[Elecraft] K2: Problem Setting AGC Threshold

[Don Wilhelm]

Doug,

First be certain you have the correct pin on U2 - it is the uppermost 
pin of U2 and nearest the right side panel.
If you remove the screws from the control board and tilt it slightly 
forward, you can easily probe that pin from the back side of the board.  
Then check to be certain U2 is oriented correctly.

If those checks are both OK, go on.
Next remove the control board from the K2 and measure the resistance 
from U2 pin 5 to ground as you rotate R1  It should range from 55k to 5k 
(a bit lower is likely OK).
With the control board still out, check the resistance to ground from RF 
Board J6 pin 6 while rotating the RF Gain control.  It should range from 
zero to 5k.
If those checks are good, look at the soldering of CB RP6 - check 
carefully for unsoldered pins and look for a solder bridge between pins 
3 and 4.

If that does not cure it, then look at the schematic for the control 
board (lower left) and you should see that the U2 pin 5 voltage is 
controlled by a simple 3 legged voltage divider.  The voltage (8 volts) 
comes in at RP6 pin 3 and then goes to R11 in series with the resistor 
in RP6 pins 7 and 8, and also goes to  the RF Gain control through the 
series resistor in RP6 pins 5 and 6.  One resistor RP6 pins 3 and 4 
between the 8 volt supply and U2 pin 6 - so it is a relatively simple 
voltage divider.  (U6 does not supply any current, so its function is 
not relevant to determining the voltage).

73,
Don W3FPR

Doug Person wrote:
> Hi Folks,
>
> I'm very leisurely building K2 6798.  I've reached page 48 where I am 
> performing "Setting the AGC Threshold".  I have a confusing situation.  
> Pin 5 of U2 reads 7.85 volts and this value does not change when R1 is 
> adjusted.  I don't see any incorrectly placed components.  All solder 
> connections are perfect (I always do a final examination with a quality 
> magnifying glass).  I greatly appreciate any assistance with 
> understanding this problem.
>
> Thanks
>
> Doug -- K0DXV
>   
>