[Elecraft] new rev B K2 build: low sensitivity

[Wallace, Andy]

Have hashed this around with several folks and I might get somewhere
by bringing the List onboard. 

Finished 40m rx construction of K2 #31xx and sensitivity is low.
40m SW broadcasters are barely nudging the meter past S9 +10 dB,
with the preamp on. I can barely hear normal band hash in my location
with the preamp off. 

(This was a week or two ago, not the lousy condx this weekend!)

Signal tracing, revD manual Appendix E page 10, using a good sig generator:

Measurements with RF probe match all the way up to the
Mixer IF Amp Xfil stage, page 11.

Step 3: composite mixer output is 0.060V (s/b 0.079)

4: Post mixer Amp output 2.63V (s/b 2.20)

5: -5dB pad output, 1.92V (s/b 1.40)

6: Xfil output, 0.60V (s/b 0.35!)

7: T7 step-up ratio: 2.14V (s/b 1.36!)

I could not seem to do the IF AMP saturated test. Seemed like the signal
generator output wasn't high enough. I could only get U12:08 to
0.10V not 0.15 as specified. So the 2nd xfil output at u11:01 was
only 0.16 (s/b 0.27) but that may not be valid. 

Because of that, I couldn't get the AGC saturation test, either. 

So what do you think? Too much getting through the filter????
Steps 6 and 7 are way off. I'm using a good Tektronix DMM with
the Elecraft probe kit. 

I played with the generator some more and got these readings. The
S meter bars were JUST coming on. Here's the bar number and microvolt
reading from the URM-25. 

1  4.5 uV
2  9.5
3  20
4  45
5  97
6  300 (s-9)
7  800
8  1800
9  5000
10 18000

This was with the preamp off. 300 uV for S-9! Ouch. 

Double checked T7 and rewound it anyway. It bugs me the output
is so high for no good reason. Visually inspected for correct parts
and solder joints all over the RF board. Nothing seen.

Note that this was the same K2 which had the AGC threshold issue,
can't go above 3.8V at R1 fully CCW. Also 8A line is 7.991V.
(Two other list members said they had the same problem. I heard from
one yesterday who said his rig is plenty sensitive, so the AGC setting 
is probably not at issue. With my AGC off, the strength (audio) does
go up slightly but not a huge amount like the AGC is being held down.
S-meter calibrated fine -- settings were about 20 and 183 if I remember
right. 

Bugs me that out of the TUF-1 mixer I get 0.060 instead of 0.079.
Gary says he seldom sees bad TUF-1s. 

Gary said it may help to continue construction to see if the transmit
section had a problem (in common), so I did. Finished that yesterday,
and the rig puts out 10+ watts on all bands, so it's not the bandpass
filters. It's insensitive on all bands. With the preamp on, it's ALMOST
as sensitive as my K1.

Filters seem aligned fine. Peaks at the sideband offset just fine.
Background
noise pitch is what I'd expect for my settings, sidebands are correct,
etc etc. 

Am about to hit it with the scope again and may take some pictures. 
Any help appreciated. I am positive nobody's like the K2 so much if it
was as deaf as mine, so I know there's a problem someplace.

Andy