Re: KPA100 - 80m poor quality SSB
Posted by
Stewart Baker on
Dec 29, 2004; 6:29pm
URL: http://elecraft.85.s1.nabble.com/KPA100-80m-poor-quality-SSB-tp372990p372993.html
>>> 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
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