K2 AGC Threshold Adjustment

[Jim Stoneback]

Hi,

When I built my K2/100 the threshold adjustment didn't go right, I turn
trimmer R1 full CCW to get 3.74v on pin5 of U2 (full CW pin 5 goes to 2.6v).
This adjustment is supposed to swing U2 pin 5 fom from 4 to 8 v.  I checked
diode polarity, resistor values, everything but never did find the problem.

Now I'm building another K2 and have encountered exactly the same result.
Anybody got a clue?

73,

Jim
Jim Stoneback
K4AXF
Drake SW8
K2.ssb.100 sn 6194
K2 sn 6263 (underway)

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[Don Brown-4]

Hi

This voltage will depend on the tolerance of the 8 volt regulator. If the 8
volt supply is on the low side of the tolerance then you will not be able to
reach the 3.8 volts. However 3.75 should be close enough as this is just a
ballpark adjustment. The final adjustment will be made later by turning the
AGC on and off and adjusting for no change in the background noise.

Don Brown

KD5NDB





----- Original Message -----
From: "Jim Stoneback" <[hidden email]>
To: <[hidden email]>
Sent: Monday, October 08, 2007 1:53 PM
Subject: [Elecraft] K2 AGC Threshold Adjustment

> Hi,
>
> When I built my K2/100 the threshold adjustment didn't go right, I turn
> trimmer R1 full CCW to get 3.74v on pin5 of U2 (full CW pin 5 goes to
> 2.6v).
> This adjustment is supposed to swing U2 pin 5 fom from 4 to 8 v.  I
> checked
> diode polarity, resistor values, everything but never did find the
> problem.
>
> Now I'm building another K2 and have encountered exactly the same result.
> Anybody got a clue?
>
> 73,
>
> Jim
> Jim Stoneback
> K4AXF
> Drake SW8
> K2.ssb.100 sn 6194
> K2 sn 6263 (underway)
>

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[n6wg]

Jim
Since the problem is the same between two radios built
some time apart, I have to ask, are you using the same
meter?  If so, try borrowing one from a friend and make
the measurement.  It is possible your meter is a bit off.
At least, it's worth a simple test.
Good luck and 73
Bob N6Wg

----- Original Message -----
From: "Jim Stoneback" <[hidden email]>
To: <[hidden email]>
Sent: Monday, October 08, 2007 11:53 AM
Subject: [Elecraft] K2 AGC Threshold Adjustment


> Hi,
>
> When I built my K2/100 the threshold adjustment didn't go right, I turn
> trimmer R1 full CCW to get 3.74v on pin5 of U2 (full CW pin 5 goes to
2.6v).
> This adjustment is supposed to swing U2 pin 5 fom from 4 to 8 v.  I
checked
> diode polarity, resistor values, everything but never did find the
problem.

>
> Now I'm building another K2 and have encountered exactly the same result.
> Anybody got a clue?
>
> 73,
>
> Jim
> Jim Stoneback
> K4AXF
> Drake SW8
> K2.ssb.100 sn 6194
> K2 sn 6263 (underway)
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

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[Don Wilhelm-4]

Jim,

The range of voltage control at U2 pin 5 is from about 2.5 volts up to
about half of the 8 volt supply line.  This is a simple resistive
divider involving the RF Gain control as well as the AGC Threshold pot.  
So there is no way you will get a 4 to 8 volt adjustment swing out of it
(perhaps you had a typo).

Set the pot to max for now and after everything is built and aligned,
you can adjust the AGC Threshold to optimize your receive as Don Brown
indicated in another response.

73,
Don W3FPR

Jim Stoneback wrote:
> Hi,
>
> When I built my K2/100 the threshold adjustment didn't go right, I
> turn trimmer R1 full CCW to get 3.74v on pin5 of U2 (full CW pin 5
> goes to 2.6v). This adjustment is supposed to swing U2 pin 5 fom from
> 4 to 8 v.  I checked diode polarity, resistor values, everything but
> never did find the problem.
>
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