Posted by
Darrell Bellerive on
May 21, 2006; 1:50am
URL: http://elecraft.85.s1.nabble.com/K2-AGC-Off-Problem-tp389830p389832.html
Ken,
Thanks for the message.
A 10 pf at C8 did the trick. The receiver is now working properly in AGC off
mode.
What are the implications of this substitution? Does changing C8 from 39 pf to
10 pf cause any other problems?
Since I robbed the 10 pf from it's future spot in the 10 meter low pass
filter, a replacement part is needed in any case. Is it better to replace X1,
and go back to the original 39 pf capacitor, or should I just get another 10
pf for the low pass filter and leave the 10 pf at C8?
Darrell
On May 20, 2006 04:16 pm, Ken K3IU wrote:
> Hi Darrell:
>
> Your report is very similar to what I experienced.
>
> >From an earlier email....
> > Wayne suggested this too:
> >
> > "The simplest cure for this may be a different capacitor in the
> > oscillator that makes it too inefficient at reduced bias to even start.
> >
> > You might try 10 pF at C8 or 680 pF at C7
>
> I installed a 10pf cap at C8 and it worked. I'm sure someone else did the
> same thing with success.
>
> Good luck.
>
> 73, Ken K3IU
>
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]] On Behalf Of Darrell Bellerive
> Sent: Saturday, May 20, 2006 5:37 PM
> To:
[hidden email]
> Subject: [Elecraft] K2 AGC Off Problem
>
> Just finished alignment and test, part II of K2 #5093. All seems to be
> working as it should, except for the AGC off function. This problems
> appears to be the same as a few others have had, the cause of which was a
> bad 5.068 MHz crystal (X1). Perhaps I have the same problem.
>
> When the AGC is turned off the S-meter pegs at 40 over and the receiver is
> muted.
>
> The AGC threshold at pin 5 of U2 measures 3.8 volts whether AGC is on or
> off.
>
> The voltage at pin 1 of U2 goes from 3.8 volts with the AGC on to 5.4 volts
> with the AGC off.
>
> All components in the AGC circuit have been inspected again for correct
> values and placement. All solder joints have been reheated.
>
> I cannot measure the 5.068 MHz oscillator with my frequency counter
> (MFJ-259) at pin 6 or 7 of U1. Perhaps this counter loads down the circuit
> too much for it to oscillate?
>
> Listening with a portable shortwave receiver (5 kHz steps) finds a weak
> carrier at 5.070 that goes on and off with the power on the K2.
>
> Before I email Elecraft support, what else should I check?
>
> Darrell
> VA7TO
>
>
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Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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