What is the purpose of Diode D511 ?

Helmut Wabnig

During a ic-7000 repair I found a mysterious circuitry in the PA unit.
The board sits on the bottom side of the radio.
Inside and invisible it carries the shielded preamp for 2 meters and the preamp FET Q551 type 3SK291.
Had to replace the FET after a lightning event.

On the way there I followed the rF signal from the antenna connector to the preamp
and found this strange little diode which is a mysterium to me.
The Diode D511 sits on the visible (outer) side of the PA board.
A small cap goes down to the RF signal after the two lowpasses
and a DC logic signal is fed there via a small opamp fed from a digital counter.

What does it do?

I ask the interested reader to look into the service manual PA UNIT 2
circuit diagram. A search within the PDF for "D511" will open the
locations of the diode in the diagrams as well as on the circuit boards.

I have only little hope this mystery will be resolved,
and am prepared to die without ever knowing it :-)


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