[Elecraft] Key Line protection for MFJ-1026 Noise Cancelling Unit

[Don Wilhelm]

Peder,

Please re-read that part in the Cady manual again.
What it *should* say is that the KX3 Keyline output will *tolerate* up 
to +30 volts open circuit.
To convince yourself if necessary,  connect nothing to the KEYLINE 
output jack except your DMM.  It should read zero voltage.  Then set the 
DMM to read resistance - the resistance should be high, even infinite. 
When the KX3 goes into transmit, the resistance should be quite low, 
even zero.

The Keyline output is an open drain FET and produces no voltage by 
itself - the voltage is produced by the device connected to it.

Measure the open circuit voltage of the KEYLINE *input* on the MFJ unit. 
  In other words, do not connect anything to the T/R phone jack and 
measure the voltage with your DMM.  If it is less than 30 volts, it can 
be used with the KX3.

73,
Don W3FPR

On 3/27/2017 4:52 PM, pentrose wrote:
> I am trying to use an MFJ-1026 "Noise Canceling Signal Enhancer" with my KX3
> serial #08668.
> The MFJ unit has a T/R phone jack for protecting its circuits from RF
> currents when transmitting.
>
> Q: The voltage seems to be too high according to the Cady book.  I need to
> use the Key Line to signal the MFJ unit to bypass sensitive circuits during
> transmission.
>
> Pg 150 of the Elecraft KX-Line Fred Cady book says the ring has +30 volts on
> the circuit when open.  The MFJ-1026 Manual says to never put over 15v
> positive to the T/R phone jack?
>
> From the MFJ-1026 Manual
> “T/R Control: We Strongly recommend using this phono connector to switch the
> MFJ-1026 to the transmit (or bypass) mode.  The MFJ-1026 switches
> out-of-circuit whenever this line is pulled low (below one volt).
> Never apply negative voltages, or over 15 volts positive, to this jack.”
>
> I have the E980232 Cable adaptor with Key Line information on the Ring.  I
> think I could use the phone jack to send the low signal through the Ring.
> But when it opens up the voltage is too high?
>