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
---
http://www.elecraft.com
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