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The 820 is a nine transistor from 1964, and the 78 is an 11 transistor from 1967, so two completely different chassis. Zenith used a three digit model designator up to 1966, then went to two digits. An example is the Royal 810 and 51, the same radio just different model numbers. The 820 is a 9v chassis, while the 78 is 12v. Top. ZenithRadios.