Adi,
Stewart Baker G3RXQ has also been fighting this problem and he sent me a
note a week or so ago saying that he understood that on Dec 14th Eleraft
issued an Errata for the KPA100 which changed R4 in the HV osc from 100k to
39k and ** added a 1k resistor in parallel with RFC3 ***. There was no
reason given in the errata. Stewart put these changes into his KPA100 and
so have I. Our 80 meter performance is now normal. These two changes are
obviously aimed at fixing the T/R instability in some KPA100's caused by
resonance in or near the 80 meter band involving RFC3 and the junction
capacitance of D14. The 1k resistor spoils the Q of the resonant circuit
without affecting receiver sensitivity or bias voltages and the change in R4
is probably to increase the design margin of the HV bias in some units with
a 'lazy' oscillator.
I had been waiting for an official Elecraft recommendation to the list but I
finally got tired of waiting. In my opinion the 1k resistor across RFC3 is
a very good solution. The change in R4 is not necessary if your bias
voltage is already within the range recommended in the instructions.
Don K7FJ
-------------------------------------------
So K2 Gentelmans...one question..
To fix RF switching problem in KPA100 there are 3 way's available (posted)
1. 12pF to ground from 1n4007's junction (just mooves a problem arround)
2. 1K resistor instead of RFC3 (affects RX preformance and marginal
switching at high power levels)
3. Change RFC3 100uH with 1mH choke (dont know abt side effects but shure
want to know the current rating of choke, cause i suppose must be at least
150mA or more?)
Which way is the best for now?
All kind of Help is Wellcome!
Thanks in advance!
S55M-Adi
K2-3204
----- Original Message -----
From: "Stewart Baker" <[EMAIL PROTECTED]>
To: "Don" <[EMAIL PROTECTED]>; "Elecraft Reflector"
<[email protected]>
Sent: Wednesday, December 29, 2004 7:29 PM
Subject: Re: [Elecraft] KPA100 - 80m poor quality SSB
Although the 1k resistor mod seems to work there is a noticeable drop
in
received signals.
Modeling the KPA100 T/R switching showed this additional loss.
As a temporary measure until the official Elecraft modification comes
out
I have replaced RFC3 (100uH) with a 1mH RFC.
My simulations show that the circuit resonance is now around 1.2MHz-
maybe
lower. Tried all bands, can't find a problem.
73
Stewart G3RXQ
On Tue, 28 Dec 2004 18:11:15 GMT, Stewart Baker wrote:
On Tue, 28 Dec 2004 09:36:00 -0800, Don wrote:
Stewart,
I have been in contact with Gary intermittantly since first coming
across
this problem in October. He assures me that Elecraft is working on a
solution and will have something to say soon. Both the "12 pF" fix and
the
"1000 ohm resistor" fix have proved effective but are probably not the
most
elegant or foolproof ways of solving the problem. For instance, the
1000
ohm resistor alters the reverse bias on D14 in a way that makes the T/R
switch marginal at high power levels. I suspect that Elecraft is taking
their time and checking out all of the possible effects of design
changes
and mods before committing to a final solution.
We will just have to be patient.
Don K7FJ
The guests having departed - the Turkey finally disposed of - the booze
consumed
- thoughts turned to "Playing radio".
I have been getting reports on 80m that my SSB signal had some "edge" to
it.
At first I put it down to RF feedback, but that was not the problem.
I used my FT1000MP to listen to the 2nd harmonic of the 80m signal.
Having read recent mails I first monitored the 3.5MHz signal and found
the
"Buzz" that others had reported.
I then tried the following frequencies on 80 and noticed that as I
increased
the
power control on the K2 the power output would suddenly jump, then when
the
control was turned ACW the power would reduce smoothly and then suddenly
drop.
It was also observable on the FT1000MP S-meter, accompanied by a click.
It was like a form of hysteresis. All other bands gave smooth power
control.
The results were as follows:-
3.8MHz Low point=20W High point=70W Hysteresis=50W
3.7MHz Low point=25W High point=70W Hysteresis=45W
3.6MHz Low point=35W High point=70W Hysteresis=35W
3.5MHz Low point=40W High point=70W Hysteresis=30W
Very unsatisfactory...
I substituted a 1k resistor for RFC3, and all the 80m problems went
away.
Smooth power control on all bands and a better SSB signal on 80m.
Many thanks to those who found/cured this problem that first showed
itself
on
CW, I would certainly have spent a lot of time tracking this one down !
I would suggest that owners of K2/100's check to see that their rig does
not
exhibit these symptoms as it can make quite a big difference to
transmitted
signal quality.
I, like a previous correspondent think that Elecraft should make some
comment on
this design problem, and it's solution.
73
Stewart G3RXQ
Thanks for the "Heads up" Don.
I will look forward to the official modification when it is released.
73
Stewart G3RXQ
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
