[Elecraft] S-meter activation, a simple cure

[Marteinn Sverrisson]

S-meter/AGC activation from strong out of band stations has long been
a problem for me when I use the OP1 filter.

Now I have hopefully solved the problem with the S-meter/AGC 
activation.

The solution is to "clean up the AGC IF signal" by
inserting a simple 153kHz bandpass filter between the AGC IF amplifier U2B
and the AGC rectifier D1.

The filter does not interfere with normal AGC action as far as I can tell.
I do not hear any plopping on SSB or CW with the filter in.

The official Elecraft solution to the problem is to make the 20m/30m/17m etc
BP filters narrower and should of course be done first.

THE FILTER

Here is a ASCII schema of the filter, it is very simple and does not
need trimming, it uses 4 standard value components 
a 2.2k resistor to control the Q of the filter and 10nF DC blocking cap 
and a resonator which uses standard values of 3.3nF and 330uH, 
it is inserted between pin 7 of
the AGC IF amplifier U2B and R3 the 10k resistor.

  pin7 on U2B ---2.2k---------R3------ D1-------
		         |
			 |
                       10nF
		         |
			 |
                      -------
		      |     |
		    3.3nF  330uH
		      |     |
		      -------
		         |
		       Ground

3.3nF and 330uH are 5% tolerance. The -3dB BW is approx 20kHz, -20dB
at 100kHz and 350kHz. Insertion loss about 1dB.

How I installed the filter:

Just remove the top (if you have KPA100 remove the small top plate)
of the K2, then you have access to the Control
board, no boards have to be removed.
I unsolder-ed one end of the R3 10k resistor, which is located
near the top left on the Control board (viewing the component side) 
and inserted the 2.2k in the hole and
connected the two together, the resonator circuit is then connected
via the 10nF DC blocking cap. No traces to cut!!

You should readjust the AGC threshold - R1 on the control board -
to compensate for the insertion loss of the filter. (tnx Sverre LA3ZA)

Details on: http://www.raunvis.hi.is/~matti/TF3MA/agc.html

BTW if you have a AM LW radio try listen to the AGC IF signal on 153kHz,
    just hold the radio close to the left upper side of K2,
    makes it simple to listen to AM!!

73
Matti, TF3MA