AGC Threshold Adjustment by Changing R1

[Mike Walkington-3]

Hi,

In following the changes in the A to B mod instructions for adjusting R1 to
fix AGC threshold I ended up reducing R1 to 47K.  Is this typical of other
K2 user experience?

Mike
VK1KCK
K2 #2599


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[Mike Walkington-3]

Hi Bob,

 

I have an A model K2, so R1 was originally installed as a 51K.  I removed it
and put in a variable and determined on a quiet band, subjectively by ear,
where noise with AGC on and off seemed the same.  I got different readings
each time I did the test, but a mid 40k reading seemed to occur the most
often so I went with that.  Now trying to see what others got as I'm still
not 100% confident.

 

Mike

VK1KCK

K2 #2599

 

  _____  

From: [hidden email] [mailto:[hidden email]]
Sent: Thursday, 6 January 2005 9:45 PM
To: [hidden email]
Subject: Re: [Elecraft] AGC Threshold Adjustment by Changing R1

 

Hi Mike,

 

In the latest revision of the K2 manual version F, 29 January 2004, R1 is
given as 50K preset. In the world outside the USA this would indeed equate
to 47K. The preset has one end strapped to the wiper so would be a
continuously variable resistor from in your case 47K down to zero which is
in series with the 5.1K in RP6. The preset apparently normally comes out at
somewhere near half rotation or in other words about 23 to 25K.

 

Believe the earlier versions of the K2 had a fixed resistor for R1 selected
on test (SOT). Not sure of the start value used, though there seemed to be
less emphasis on the adjustment then to what is current now to ensure AGC is
fully off with no signal.

 

Seems to have been a few reports recently with the resistor at one end
though they did not quote which end to know the value.

 

Regards,

Bob, G3VVT

K2 #4168



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[G3VVT]

In the reply to Mike, VK1KCK I said "R1 is given as 50K preset.  In the world
outside the USA this would indeed equate to 47K"
 
I found that this is not quite correct as although for carbon presets and  
controls have changed to the same decade values as fixed resistors in  Europe at
least, cermet presets which most probably Elecraft will be  using for R1,
still retain the older values such as in this case 50K for some  reason.
 
A visual check on my K2 is that R1 when set for 3.70V at U2B-5,  being the
optimum AGC threshold setting for my K2, the control  is not far from half
rotation, i.e. about 25K ohms. Working out the  maths for the various resistors in
series and parallel, including the RF gain  control to form the potential
divider from the 8A rail comes out fairly close to  this voltage.
 
The preset R1 is as shown in the K2 Control Board schematic, maximum  
resistance for fully CCW.
 
Bob, G3VVT
K2 #4168
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