[Elecraft] K2 S-Meter Sensitivity

[Bill Coleman]

On 6/3/02 6:39 PM, k6se@juno.com at k6se@juno.com wrote:

>The S-meter on my K2 (with pre-amp on) seemed to be quite stingy compared
>to my FT-1000MP, so I searched the archives and found a wealth of
>information on this subject.

I had this problem with my K2. I would tune to SWBC stations on 40m, 
which read S9+40dB on my TS-430S, and the K2 would register them as S5.

>Most commonly, D1 on the control board was blamed for this problem and
>selecting a diode with a higher forward voltage drop was the fix.

I did this, but it didn't have much effect in my unit.

>In the archives, Gary Surrency said that if the output of the 8-volt
>regulator on the control board was on the high side (mine reads 8.2
>volts), it could also contribute to the problem because it would elevate
>the AGC threshhold voltage.  He went on to say that if you notice a large
>drop in IF hiss when you turn the AGC off that it would be beneficial to 
>reduce the value of R1 on the control board from 51k to 33k.

I did this mod, and it didn't change much of anything.

I do think he meant a large increase in hiss with AGC off (eg more IF 
stage gain).

I haven't done any measurements, but I did find a solution. The 
transistor isolation amp in the AGC, which I think is Q12, was bad in my 
unit. Since I didn't have any PN2222As, I replaced it with a 2N2222, 
which is in a TO-18 (metal case) package. (You have to be sure to get the 
leads in the right holes, too)

After this, S-meter response seemed much more lively. Those SWBC stations 
on 40m are typically S9+10-20dB.



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901