[Elecraft] Re: Mod for more AF gain on K2

[wayne burdick]

On Oct 22, 2004, at 1:17 PM, Mike S wrote:

> It seems that some people have problems with low audio out on some 
> K2's. There was a recent thread where I mentioned that it may be 
> possible to easily substitute an LM386 for the LM380 AF amp and get 
> more gain. I'm happy to report that this works FB.

Mike,

I'm glad that this worked for you.

However, because of the problem with clipping at high signal levels, I 
definitely don't recommend that anyone do this unless they're desperate 
for more audio. The LM386 is a lower-power part and is also noisier 
than the LM380, partly because of all that extra gain.

AF output can be increased in less drastic ways. Here are my favorites:

- On older K2s, make the 2nd XFIL modification. This allows you to 
properly align L34, minimizing loss through the 2nd crystal filter in 
both SSB and CW modes.

- Verify that the band-pass filters and L34 are peaked correctly.

- Use Spectrogram to properly align the BFOs as specified in the manual.

- Adjust the AGC threshold so that it isn't activated by low-level band 
noise.
   (First, turn AGC OFF by holding PRE and ATTN. If the background noise 
comes up,
   you may have the AGC pot set too aggressively.)

- Add the KDSP2 option, which provides programmable AF gain well above 
the standard level.

- Add the KAF2 option, which also increases AF gain (and it can also be 
adjusted by changing a couple of resistors).

- The gain of the preamp can be increased by a few dB with little 
overall change in RX performance. Simply change R76 from 10 ohms to 
about 5 ohms. You can substitute a 5.6-ohm resistor, or just parallel 
another 10 ohm resistor across the existing one. In fact you can do 
this "live" to see what the effect is.

- If all else fails, signal trace through the receive chain as 
described in appendix E. You could be losing RX signal in the T-R 
switch or elsewhere.

Note that you can check your K2's performance easily with an Elecraft 
XG1 1-microvolt receiver test oscillator.

73,
Wayne
N6KR


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