Re: [K2] K6XX zero beat indicator and the "Spot" function

Posted by Don Wilhelm-4 on Jul 01, 2011; 4:00pm
URL: http://elecraft.85.s1.nabble.com/K2-K6XX-zero-beat-indicator-and-the-Spot-function-tp6538059p6538303.html

  Chris,

If the K6XX tuning indicator is turning the LED on when you have a tone
in the audio of the correct pitch, then it is working just fine and is
not defective at all.

Your challenge is to find the correct resistor to reduce the number of
false detects.  The indicator requires only about 20 millivolts at the
AF input to trigger, and you have more audio at the hot end of the AF
Gain control than that.

If the simple series 100k resistor did not do it for you, then you will
have to construct a voltage divider of 2 resistors.  You might try a 47k
from the hot side of the AF gain control to the input of the detector
and then another from the input of the detector to ground.  That will
reduce the audio input to the detector to half that at the AF Gain
control.  Vary the two resistors, keeping the total resistance in the
vicinity of 100 k to keep the resistors from loading down the normal AF
signal in the K2.

You might want to connect a 100k pot initially (the detector input
connects to the wiper) so you can adjust it until you are satisfied with
the detector sensitivity, then measure the resistance from the wiper to
each end of the pot and substitute fixed resistors for the pot.

73,
Don W3FPR

On 7/1/2011 10:52 AM, Christopher Kimball wrote:

>
> Thanks for all that have responded.
>
> I've installed a 100 K series resistor in the audio input as suggested.
> The problem continues, however.
>
> In messing around with the unit, I attached an EZhook cliplead to the
> audio input at the board-connector.  The cliplead is attached to a wire
> about 1 ft long with another EZhook at the end.  Nothing is attached to
> the other end, however.  When this is done, the indicator works
> correctly, with no-signal responses to very few noise bursts and correct
> operation on signals, at full RF gain.
>
> The cliplead seems to be injecting noise into the audio line.  If the
> other end is connected to a 1 M resistor to ground, the needless
> blinking resumes.
>
> This suggests that the K6XX board may be defective. Further ideas would
> be appreciated.
>

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