conelrad symbols themselves. One version of the K731, probably a slightly older version than mine, has the CD symbols on the AM tuning scale (my set does not have these icons, so perhaps it is one of the newer models made after 1963, the year Conelrad was abolished in the U.S. in favor of the Emergency Broadcast System).