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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 :-)

OE8UWW
Chuck Scott <cscott@...>
 

Helmut:

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 :-)

OE8UWW
Prabhu
 

Excellent Chuck 👍👍👍


On Sat, Mar 21, 2020, 10:22 PM Chuck Scott <cscott@...> wrote:
Helmut:

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 :-)
>
> OE8UWW
Helmut Wabnig
 

On Sat, 21 Mar 2020 12:52:18 -0400, you wrote:

Helmut:

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).
........
Am talking about Diode D511.

The atten is described in the circuit description service manual,
but not D511.

OE8UWW
Steve Murphy
 

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.

Steve,
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 :-)

OE8UWW
Helmut Wabnig
 

On Sat, 21 Mar 2020 17:26:53 +0000 (UTC), you 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.
Steve,73 de KD6NQA
Ah, I did not get that shift and hold register.
I was seeing it as a counter, sending pulses.
But your description is fine.
So it tunes the passband filter to various frequency ranges.
Will have to check that.
After the lightning I had to replace this D511 diode together with the
preamp FET and now sensitivity is down. Possibly another diode fault
or does it just need the receiver adjustment procedure, will see.
D511 was physically cracked in two parts, easy to spot.
ATTEN and PREAMP switching works obviously.

Must get that 7000 sensitivity back again.

Thank you for your help.

OE8UWW
Steve Murphy
 

Helmut,

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!
Steve
KD6NQA
       


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.

Steve,
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 :-)

OE8UWW
Chuck Scott <cscott@...>
 

Helmut:

Well, that's a bit more curious. Looks like the P2R8 control line enabling that line during receive (I suppose) to apply another 10 pF to that part of the circuit. Why? That's a good question. Perhaps they were concerned about some stability issue or needed to shift the bandpass of that circuit between RX and TX. In any case, it look like it just adds some capacitance. Why, perhaps someone else knows.

Chuck - N8DNX

On 3/21/2020 1:06 PM, Helmut Wabnig wrote:
On Sat, 21 Mar 2020 12:52:18 -0400, you wrote:

Helmut:

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).
........
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