[Elecraft] K1 Receiver Sensitivity Issue.

[Don Wilhelm]

Les,

Yes, the NE612 should show plenty of gain.  When it does not, the usual 
reason is little or no oscillator signal to pin 6.

The Pre-mixer Bandpass filter must be aligned before you will get much 
of a signal at U1 pin 6.

Since you have a 'scope (hopefully with a 10X probe), I will give you an 
alternate method of tuning the Pre-Mixer Bandpass filter.

Since you have the 4 band board, you must follow the proper band order - 
adjust 15 (or 17) meters before adjusting 20 meters, and likewise adjust 
30 meters before 40 meters.
Make a check before starting - align all the slots in the trimmer 
capacitors parallel with the long edge of the band board.  Then check 
the activity of the crystal for each band - put your probe on band board 
P1 pin 4 and not the amplitude of the signal as you switch through the 
bands - you should have greater than 200 mV peak to peak.  The position 
of the trimmer slots when tuned should be no more than 30 degrees from 
the center position if your toroids are correct.

Then switch to 15 (or 17) meters, and with your probe on P1 pin 6, 
adjust the Pre-Mixer trimmer caps for 15 meters for the maximum 
amplitude.  Switch to 30 meters and do the same for that bands 
trimmers.  Do the same for 20 meters and 40 meters.  You should be able 
to get 200 mV peak to peak there on all bands.
Remember that your 'scope probe will load the Pre-Mix filter a bit, so 
the trimmers to the right of the filter will not be at their optimum 
setting, but it will be enough for receive - you will set the optimum 
position when you adjust for transmit.

You should check to see that the Pre-Mix signal is getting to U1 pin 6 - 
if it is not, suspect the coax link on the bottom of the board between 
J9 and J10.

Once you have a good signal at U1 pin 6, you should see that the output 
at pin 4 is much greater than the input at pin 1.

73,
Don W3FPR

On 8/18/2014 5:20 PM, Les Garwood wrote:
> Dear Fellows,
>
> I Bought a K1 kit at a ham fest several years ago  (SN 02197) and recently
> finished the receiver section.  Although audio is strong, as evidenced by
> the hiss, I have nearly no signal on all 4 bands.
> I can pump strong rf into it and hear a good side tone/bfo but not
> strongly. Once, I heard a very faint CW station that was loud on my big
> rig. But no others.
> >From the best of my repeated testings, all voltages and resistances
> provided in the manual match.  I have not built an rf probe for my DMM yet
> but will.  However, I also used the signal generator kit from Elecraft
> (the KXGI?).  It is only weakly audible too, and only on the 50 microvolt
> setting.  The AGC setting has no effect.  The attenuator kills the weak
> probe signal when on.
> One post I found after scouring the  Net spoke of a very similar problem
> that turned out to be a bad 2N7000, in the A Preamp stage,  but I can't see
> how that would affect signal sensitivity, only audio gain.
> I also put a scope on pin one and pin 4 of U1, the Receiver Mixer.  The
> relative voltage was 3x higher on pin 1 than on pin 4.  I would at least
> have expected gain instead.
>
>