Posted by
Sarah K. on
Nov 24, 2007; 12:19am
URL: http://elecraft.85.s1.nabble.com/K2-AGC-Threshold-Too-Low-tp367004p367013.html
On Nov 23, 2007, at 3:31 PM, Don Wilhelm wrote:
> Sarah,
>
> Congratulations on getting your license and venturing to build your
> K2.
Thanks!
> The AGC Threshold high limit is directly proportional to the 8 volt
> regulator actual output - if you follow the circuit you will
> discover that that voltage is really the result of a 3 legged
> resistive voltage divider. With a full 8 volts out of the
> regulator, the typical highest voltage is about 3.85 to 3.9 volts
> depending on the exact values of the 5% tolerance resistors. Since
> your 8 volt regulator is near the lower edge of its tolerance range
> (actually a bit lower than it should be), you can set R1 to its
> full limit and things should work fine.
>
> I would recommend that you replace the regulator since a 5%
> tolerance would produce an output between 7.60 and 8.40 volts.
Thanks! I'll email
[hidden email] and see if they'd like to
send me another one next week; I don't have one of those laying around.
In the meantime, I'm proceeding with the RF board, since it sounds
like it will work ok for the time being.
> 73,
> Don W3FPR
Thanks again!
Sarah AF6FH
> Sarah K. wrote:
>> Hi,
>>
>> I'm Sarah, AF6FH! I just got my license a couple weeks ago, and I'm
>> now assembling K2# 0622, which will be my first HF radio. I started
>> assembly on Wednesday night, and so far, everything's been going just
>> fine.
>>
>> But...
>>
>> I'm having a little issue with setting the AGC Threshold.
>>
>> The instructions (pg. 48) say to set the RF GAIN control fully
>> clockwise, measure the voltage on pin 5 of U2 on the control board,
>> and adjust trimmer R1 for a reading of 3.80 volts.
>>
>> I did my voltage measurements with my own (cheap) DMM, not the
>> internal voltmeter.
>>
>> My concern is that, by adjusting R1, the highest I could get the
>> voltage to go was 3.59v, so for the moment, that's where it's set.
>>
>> I measured the voltage on the LM29307-8, and it has 12.23V at the
>> input, but only 7.53V at the output. The 78M05 has 4.93V at its
>> output.
>>
>>
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