[Elecraft] What value for R88 with the KN2B

Fri Apr 4 20:52:00 2003

OK.  I am now very confused. (It doesn't take a lot to confuse me I'm
sorry to say). I have K2 #3209.  I finished the radio a couple of days
ago and now I'm also about done with the noise blanker.  So, do I
install the 2.K resistor or not?  Although the schematic with my rig is
Rev C, it appears to have the same instructions as Rev E.  That is R88
and R90 removed and R89 replaced with a jumper.  There is no mention of
installing any replacement resistor at R88 and yet, the earlier
instructions that came with the NB say to install the 2.7K.  One reply
to the earlier questioner said no and one said yes.  So . . . what
should I do , i.e., is there a consensus?

Bruce - W8FU

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Bob - AG5Q
Sent: Saturday, March 29, 2003 11:00 AM
To: Larry Benko; [email protected]
Subject: Re: [Elecraft] What value for R88 with the KN2B

Hi Larry,

You do need the 2.7K resistor for R88 to provide a DC path to ground for
the leakage current of varactor diode D29 in the crystal
filter.  Without this resistor, D29 will not perform it's desired tuning
function.  Since R88 is across a tuned circuit, there may
be some other useful effect, such as broadening the response of the
circuit.

In general, you can't connect a diode to a capacitor without having a DC
path to some voltage source (or ground).  Otherwise, the
diode leakage current charges the capacitor. This current is highly
variable so it's better to just eliminate its effect with a
resistor. The resistor can  usually be very large compared to other
impedances in the circuit because the leakage current is very
small; for example, it's less than 50na for D29.

73/ Bob - AG5Q

----- Original Message -----
From: "Larry Benko" <[email protected]>
To: <[email protected]>
Sent: Saturday, March 29, 2003 9:01 AM
Subject: [Elecraft] What value for R88 with the KN2B

> I posted a question yesterday and have received a couple of
> comments and questions but no definitive reply.  I don't think
> that I made the original question clear enough so I will try
> again.
>
> Specifically, Rev. E of the K2 schematic, dated Feb. 16, 2003,
> says on sheet 2 of 4 in the upper right corner that if the KNB2
> Noise Blanker is installed to remove R88 on the K2 RF board and
> makes no mention of putting any component back in its place.
>
> Rev. D of the Noise Blanker documentation, dated Aug. 20, 2002,
> says to remove R88 on the K2 RF board and to then install a
> 2.7k resistor in the R88 location.  The KNB2 kit does provide
> this 2.7k resistor.
>
> Which is correct?  Do I follow the instructions for the KNB2 or
> the NEWER schematic for the K2?
>
> Apparently the original components R88, R89, R90 make up a 5dB
> pad which also provides some impedance stabilization to drive the
> crystal filter.  The KNB2 has a bandpass filter which appears to
> do the same function perhaps at the same level or not.  The
> presence of R88 would change the level to the crystal filter
> slightly.
>
> Thanks again and 73,
> Larry Benko, W0QE
>
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