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2A7 Frame < 1976
2A7 Frame > 1976

FH-Line Control Cabinet

The biggest change within the Control Cabinet was in the Relay Logic Assembly known as the 2A7 Frame.  Earlier versions of the 2A7 Frame had IBM style relays that had wire contacts which were quite troublesome.  The newer units contain more reliable relays of a different manufacturer so were consequently far less trouble.

 

You can tell what relay assembly you have in your transmitter by looking at the RCA MI number of the control cabinet.  MI-560800 was manufactured between 1969 and 1974. MI-560800A was manufactured between 1974 - 1976.  Both these cabinets had the older style IBM relays.  If the control cabinet is marked MI-560800B then you have the newer more reliable relays.