Don't apply much more than 10V, the zener is (I am not positive for a C line) inside the PTO unless you use a series current limiting resistor,
or you might over heat the zener.
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The RIT is the wire on the side of the PTO.? All Drake PTOs have had a terminal there to be used for optional RTTY FSK use, RIT uses the same
capability.
73,
Gary
WB6OGD
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Gary WB6OGD