[Elecraft] KX-1 20 Meter Deafness

[Don Wilhelm]

Kurt,

 From all you have said, I would believe that the problem is in the 
KXB3080 low pass filter.
While I agree that your continuity test indicates no apparent problem 
with the toroid lead tinning, check all the connections to be certain 
there is a bit of solder on both sides of the board - if not, view that 
connection with suspicion.

Make certain there is no contact with the green wire from L2 to the 
lower connection of the red wire.  The green wire must wrap around to 
the back side of the LPF board and can inadvertently contact the #4 
termination of the wire for the red winding.

The best LPF testing is visual - look for any suspicious solder 
connections.  Yes, you will see a 17 MHz cutoff on the LPF if you 
measure it without regard to disconnecting it from the rest of the 
circuit and failing to properly terminate the filter (it is not 50 ohms 
at the PA transistor side, but I have not ever calculated the proper 
termination).

Perhaps it is helpful if I tell you that the green winding is the only 
active portion on 40, 30 and 20 meters.  The red winding is used for 80 
meters and SWB band below about 6 MHz only.

Caution:  The sensitivity of the KX1 is not as great on 20 meters as it 
is on 30 or 40 meters.  Bringing a finger (or antenna) close to T2 or 
many of the receiver tuned circuits after the LPF  will normally result 
in an increase in receiver total output, but that is actually an 
increase in broadband noise - under careful measurement conditions, the 
desired signal strength will not increase even though the overall noise 
level does.  Spectrogram will show that is true.

73,
Don W3FPR

Kurt Pawlikowski wrote:
> Hi,
>
>    First, thanks for reading. Hopefully, it won't be boring for those
> lucky K3 guys and gals...
>
>    CONFIGURATION
>
>    KX-1, 3080 upgrade. ATU currently removed and antenna jumper
> installed for testing.Using XG-2 as a signal source. I have confirmed
> the XG-2 seems to be working about as expected with another receiver
> (about S-10 at 50uv and S-0 (but still in the speakers) at 1uv).
>
>    SYMPTOM
>
>    With normal operation, receive levels appear normal on 80 and 40 (I
> can make contacts), but the XG2 is not peaking the KX-1's meter (I used
> the XG-2 to calibrate the meter). 20 meters has always seemed deaf to
> me. I don't remember how well the XG-2 was received on 20 when I
> calibrated it. Here's the real "kicker": On 20 meters, with the XG-2
> attached, I hear *no* volume difference between 50 and 1uv (which means
> the KX-1's receiver is picking up the signal by proximity, not through
> the antenna connection). If I touch the RX antenna connection almost
> anywhere between the low-pass filter and the input of the Rx Mixer (U6,
> pin 1), 20 meters comes way up (from nothing to receiving moderately
> weak signals and noise). Touching the RX antenna line on 80 and 40
> appears to have little or no affect. Output power on 20 meters does not
> register full power, but I'm not sure if this is a calibration problem
> as I don't have an accurate power meter. Also, when I couple a grid dip
> meter into the RX, when tuned to 30 or 20 meters, I get a marked "dip"
> just at about 17 MHz.
>