[Elecraft] K2 tx/rx switching problem - help please ;-)

[Don Wilhelm]

Vic,

Yes, the T/R switching in the K2 is entirely electronic - no relays are 
used.  You may end up discovering an unsoldered connection or a poorly 
soldered one somewhere in the RX chain, but you can make some other 
checks before searching for an elusive problem like that.

Since you do not have the problem in CW mode, that leaves the KSB2 as 
the major suspect - but there can be a few other potential problem 
spots.  I do not believe it is the AUX BUS.

Does the problem occur if you use a SSB filter other than the OP1 
filter?  If so, the most likely problem is that the voltage at KSB2 U1 
pin 21 (the RXS signal) does not always come up to 5 or 6 volts when the 
K2 switches to receive OR a problem with the diode switching of the OP1 
filter on the KSB2 board - check the soldering of those diodes and other 
components adjacent to the filter.

No problem there?  Go on to the next paragraph.

Open the K2/KPA100 and leave the KPA100 connected (put it on a book or a 
box to the right of the K2).
First thing to check is the state of the 8R and 8T voltages during 
transmit and receive.
An easy place to monitor the voltage of 8T is at the anode of D7 and 8R 
at the anode of D6 on the RF board.
Is the 8T voltage very close to zero during receive?  Is the 8R voltage 
close to zero during transmit?
If the 8R voltage does not drop to zero during transmit, it can be due 
to an old KSB2 board which does not have D5 installed - install a 1N4148 
in the path between P1 pin 3 and the collectors of Q2 and Q3 (yes you 
will have to cut the trace).
The other reason that 8R does not drop to zero is a damaged RF board Q23.

That is the best I can offer until I have more detailed information.

73,
Don W3FPR


On 7/20/2014 2:20 PM, Vic wrote:
> Hi All,
>
> I have a K2/100 feeding a KPA100 in an EC2 case. I have a foot switch and cw
> paddle connected to the key socket at the rear of the K2.
>
> I have a problem with the tx/rx switching. When on cw the tx / rx switching
> works just fine, but when  on SSB and I press the footswitch OR key the
> paddle OR the PTT switch on the mic socket ,  the K2 switches into tx mode
> OK but when I release Ptt switch (any of the three ) the rig returns to rx
> but the received volume is severely attenuated. Only careful listening
> confirms that it is actually receiving.
>
> I can resolve the issue by repeatedly pressing the PTT switch (any of them)
> many times or (oddly) by pressing certain  buttons on the K2 eg Mode , band,
> Pre amp, or even turning the power control up or down so it switches the KPA
> in or out . Not all buttons clear the issue but most do.
>
> I have no idea where to start looking for  the fault and I would be very
> grateful for any suggestions of what this might be. I have a suspicion it
> may be an Aux Bus issue.
>
> I don't think it is a sticky relay as the eventual return to full receive is
> not accompanied  by a relay click and anyway I think the entire tx/rx
> switching is solid state.
>