[Elecraft] RE: new rev B K2 build: low sensitivity

[Wallace, Andy]

> Finished 40m rx construction of K2 #31xx and sensitivity is low.

OK, just had some fun with a scope and an old URM-25 sig gen. Remember, this
tx works fine, but rx seems very low. Nothing pegs the S meter. Barely hears

local QRN without the preamp on.

Measurements done with Tektronix scope using its "measurements" function
for RMS -- I didn't calculate each one! 

Going by the manual and putting in a 0.14V RF signal at the antenna jack,
I see the following with an oscilloscope, at the TUF-1 composite mixer, 
with preamp ON:

1) RF line... 265 mV RMS, wave is a little square. (Input at ant is perfect)
2) IF line... 225 mV RMS, weird waveform. Guess I am not used to seeing the
IF.
Signal does go to zero with no signal at antenna
3) GND
4) LO in.... 310 mV RMS very nice waveform.

Preamp off, I see 86mV/72mV/GND/310mv respectively.

Going through the signal tracing again: note that this was with 
0.14V in as measured at the antenna jack, with a scope, not the Elecraft
RF probe. Manual's good values in parenthesis. VALUES WHICH ARE OFF 
BY A FAIR AMOUNT ARE MARKED ****

Page 10 of the appendix...

Reference osc output: 1.02V (0.8-1.8)
VCO output: 0.377V (0.3-0.4)
VCO buffer out: .765V (0.6-0.75)
BFO output: 0.7V (0.2-0.7)
BFO buffer output: 0.080 (0.025-0.70)****
Low pass filter output: 0.15V (0.13V)****
TR Switch output #1: 0.136V (0.093V)****
Band pass filter output: 0.089V (0.086)
TR switch #2 output: 0.084V (0.077)
Atten off test: 0.086V (0.077)
Preamp off test: 0.086V (0.077)
Composite mixer out: 0.072V (0.079) not a very clean looking sig
Post mixer amp out: 1.83V rms, 6.84V p-p awful looking (2.20 RMS)****
-5dB pad: 1.46V (1.40)
Crystal filter output: 0.048V (0.035)**** SEEMS TO BE FLAT TOPPING ALSO
T7 step up ratio: 1.4V (1.36)
IF AMP sat output: 0.165V (0-0.30)
2nd xtal fil out: 0.25V (0.27)
Prod det sat out: 0.58V (0.58) 
No sig max IF gain: 3.82V (3.6V)****
No sig min IF gain: 5.00V (4.6V)****
AGC at sat: 6.6V (6.9V)****
IF amp AGC out: 5.1V (5.0)
Af gain, rf gain at min: 1.52V (1.6)
AF out, min IF gain: 0.003V (0.001)
AF out, max IF gain: 0.011V (0.007-0.013)
Preamp noise gain: 0.050V (0.030-0.060)

Checked AF amp -- sidetone level 255, no speaker or headphones connected, 
voltage at speaker connector is 0.56VAC. At setting 128, it's 0.29VAC.

Ideas? I am suspicious of the flat topping of the xtal filter output.

Perhaps these numbers are all inconclusive because I was using a scope
instead of an RF probe. At the high level 0.14V in, it sure sounded like the
whole thing was overloaded. 

Hate to give up and send it in to Gary, and I also wish I could pinpoint
a particular stage and/or chip. I do have scope shots of the input/output
of the filter, product detector, etc. if someone wants to look. 

Andy