[Elecraft] K2 AGC Threshold

[k6se@juno.com]

Back to the AGC threshold discussion on the K2.

I have a K2, #2622.  It has a fixed 51k resistor at R1 on the control
board.  Starting with #3000, that resistor was replaced with a 51k pot. 
The following has been noted:

1) The Rev C manual (S/Ns less than 3000) shows 7.3 VDC on U2 pin 5 in
the voltage chart on page 15 of Appendix E.  This measurement is taken
with the front panel removed, otherwise U2 pin 5 would be about half of
that with the RF gain controlset to max.

2) The Rev E manual (S/Ns 3000 and up) on page 46 says to adjust R1 for
3.8 VDC on U2 pin 5.  This is measured with the front panel connected and
the RF gain control set to max.

3) The Rev E manual also says on page 6 of Appendix E that R1 should be
adjusted for 3.9 VDC on U2 pin 5 (0.1 VDC higher than the procedure on
page 47).

4) Page 6, Appendix E, Rev E manual goes on to say, "You can set R1 for a
slightly higher voltage at U5 pin 5 (obvious typo - should be U2 pin 5)
to increase the no-signal I.F. gain."  (However, setting U2 pin 5 to a
lower voltage would increase the S-meter sensitivity.)

5) It has been implied by others on this list that R1 should be set so
there there is little or no difference in the no-signal noise level when
turning the AGC on and off.  It seems to me that this would result in
poor S-meter sensitivity.

6) Once, someone stated on the list that if the regulated +8.0 VDC is a
bit high (mine is about +8.2 VDC) that R1 should be adjusted for some
other level as measured on U2 pin 5 (forget what voltage would be right).

It appears from the above that decreasing the value of R1 will increase
the AGC voltage thereby increasing the S-meter sensitivity, however it
will decrease the no-signal I.F. gain of the receiver.

I've decreased the value of R1 in my K2 to 33k and that indeed has
increased the sensitivity of the S-meter.  However it has decreased the
gain of the receiver, noticeable when trying to receive weak signals with
the AGC turned on.

This has been a dilemma with me since my K2 was born.  Has anyone delved
into this problem more deeply?

I know the S-meter reading is only relative, but it would be nice to have
it not be as stingy as mine is without sacrificing receiver I.F. gain.

73, de Earl, K6SE