[Elecraft] Re: Instability on 7MHz with KPA100 - follow up and a cure ?

[g3pjt]

John,

I seem to have 'cured' the problem and will post this comment to the
reflector today.

Following the suggestion of Phil Wheeler who said it might be due to
feedback from RFC10 to the 7MHz input citcuits through the cutout in the
shield to clear the nuts which secure J2-SO239.
Phil recommended I position RFC10 as close as possible to the PCB ie as far
as possible from the other 7MHz circuits. I looked at mine and I had in fact
already done this
But I did not see any reason why I could not blank out the cutout with a
better fitting screen. I therefore made from a piece of thin brass a shaped
screen which filled the cutout except for 2 notches to clear the nuts
securing J2. I soldered the brass to the screen. I also positioned the
entire screen as close to the back panel as the clearance hole in the screen
would allow.
This seems to have cured the problem. I have not removed the brass to see if
the oscillation returns!
I do not have access to a spec anal here so I could not find out where the
oscillation was - I didn't fancy manually tuning my GC RX whilst the amp was
blasting away at 150 watts output !

Thanks for the help,
I will let you know if it returns.

73
Bob G3PJT

----- Original Message -----
From: "John Grebenkemper, KI6WX" <KI6WX@pacbell.net>
To: "g3pjt" <g3pjt@whsmithnet.co.uk>
Sent: Monday, August 05, 2002 9:53 PM
Subject: Re: [Elecraft] Instability on 7MHz with KPA100


>
> Bob;
> Does the oscillation continue even when you aren't transmitting a signal?
> Do you have access to a spectrum analyzer to see what frequencies are
> present in the oscillation?  I was involved in the KPA100 field test and
> took a careful look stability issues.
> -John
>  KI6WX