Re: What is the purpose of Diode D511 ?

Chuck Scott <cscott@...>


The two diodes D551 and D552 on the PA board are used for the "ATT" function, which is used to attenuate the incoming VHF signal (the other option on the preamp select button).

When those diodes are forward biased, they tie the low ends of R552 and R553 to ground so those resistors can attenuate the incoming VHF signal. The voltage for D551 and D552 comes from the AT13 line on the schematic which has voltage on it when the ATT function is selected.

Also notice that there's the same below that on the schematic where D451 and D452 perform the same function for UHF. Those diode also are biased by the same AT13 voltage.

Are you still having receive problems with the radio? If so, there's other parts in there that might have been affected if the preamp transistor was bad.

So, you can now pass on to DX in the next world knowing what those two diodes do. Not too soon we all hope.

Chuck - N8DNX

On 3/21/2020 10:46 AM, Helmut Wabnig wrote:
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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