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

Sun Feb 2 20:28:01 2003

Thought the problem was related to the low AGC voltage, caused by the low 8
vdc regulator? As for the solution, still waiting for that!

----- Original Message -----
From: "Wallace, Andy" <[email protected]>
To: "Wallace, Andy" <[email protected]>; "'Elecraft'"
<[email protected]>
Sent: Sunday, February 02, 2003 5:11 PM
Subject: [Elecraft] RE: new rev B K2 build: low sensitivity

> > 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
>
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