Hello Chris,

Is anything else picking up RF? I had a sort of similar issue ages ago
whereby using an autotransformer to impedance match to the loading
coil RF seemed to abound everywhere it shouldn't and the workshop
alarm would trigger as well as the house mains panel RCD tripping.
Then it killed the TV aerial distribution amp in the loft... The
computer broadband would go slow or just stop, lots of issues. I cured
all in one fell swoop by changing from an auto transformer impedance
match to a conventional isolation transformer. I still had the odd FET
failure at the end of a WSPR transmission when the gate signal would
go wild, that was fixed by lots of 0.1uF ceramics on the 5 and 12V
rails  to  any  IC's  in  play, as near the pins as possible. Have you
considered  a  QRP Labs U3S as the exciter, they seem pretty immune to
anything  like  you  see from the PC sound card if nothing else works.
Good luck!

Saturday, June 17, 2017, 1:26:21 PM, you wrote:

> Hi Stefan,

> Many thanks for the prompt reply.
> I shall do as you suggest and hopefully cure the problem.

> 73
> Chris


> On Sat, Jun 17, 2017 at 12:57 PM, DK7FC <[email protected]> wrote:

> Hi Chris,
>
>  Try a 1:1 RF ferrite decoupling trasformer, right behind the PA.
> Maybe also on the PA input. Try to use a battery for the PA, just
> for one WSPR sequence, to see if that solves the problem. If it
> does, then use common mode chokes in the PA supply cable. Avoid
> ground loops. Use shielded circuits only.
>
>  73, Stefan

>
>  Am 17.06.2017 13:20, schrieb Chris Osborn:
>
>  GA LF,
>
>  I have a recurring problem with my home made LF WSPR TX  :-
>
>  As the PC's audio signal shuts down (after the nominal 2 minute 
> transmission time) it picks up RF  -  this is fed back to the TX
> input causing several seconds of instability.
>
>  Has anyone else had this trouble and/or knows of a simple solution?
>
>  I've tried putting ferrite rings around the PC's input and output
> cables which solved the problem for a while.
>
>  In desperation I'm considering adding a monostable to ground the
> TX input for a few seconds after the PC's AF shuts down, but this will be a last resort.
>
>  Any useful advice or comments would be most welcome.
>
>  73
>  Chris (G3XIZ)

--
Best regards,
 Chris                            mailto:[email protected]