[Elecraft] K2 unadvertised feature ?

[Don Wilhelm]

Steve,

The VRFDET is secondary.  If the 8R voltage rail is going low, the K2 is 
either going into transmit state or the microprocessor is dropping its 
RX output.
The control for 8R is on the K2 Control Board - Q2 and Q4.  Check the 
gate of Q4 to see if the voltage there drops - that causes 8R to drop.
Also check the state of the TX signal from the microprocessor (gate of 
Q3) - if that signal is above 3 volts and the RX signal is zero, the K2 
is definitely in transmit state, whatever the cause.

The K60XV uses the 8R input dropping to switch its circuits from receive 
to transmit, and will produce energy at the output - it is operating 
into an open load (your wire antenna), and as a result, VRFDET will 
become quite high because the output is measured assuming a 50 ohm load 
on that K60XV low power output.

Put a piece of coax on the transverter output terminated in 50 ohms and 
see if the behavior continues - that should simulate normal operating 
conditions better than your wire.

If the behavior continues with the coax attached, you will have to find 
how the signal is being coupled into the K2.  If you find the 
microprocessor TX output on and RX off, then I would look at possible 
coupliing from the PTT input or the paddle input (the PTT and DOT paddle 
lines are the same inside the K2), and RF coupling into them could 
induce 'strange happenings' even though the DC voltages may be close to 
the normal 5 volt level.  My guess that the PTT input may be the culprit 
is only a guess, but it is possible for severe noise (high RF levels) on 
the PTT line to cause the microprocessor to become 'mixed up'.

73,
Don W3FPR

Steve Kavanagh wrote:
> Thanks for taking the time to consider my problem, Don.  Here's a few more observations.....
>
>   
>> If the 8R line in the K2 goes low, it is because the K2 has
>> entered the transmit state.
>>     
>
> You'd think so, but since the display does not show a frequency change between the receive state and the anomalous state when the RIT is on (and offset) this is not entirely the case.
>  
>   
>> As a
>> further thought, the driver for VRFDET on the K60XV is a
>> tri-state driver and must have an enable input to drive the
>> VRFDET line.
>>
>> if you do not
>> have a transverter band selected on the K2, the input should
>> be grounded by the K2 relay on the K60XV option.
>>
>> If you *do* have the K2 set to a transverter band and the
>> low power output is selected, then yes, the RF input on the
>> output line could be coupled back into the K2 bandpass
>> filter - the obvious result should be receiver overload, but
>> not changing into transmit state.
>>     
>
> I am thinking of the VRFDET detector in the K60XV, not the one on the K2 RF board, as the source of my problem.  Its enable signal is the 6LP power supply to Q2/U2, which is on in both receive and transmit states.  The VRFDET output from Q2 (P1-pin7) is about 0.4 V in receive and goes up to over 2 V when I transmit on 6m, so there is certainly RF being detected.  In any case, when I switch to a transverter band which does not use the low power output (thus turning off the 6LP voltage), the problem goes away completely.
>
> I tried connecting the cliplead "antenna" to many points on the K60XV (with the cover off the K2), including XVTR IN, all 3 pins of Q1, P1-pin1 (currently fitted with an extra 4.7nF bypass to pin2), and the junction of D1 & D2.  The only one which resulted in the anomalous state was XVTR OUT.  I'm thinking I should try the VRFDET detector diode on the K2 RF board, though.
>  
>   
>> I would suggest that you work to reduce the stray RF energy
>> in the shack, and if that is not possible, 
>>     
>
> It really isn't practical in my apartment station or in the rover vehicle.  Space limitations force operation close to the antennas.
>
>   
>> try ferrite cores
>> on all inputs to the K2 including the power connection.
>>     
>
> In this simplified K2-only test configuration the only connections to the K2 are the pickup antenna to the XVTR OUT and the power connection, and the power lead does in fact have a clip-on ferrite choke in use for this testing, which ought to do something at 50 MHz.
>
>   
>> Try running the K2 and the 6 meter transmitter from different
>> power supplies, 
>>     
>
> The power supplies (at the home station) both plug into the same AC power bar, but otherwise the 6m setup is completely separate. 
>
>   
>> and/or try bonding the other transmitter to
>> the K2 to see if there is any difference.  Often slight
>> ground potential differences can cause "strange happenings".
>>     
>
> Agreed.  I have tried various different ground connections, and different sensitivities to the RF fields result, presumably due to different RF potentials on the K2 case compared to that on the XVTR OUT jack. But there was no magic cure, so far. And this situation will be completely different in the car, anyway...I need to find a more robust solution.  I am thinking of perhaps some sort of bandpass or low pass filter in the TX IF line, though the design is complicated by the fact that the terminating impedances aren't all 50 ohms.
>
> 73,
> Steve VE3SMA
>   
>