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Maxtrac 820 is a trunking radio; the only freqs you enter are the control channels that are entered in the system info screen. You also need a system key to do this.
If all you see after reading the radio is the "F2 - Radio Wide Configuration: Time Out Timer" under the "Change/Create/View" menu, then you are both trying to read a Trunking Maxtrac with Conventional Maxtrac RSS (RVN-4019). You need to use the separate Trunking Maxtrac RSS, part# RVN-4043.