Re: What is the purpose of Diode D511 ?

Steve Murphy


Giving this a little more thought...

Since the IC-7000's receiver is full coverage from 30 kHz to 199.9999 Mhz I suspect C510 is switched in at 
some point when the RX VFO is tuned below some VHF frequency. 

This might be a good starting point to determine the exact function. Set the radio band to GEN (general) or 144 Mhz then enter a
frequency of 199.999 Mhz USB. Now, begin running the VFO frequency down while watching the state of Q7 and see if it changes
at some point.

Have fun!

On Saturday, March 21, 2020, 10:30:10 AM MST, Steve Murphy <kd6nqa@...> wrote:

D511 is a Toshiba RF PIN diode. The circuit switches capacitor C510 (10 pF) in parallel with C517 (12 pF)
capacitor. This changes the characteristics of the low pass filter formed by C514, L505 and C517.   

D511 is biased through R518 (1K)  and source driver Q988 (not an op amp).

The Q988 driver is controlled by the Q7 output of IC981 which is a 4094 8-bit shift and store register. Control data is shifted serially in to the register to control the Q0 - Q7 outputs. 

Source power to driver Q988 is supplied by 'P2R8'. My best guess is this indicates Power, 2 meter Receive, 8 volt. 

I would look at Q7 with a scope or DVM and see what it does under various conditions of the transceiver.

Good luck figuring it out.

73 de KD6NQA

On Saturday, March 21, 2020, 07:46:51 AM MST, Helmut Wabnig <hwabnig@...> 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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